Global seaweed farming and processing in the past 20 years

Seaweed has emerged as one of the most promising resources due to its remarkable adaptability, short development period, and resource sustainability. It is an effective breakthrough to alleviate future resource crises. Algal resources have reached a high stage of growth in the past years due to the increased output and demand for seaweed worldwide. Several aspects global seaweed farming production and processing over the last 20 years are reviewed, such as the latest situation and approaches of seaweed farming. Research progress and production trend of various seaweed application are discussed. Besides, the challenges faced by seaweed farming and processing are also analyzed, and the related countermeasures are proposed, which can provide advice for seaweed farming and processing. The primary products, extraction and application, or waste utilization of seaweed would bring greater benefits with the continuous development and improvement of applications in various fields.


Introduction
In the past 20 years, the seaweed farming and production process have increased significantly, and play the important role in the fishing industry by country (Cai et al. 2021). According to the Food and Agriculture Organization (FAO) data, the global seaweed output (both aquaculture and wild) has increased nearly threefold from 118,000 tons to 358,200 tons from 2000 to 2019 (FAO 2021) (Fig. 1). In 2019, 97% of the global aquaculture output came from artificial farming. The world's seaweed production mostly comes from the five major continents with Asia accounting for 97.38%. In Asia, 99% of seaweed is cultured artificially. In particular, China ranks first in the world in terms of aquaculture production, accounting for 56.82% of the global aquaculture. The main algae are Japanese kelp (Laminaria japonica), Gracilaria seaweeds (Gracilaria spp.) and nori Nei (Porphyra spp.). The second is Indonesia, another major seaweed farming country, which accounts for 28.6% of the global breeding. Eucheuma seaweeds nei (Eucheuma spp.) and Gracilaria seaweeds (Gracilaria spp.) are the main species. South Korea has a developed seaweed culture industry and many seaweed species, accounting for 5.09% of the world, including brown, red, and green seaweeds (excluding microalgae). Among them, Japanese kelp (Laminaria japonica) is the most cultured, followed by laver (Porphyra tenera) and wakame (Undaria pinnatifida). The aquaculture in the Philippines accounts for 4.19% of the global market, mainly planting Elkhorn Sea moss (Kappaphycus alvarezii), accounting for more than 90% of the country. North Korea accounts for 1.6% of the global aquaculture and mainly grows Japanese kelp (Laminaria japonica). Japan accounts for 1.15% of the global seaweed production, mainly planting laver (nori, Porphyra tenera), wakame (Undaria pinnatifida), and Japanese kelp (Laminaria japonica). Malaysia accounts for 0.53% of the global aquaculture, mainly planting Elkhorn Sea moss (Kappaphycus alvarezii). North America accounts for 1.36% of the world's seaweed, and 95% of the seaweed in North America is obtained from natural resources. In terms of seaweed cultivation, Chile is the main producer, accounting for 0.3% of the global In Asia and South Africa countries, seaweeds (such as brown algae, leafy algae, and kelp) are often used as fish feed like Laminaria and Sargassum in China, Kappaphycus are used as seaweed fertilizer in India, and made into livestock feed in most European countries (Fig. 3).
Excepted for commercially important brown algae species, research on green algae have so far focused on Ulva lactuca, Enteromorpha prolifera, Monostroma nitidum, Chlorella pyrenoidosa, and Ulva conglobata, with the bioeffects of regulating intestinal flora and improving immune function Zheng et al. 2020aZheng et al. , 2020b. In contrast, the development of edible green algae resources is not enough. More nutrients in seaweed are discovered now, and the potential demand for algal compounds and other chemicals generated by biotechnology is growing In the future, it is expected that research and utilization of edible green algae will attract increasing attention.
Seaweed contains a wide range of bioactive compounds as well as nutritional benefits. Furthermore, algae can produce far more biomass than terrestrial plants and may be cultured successfully in fresh or seawater without the use of antibiotics or pesticides, which lead to an increase in consumer demand and economic interest over the last two decades. In this review, the development of global seaweed cultivation production and processing in the past 20 years was reviewed, and the latest situation and technology of seaweed farming  (FAO 2021) polyphenols, lipids, sterols, carotenoids, and other active compounds, showing a variety of pharmacological properties, so it is called the twenty-first century medicinal food (Yende & Chaugule 2014). Some bioactive compounds of brown seaweed and their important bioactivities are summarized in Table 1.
Polysaccharides are the main components of brown algae. The concentration of total polysaccharides in common seaweed species ranges from 4-70% of dry weight, and the main bioactive polysaccharides include alginate, fucoidan, mannitol and laminarin (Mohd et al. 2021;Holdt & Kraan 2011;Shen et al. 2017). Alginates, the only polysaccharide with carboxyl groups in monomers, provide several health advantages, including anti-inflammatory, antioxidant, antiobesity, antiallergic and immunomodulatory (Feng et al. 2020;Horibe et al. 2016;Wang et al. 2018Wang et al. , 2021Yu et al. 2020). Fucoidan is a sulfated polysaccharide found in large quantities in the cell wall. It has the potential to be employed as an antioxidant, anticancer, anti-angiogenic, antiphotoaging and antitumor drug, according to several in vitro investigations (Cong et al. 2016;Jing et al. 2021;Palanisamy et al. 2018;Park Fig. 4 Overview of the global state of farming, extraction, and applications of seaweeds Page 6 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23  Page 7 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23  Page 8 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23  Sterol Sargassum horneri Antidepressant The levels of major monoamine neurotransmitters and their metabolites in the mouse brain were also evaluated by structural characterization Zhao et al. 2016 Jolyna laminarioides Hypolipidemic Solvent fractions (n-hexane, chloroform, methanol) and fractions containing alginate sterols were evaluated, and marker enzyme levels in the heart and liver were compared Ruqqia et al. 2020 Fucosterol and phlorotannins Eisenia bicyclis Anti-inflammatory The expression of inflammatory genes regulated by NF-κB was inhibited by ROS inhibition Jung et al. 2013 Page 9 of 30 Zhang et al. Food Production, Processing andNutrition (2022) 4:23 et al. 2011;Shannon et al. 2021;Shen et al. 2017). Mannitol is a monosaccharide found in the cytoplasm of brown seaweeds. It has a high permeability and shows promise as a free radical scavenger, lowers strokerelated edema and tissue damage, and is frequently employed in the production of chewing gum, diabetic foods, and various tablets (Bereczki et al. 2007;Cavone et al. 2012;Dai et al. 2017;Ruiz et al. 2017;Holdt et al. 2011;Karthikeyan et al. 2010). Laminarin is a storage β-glucan containing up to 35% laminarin (on a dry weight basis) and is increasingly recognized for its biofunctional activities (Kadam et al. 2015). Laminarin has been reported to have biological functional activities, including anticancer, antioxidant, anti-bacterial and immune stimulation (Bae et al. 2020;Hu et al. 2012;Ji et al. 2020;Liu et al. 2017). Brown seaweeds usually have a small protein content ranges from 5 to 15% of its dry weight. It has been reported that peptides isolated from Undaria pinnatifida have hypotensive effects on blood pressure in spontaneously hypertensive rats (Sato et al. 2002;Suetsuna et al. 2004). Furthermore, lectins, a functional active protein isolated from the brown seaweed Hizikia fusiformis, have been demonstrated to have high antioxidant capacity and robust free radical scavenging activity (Wu et al. 2016a(Wu et al. , 2016b. Various bioactive substances, including omega-3 PUFAs, omega-6 arachidonic acid (ARA), and fucoxanthin, can be Ecklonia stolonifera Anti-Tyrosinase, antioxidant Reduce melanin content and tyrosinase activity, as well as down-regulated production of melanin-producing enzymes (tyrosinase, TRP-1, and TRP-2)

Manandhar et al. 2019
Eisenia bicyclis Anti-obesity Inhibition of pancreatic lipase activity Eom et al. 2013 Ishige foliacea Memory improvement The ERK-CREB-BDNF pathway is regulated to improve scopolamine-induced memory impairment Um et al. 2018 Page 10 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 found in brown seaweed lipids (Miyashita & Hosokawa 2013). The active forms of omega-3 PUFA include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which lower the risk of cardiovascular disease (Lavie et al. 2009;Ruxton et al. 2007;Yanai et al. 2018). Additionally, ARA is crucial for the functioning of the immune system, thrombosis, and the brain (Miyashita & Hosokawa 2013). Fucoxanthin is the most abundant pigment of all carotenoids in brown seaweed and has been shown to have anti-inflammatory, anti-obesity, antioxidant, and antidiabetic properties (Jung et al. , 2014a(Jung et al. , 2014bLee et al. 2003;Yoo et al. 2012). Furthermore, sterols have been found in brown seaweed and have been associated with antidepressant and lipid-lowering properties (Ruqqia et al. 2020;Zhao et al. 2016). In particular, fucosterol, which is the characteristic sterol of all brown seaweed phylum, has antioxidant, anti-diabetic anti-inflammatory and other biological activities (Abdul et al. 2016;Jung et al. 2016Jung et al. , 2014aJung et al. , 2014bLee et al. 2003;Sun et al. 2015;Yoo et al. 2012).
Most brown seaweeds contain fucoxanthin pigments and brown tannins, which give their distinctive greenishbrown hue. Among these, phenolotannins are the most abundant phenolic chemicals in brown seaweed, accounting for 25% of dry weight (Qin 2018). Phlorotannins are found only in brown seaweed and have the 2-tenfold antioxidant activity of ascorbic acid and tocopherol (Bogolitsyn et al. 2019). In addition to the anti-oxidative properties of phlorotannins from brown seaweed, phlorotannins also can prevent obesity by inhibiting the adipocyte differentiation of stem cells (Suzuki et al. 2016). In particular, tannins can improve memory by modulating the ERK-CREB-BDNF pathway (Um et al. 2018).
At present, many active ingredients in brown seaweed have been proved to have functional activities such as antioxidant, anti-inflammatory, anti-tumor, and anti-diabetic. For example, mannitol has been used medicinally as a good diuretic and hyperosmolar antihypertensive agent, but verification of these functional activities with mannitol extracted from brown seaweed is rare. In addition, studies on functional components in brown seaweed mainly focus on macromolecules, such as polysaccharides, sulfated polysaccharides. There are few reports on the mechanism of action of alginate and fucoidan contained in polysaccharides in brown seaweed.

Red seaweeds
The majority of red seaweeds grows in the deep sea, which is the largest group of marine macroalgae. The bioactive compounds of red seaweed and their important bioactivities are summarized in Table 2.
Red seaweed contains comprises polysaccharides (carrageenan or agar), proteins, amino acids, sterols, carotenoids, bromophenols, and other natural bioactive compounds. Polysaccharide is the most developed molecule in the cell wall of seaweed, accounting for 40-50 percent of its dry matter. It is worth noting that carrageenan and agar are the most relevant and developed compounds in red seaweed (Carpena et al. 2022). Carrageenan, the primary algal group of red seaweed, has been extensively studied for its wide range of biological activities, including its antitumor, antiproliferation, antiviral and anticoagulant activities (Guo et al. 2019;Cotas et al. 2020aCotas et al. , 2020bCotas et al. , 2020cJazzara et al. 2016;Gomaa & Elshoubaky et al. 2016;Carlucci et al. 1997). Agar is a mixture of polysaccharides with similar functional properties to carrageenan, exhibiting antiviral, anti-diabetic, anti-colon cancer and anti-inflammatory properties (Ślusarczyk & Czerwik-Marcinkowska 2021;Geetha & Tuvikene 2021;Hardoko et al. 2015;Yun et al. 2021;. The amount of protein in red seaweed varies depending on the species and other conditions such as season, temperature, and light (Cotas et al. 2020b). Protein content ranges from 35 to 47%, which is comparable to or higher than that of legumes and soybeans (Murata & Nakazoe 2001). Most total proteins found in red seaweeds are phycobiliproteins, which also give these species their characteristic red color and are widely used as natural colorants in food and cosmetics (Francavilla et al. 2013). This compound has medicinal potential due to its antioxidant and anti-inflammatory activities Lee et al. , 2017. Recently, bioactive peptides with therapeutic and anti-inflammatory effects have attracted particular attention (Lee et al. 2015). In addition, it has shown great therapeutic potential in the lectin of red seaweed, which has anti-inflammatory, hypoglycemic and antioxidant effects (Alves et al. 2020;Mesquita et al. 2021). Furthermore, red seaweed contains numerous glycine, arginine, alanine, and glutamic acids, including mycosporine-like amino acids, which have been demonstrated to provide UV protection and antioxidant characteristics (De et al. 2009;Karsten et al. 1998;Sun et al. 2020).
Palmitic acid, EPA, arachidonic acid, oleic acid, linoleic acid (LA), and alpha-linolenic acid (ALA) are the main fatty acids in the red and brown seaweeds. Red seaweed in Japan and South Korea mainly contains docenoic acid, and Undaria pinnatifida and kelp mainly contain arachidonic acid (Dawczynski et al. 2007;Tamama 2021). Despite the low lipid content of their constituents, they include vital fatty acids for human health (Amador-Castro et al. 2021). Most sterols present in red seaweed are cholesterol and its derivatives, such as 24-propylidene Gelidium amansii Anti-inflammatory Anti-inflammatory is achieved by increasing the production of anti-inflammatory cytokines and levels of lipolysis proteins  Page 12 of 30 Zhang et al. Food Production, Processing and Nutrition (2022)   Page 13 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 cholest-5-en-3β-ol, a compound that could be used as a potential lead molecule in the development of antibroad-spectrum drugs (Kavita et al. 2014).
Red seaweeds are mostly represented by phenolic acids and flavonoids among phenolic compounds. Additionally, In addition, phenolic compounds (phlorotannins and bromphenol) unique to Marine sources, although in Grateloupia turuturu Antioxidant, anti-inflammatory The radical removal of p-2, 2-diphenyl-1-sulfamide (DPPH) and 2,2'-azino-bis-3-ethyl benzothiazoline-6-sulfonic was evaluated. The potential to carry out acid (ABTS) free radicals and antiinflammatory activities were evaluated based on the ability to inhibit cyclooxygenase 2 (COX-2) enzyme small quantities, have strong antioxidant activity (Cotas et al. 2020a;Dong et al. 2021;Olsen et al. 2013). It possesses anti-AIDS and anti-inflammatory properties in addition to its antioxidant action Paudel et al. 2019). Carotenoids are thought to be one of the major terpenoids found in red seaweed and also contribute to their special pigmentation, mainly represented by α-carotenes and β-carotenes, lutein and zeaxanthin. This carotenoid has antioxidant, anti-inflammatory and anticancer properties, which may reduce the risk of eye disease in humans ( At present, polysaccharide is still the most studied functional component, but the research on red seaweed polysaccharide mainly focuses on monosaccharide composition, molecular weight, etc., but there are still few reports on its advanced structure. In addition, although there are many research on the active ingredients of red seaweed, there is a lack of in-depth pharmacological research and clinical trial research. In conclusion, to realize the further development and application of red seaweed active ingredients, these problems and challenges must be solved first. It is believed that soon, the application of red seaweed functional activity in the medical field will no longer be a limitation.

Green seaweeds
There are over 6700 species of green seaweed in the world, the majority of which live in fresh water. Despite their microscopic size, they are bursting with life and contain nearly all of the nutrients required for human survival. It is regarded as the most ideal diet of the twenty-first century by the United Nations FAO due to its high protein, low fat, low sugar, and low cholesterol content. The bioactive compounds of green seaweed and their important bioactivities are summarized in Table 3.
Enteromorpha, Ulva, and Chlorella are the most prevalent green seaweed. Ulva and Enteromorpha are rich sources of polysaccharides from green seaweed, especially Ulva (total polysaccharide content up to 65% of dry weight). Green seaweed polysaccharide is a kind of acidic polysaccharide (such as sulfated polysaccharides, sulfated galactans and xylans) located on the cell wall of green seaweed. It has special molecular structure and can play a variety of biological functions by regulating cell signal transduction function, such as anti-hyperuricemia, anti-oxidation, anti-coagulation, anti-virus, and blood glucose regulation Chen et al. 2022;Li et al. , 2021aLi et al. , 2021bWang et al. 2020aWang et al. , 2020bWang et al. , 2013aWang et al. , 2013bWassie et al. 2021;Wu et al. 2020). Chlorella is a type of single-celled green seaweed. It grows quickly and is the only plant that may double in size in 20 h. Proteinucleococcus has high protein content and can be used as high nutritional value food. It contains bioactive glycoproteins, polysaccharides and up to 13% nucleic acid and has antioxidant, immunomodulatory, anti-aging, and anti-tumor properties (Chen et al. 2018a(Chen et al. , 2018bTanaka et al. 1998;Wan et al. 2021;Yang et al. 2006). Lectins are one of the protein types binding to carbohydrates or substances in a reversible manner. It has been reported that green seaweed lectins can show potent anti-influenza virus activity with the help of high affinity binding to viral hemagglutinin .
Because green seaweed contains just 5% fat and very little cholesterol (mostly in the form of sitosterol), there is no risk of elevated cholesterol from animal protein.
The fats in green seaweed are mainly unsaturated fatty acids, of which EPA and DHA are important unsaturated fatty acids in marine lipids, which are mainly produced by Eustigmatophyte species. They have potential therapeutic effects in cardiovascular disease, Alzheimer's disease, hypertension, coronary artery disease, arthritis, and cancer (Leone et al. 2019;Peltomaa & Taipale 2017;Van et al. 2011).
The antioxidant qualities of β-carotene produced by the microalgal Dunaliella salina help to regulate the detrimental effects of free radicals, which have been related to a variety of life-threatening disorders, including cancer, coronary heart disease, premature aging, and arthritis. It may also aid the body in combating the effects of ultraviolet light-induced accelerated aging (Dembitsky & Maoka 2007;Miyashita 2009). Haematococcus pluvialis is known to produce astaxanthin, a blood-red carotenoid whose antioxidant properties give astaxanthin an interesting therapeutic potential as an anti-cancer, anti-diabetic, and anti-inflammatory agent (Ambati et al. 2014;El-Baz et al. 2018). Compared with brown seaweed, green seaweed contains relatively few polyphenols. Preliminary studies have shown that Enteromorpha prolifera can demonstrate anti-inflammatory and hypoglycemic effects by regulating signaling pathways (Huang et al. 2022;Yan et al. 2019).
Although there has been some advancement in recent years in the study of structure and activity, particularly in the study of the polysaccharide activity of green seaweeds, other green seaweeds active components have not yet been fully developed and utilized, and the specific biological active components have not been fully explored. Research on the mechanism of action is also necessary.
Page 15 of 30 Zhang et al. Food Production, Processing and Nutrition (2022)   Although the application of seaweed active substances has developed rapidly in recent years, the following problems still need to be solved: most studies on anti-tumor peptides from seaweed currently ignore peptide preservation, which is very unfavorable for peptides with unstable chemical properties. It is useful to improve the stability of anti-tumor active peptides from seaweed by developing existing active peptide preservation and delivery technologies, such as encapsulation of chitosan nanoparticles and administration of multi-encapsulated liposomes. Furthermore, the active substance of seaweed extract must be produced in a more efficient manner. The method of trehalose synthase catalytic synthesis is simple and low-cost. However, it has some advantages of high energy consumption material, but the enzyme also has some disadvantages, such as resistance to high temperature and pH. To tackle this challenge, trehalose synthase's structure and function need to be learned more. In addition, seaweed extracts are limited to purify the active substance. If the purification enhanced, it will be helpful to further study its function and excitation, which is also the future research direction of seaweed extract. It is worth emphasizing that most of the bioactive studies currently being conducted are non-human including in vitro and in vivo animal models. However, due to the different enzyme catalytic centers and the complexity of organisms, the applicability and safety of algal bioactive substances as drugs for further research remain to be solved. Therefore, the stability, spatial structure, and physiological function of these bioactive substances still need to be further studied.

Seaweed farming technologies
Early algae were mainly from natural growth, and people mainly collected wild algae (Tseng et al. 2001). With the growing market for food industrial and medical use of seaweed products, the wild seaweed resources are limited, which has promoted the seaweed farming. Seaweed culture can be cultivated on land, sea, desert and even in integrated aquaculture system. Even the seaweed farming technology is not different in essence, there are obvious differences in culture methods due to the different environment.

Land farming
The cultivation of algae on land is mainly in closed systems such as water tanks, ponds, lagoons, and pipelines (Sara et al. 2020). The culture method is suitable for a wide range of seaweed genera. It has simple and easily accessible equipment that allows real-time monitoring and effective regulation of seaweed culture conditions (nutrients, light, pH value, CO 2 and salinity) to produce more target products. However, land-based aquaculture occupies scarce cultivated land and water resources, which requires high maintenance cost and cannot achieve mass production. In recent decades, there has also been a way to use saline alkali groundwater for algal culture (Sara et al. 2020), using the existing salt water resources at low cost and high economic efficiency. A circular culture system was developed to reduce the cost by reducing the required medium (Sebök et al. 2017). Page 17 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 Mariculture Inshore shallow water aquaculture is carried out in the sea area close to the land and is the main method of algae culture, at sea depth of 5-50 m, using fixed piles off the bottom. It has sufficient land nutrients and benefits from moderate seawater velocity and wind wave degree. To meet the growing demand for seaweed products, the field of seaweed culture has extended outward, and there is a way of offshore deep-water culture that relies on floating rafts or long ropes to make culture rafts (Fig. 5). Mariculture does not occupy scarce land resources. Compared with shallow sea culture, offshore deep-water culture is greatly affected by uncertainties such as wind and waves, and alien species, and is difficult to manage and costly to maintain. It has high requirements for breeding equipment, seaweed varieties and technology, which has not been popularized. Semi floating rafts and supporting rafts are mostly used in intertidal aquaculture, with high requirements for the selection of sea areas. Due to the increasing breeding density, pest and disease problems occur frequently, inshore aquaculture appeared, which does not occupy scarce land resources and is free from the impact of marine turbulence. Desert farming technology through water and nutrient recycling effectively exploits the rich desert resources and alleviates the shortage of land resources (Buschmann et al. 2017).

Integrated aquaculture system
In the integrated aquaculture recycling system, the algae can absorb nitrogen and phosphorus from the waste produced by aquatic products and serve as feed when algae and aquatic products are mixed. The cultivation of seaweed in this cycle system can reduce marine eutrophication and repair the damaged marine ecological environment system better (Sun et al. 2016). Sashimi seaweed quantitative ecological breeding model was developed. The advantages of high yield and benefit by using kelp culture raft for large-scale economic algae and sea cucumber multi variety rotation culture was proved. Algae-bacteria symbiotic system is used for algae culture to treat organic wastewater and human and livestock waste. The integrated high-rate algal ponds (HRAP) system is applied for wastewater treatment and algae production. However, the system has too many links and is inefficient. It is only suitable for large-scale aquaculture in ponds and lakes, but not for industrial aquaculture. Moreover, the seaweed seedling raising technology in China is weak and the supply of seedlings is insufficient to meet the demand for year-round supply of seedlings for this system.

Seaweed processing and applications Current status of development
The Financial Times has reported that the global population will rise to 10 billion by 2050. And algae could supply the protein needed for people while conserving natural resources (Koyande et al. 2021). Because algae grow 10 times faster than terrestrial plants, less than one-tenth of the land is needed to produce the same amount of biomass. The growth of algae does not compete with other crops for land and does not require fresh water. It fertilizes more efficiently than land crops, and avoids the intensive water use, fertilizer wasting, and downstream eutrophication associated with modern agriculture (Tzachor 2019). Therefore, seaweed has aroused great interest as for these advantages around the world, especially in Asia, Europe, and South America, as well as in North America and Australia. Particular attention has been paid to seaweed resource processing and utilization (García-Poza et al. 2020). Forbes reported that the market for algal products was expected to be approximately $4 billion in 2018, growing to $5.2 billion by 2023 (Kite-Powell 2018). In the past twenty years, algae contain high levels of minerals, dietary fiber, and low fat levels, which has regarded as an attractive raw material in food, medicine, chemical industries, and even as the natural source of CO 2 and biomass energy.

Current extraction techniques
In recent decades, research on extracting bioactive ingredients from natural resources have attracted special attention. Studies have shown that many ingredients have a variety of biological characteristics and potential industrial application prospects. Therefore, it is necessary to find new technologies for improving the production of algae extracts, instead of choosing Page 18 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 extraction conditions which are time consuming, low selectivity, low efficiency and harmful for human health. In this paper, the extraction methods of algae oil and the extraction techniques of volatile substances from algae in recent years are summarized, and the new extraction techniques such as microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), supercritical fluid extraction (SFE), pressurized solvent extraction (PSE), and enzyme-assisted extraction (EAE) are introduced. Additionally, the traditional extraction technology is compared with the new extraction technology in order to better develop and utilize algae resources (Table 4). MAE is a relatively new extraction technique that combines microwave and traditional solvent extraction. The use of microwave radiation, which generates heat directly in the matrix through the friction and collision between molecules, has been used to extract seaweed hydrocolloids and other derivatives from red and brown seaweeds to obtain high quality seaweed hydrocolloids with less extraction time and solvent consumption. Compared to the traditional methods of extracting compounds from natural products, MAE has shorter extraction time, less solvent, higher extraction rate, and lower cost (Delazar et al. 2012).
UAE can be carried out at low temperatures by taking advantage of the vibration cavitation effect of ultrasonic waves, which reduces heat loss from high temperatures and prevents bioactive substances. It is suitable for the extraction of heat-resistant compounds and is simpler and faster than microwave-assisted extraction and has great potential for large-scale production (Chandrapala et al. 2013). However, the process is also affected by many factors such as extraction time, microwave power, and solid-liquid ratio. Currently, this technique has been used in the extraction of many plant materials by significantly reducing extraction time and increasing the maximum extraction rate (Ma et al. 2010;Surin et al. 2020).
SFE is a process of extracting valuable substances by using solvents at pressures and temperatures above critical points, which is environmentally friendly, inexpensive, widely available, non-flammable, and timesaving. Carbon dioxide and water are the most common uses of supercritical fluids. Hydrocolloid from marine algae contains many bioactive substances that are susceptible to degrade at high temperatures. SFE-CO 2 provides a non-oxidizing atmosphere during the extraction, thus preventing the degradation of the extract. Kumar extracted total phenols with antioxidant activity from brown seaweed (Sargassum wightii and Turbinaria), and this activity was greatly improved compared to the traditional organic solvent extraction method (Kumar et al. 2020). Although extracting algal flavor compounds usually takes several hours, it is expensive and difficult for machines to clean (Dmytryk et al. 2015).
PSE is a relatively new automated technique that extracts target compounds at 200℃ and 3000 psi, using solvents or mixtures of solvents with low boiling points. The solubility, solvent diffusivity and mass transfer rate increased significantly by PSE method, while solvent viscosity and surface tension decreased significantly. Compared with SFE, PSE extraction can use a wider range of solvents. However, PSE is not suitable for heat-resistant compounds that are sensitive to high temperature and high pressure and is not selective for SFE. It has been shown that the extraction of carotenoids from Dunaliella halogensis, as well as kavanolides from pepper, can yield higher yields with less solvents in a shorter time while maintaining chemical integrity (Hossain et al. 2011).
EAE uses specific enzymes to break down unwanted components of the cell wall, thereby releasing the desired components. Compared with the traditional water extraction method, this method has the advantage of high catalytic efficiency, and retains the original effect of the compound to a large extent. Billakanti et al. (2012) extracted haloxanthine from wakame by alga lyase hydrolysis with the optimum temperature at 37℃ and pH at 6.2 to yield well performing bioactive compounds.
It is necessary to select appropriate extraction technologies for different active substances. In particular, the combination technology has great potential to minimize the degradation of bioactive compounds caused by different extraction steps. Many bioactive substances from seaweed play an important role and have promising applications in functional foods, health care products, cosmetics, and medicine. However, more researches are needed to improve modern extraction technologies to enlarge industrial scale.

New applications Food industry
Today, seaweed is as widely used as a vegetable. In many Asian countries, seaweed is an important part of human diet in its fresh, dried, flaky, and flour form. Commercial production of seaweed has been the focus of seaweed research in the past, but recently the researches has been a shift towards high-value products with health benefits. Studies have shown that adding Chlorella to foods (such as pasta and biscuits) can improve the nutritional quality of the diet. Chlorella and Spirulina are mostly applied in tablet, capsule, and liquid form for nutritional supplements because of their high nutritional value and ease of growth. Moreover, an edible cyanobacterium Spirulina platensis has gained worldwide attention as a food additive due to its high nutritional value as a human food (Andrade et al. 2018;Batista et al. 2017;Martelli Page 19 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 Kumar et al. 2020;Dmytryk et al. 2015;Güçlü-Üstündağ et al. 2005 Pressurized solvent extraction  Hossain et al. 2011;Turner & Waldebäck 2013;Reighard & Olesik 1996;Denery et al. 2004 Page 20 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 Billakanti, 2012;Nguyen et al. 2020 Page 21 of 30 Zhang et al. Food Production, Processing andNutrition (2022) 4:23 et al. 2020). It has proven to be a rich source of protein, polyunsaturated fatty acids, and pigment. Clearly, the food industry is beginning to focus on developing highvalue non-commercial products for human health. In the future, many seaweeds are likely to become important components of functional products.

Agriculture
Farmland natural ecology is currently deteriorating due to excessive usage of artificial fertilizers and pesticides. Seaweed is abundant in unique mineral elements, nutrients, and biologically active chemicals. In recent years, agricultural output has played an increasingly vital role. Seaweed can be utilized as a protectant for diseases and as a stimulant in horticulture, promoting and enhancing all aspects of plant growth and development (Battacharyya et al. 2015). Green seaweed Ulva crude extracts and sulfated polysaccharides have antibacterial activity against common bean (Phaseolus vulgaris L.) anthracnose, as well as considerably promoting soybean growth (Paulert et al. 2009). Furthermore, seaweed can boost the ability of plants to absorb nutrients, hence improving plant quality. A new study showed that leaf spraying and seed soaking can significantly improve the yield and nutritional quality of carrots treated with seaweed (Sargassum vulgare) extract (Mahmoud et al. 2019). Seaweed is a valuable animal feeding as well as a source of agricultural chemicals. A variety of algal diets have been utilized to grow a range of fish, shrimp, crabs, and shellfish throughout the last two decades. The most commonly alga are Chlorella, Spirulina, and other microalgae ). Many minerals remain in the waste biomass after cyanobacteria recovering oil and carbohydrate, which can be used as fertilizers to improve various physical and chemical properties of soil while boosting yield and conserving fertilizer nitrogen. The Asia-Pacific region accounted for more than 15% of global seaweed fertilizer market revenue in 2017. By 2025, the global market for seaweed fertilizer is estimated to reach 17.1 million US dollars. Organic agriculture is gaining traction, and the usage of seaweed fertilizer is on the rise. As a result, seaweed processing is predicted to become a key resource guarantee in green and modern agriculture.

Biological medicine
In medicine, seaweed has attracted a lot of attention as a potential source of various drug properties. In the last two decades, seaweed polysaccharides have been shown to have a variety of promising biological activities, such as anti-tumor , immunomodulatory (Huang et al. 2015), antioxidant (Maheswari et al. 2021), anti-hyperglycemic (Pantidos et al. 2014), anti-cancer , antiviral, anti-fungal (Pallela & Kim, 2011), anti-diabetic (Lin et al. 2018Zhao et al. 2018), anti-hypertensive (Seca & Pinto 2018), anti-inflammatory, uv-protective, and neuroprotection effects (Schepers et al. 2020). Meanwhile, algal hydrogels and hydrocolloids are valuable components in the medical field, which are widely used in wound healing, drug delivery, in vitro cell culture and tissue engineering (Senthilkuma et al. 2017). These gels maintain structural similarity to the extracellular matrix in tissues and can be manipulated to perform several key roles. Although some specific drug-specific gels have been clinically used for wound healing, they play a rather passive role. In wound healing and drug delivery applications, there is a great need for precise control of single drug delivery versus multiple drug delivery, or continuous and sequential release in response to changes in the external environment, which is useful for future development of products (Lee & Mooney 2012).

Chemical industry
For nearly 20 years, the bioactive substances from macroand micro-algae are popular in the cosmetic industry. Compared with terrestrial plants, algae contain many unique and novel bioactive ingredients such as polyphenol compounds, halogen, terpenoids, sterol compounds, unsaturated fatty acids, and polysaccharides in addition to vitamin, protein, minerals and trace elements (López-Hortas et al. 2021). Various algal composition as a thickener, water binder, antioxidant, and UV blockers, are present in a variety of facial and skin care products (such as masks, eye creams, and sunscreens) to improve moisture balance, reduce wrinkles, and improve skin tone (Priyan Shanura Fernando et al. 2018;Wijesinghe & Wedamulla 2019). Thus, seaweed could be a sustainable and profitable source of bioactive substances with the growing demand for cosmetics and cosmetic ingredients.

Other applications
Algae is a decent candidate because of its renewable and sustainable features, as well as its economic viability meeting the world's demand of fuels for transportation. Algae can be used to produce biodiesel, bioethanol, biohydrogen, and biomethane. And it is particularly popular in energy applications due to its high safety, lack of competition from food crops, high reproductive capacity, and short cycle (Adeniyi et al. 2018). In addition, algae has been used in the construction industry. Vijayaraghavan and Joshi (2015) developed a new alga-based green roof growth matrix, which found that the green roof 's runoff quality improved after adding brown seaweed (Turbinaria conoides) to the growth matrix. It can improve building insulation, rainfall attenuation, sound insulation, and lessen the heat island effect and extend the life of the  Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 roof. Besides, algae can also help to tackle the problem of eutrophication in water bodies. Green algae were utilized to treat municipal wastewater in ponds (Woertz et al. 2009). They may remove up to 99% of ammonium and phosphate under cultivation conditions, and then offer valuable wastewater treatment services and supply raw materials for liquid biofuel synthesis. Seaweed would encapsulate its value in a variety of industries due to its unique composition.

Challenges and solutions
Over the past 20 years, the situations of algal breeding and processing industry have risen steadily, which playing the active role in economic, social, and ecological aspects. Especially, the marine ecological problems have been alleviated due to the positive ecological benefits of breeding algae. However, this field still faces some challenges including the lack of improved varieties, poor growth environment, immature breeding and processing technology, shortage of cultivated land resources, and restrictions of relevant policies. Due to the continuous expansion of seaweed market and the increased demand for seaweed products, it is urgent to overcome their own and external constraints.

External challenges
The external challenges are mainly in the following aspects: global warming and sea-level rise lead to the declined seaweed biomass and quality. In order to deal with the harsh environment, the cost of breeding management including equipment maintenance, renewal and growth environment management has been increased. However, it is a burden for many low-income areas for the increase of technical investment in the development of high-quality seaweed species and processing products. Seawater eutrophication and the emergence of harmful algal blooms are happened due to rapidly expanding human activities (Yu et al. 2016). Harmful algal blooms usually contain the toxic substances. Accidental poisoning may occur by eating contaminated food (Lewitus et al. 2012). The environmental problems caused by the development of algae breeding and processing will eventually become factors affecting themselves. For example, halogenated hydrocarbons produced by many algae will affect the flux of ozone and ultraviolet rays. The density on growing environment is easy to bring about the invasion of alien species. The breeding equipment is the attachment base of the green algae. And the water body in the breeding area is relatively stable because a large number of rafts hinder the flow of sea water. With the increase of temperature, these large amounts of green algae reproduce and eventually enter the ocean to form a "green tide" (Yu et al. 2016). Microplastics in the ocean will be captured by cultured algae and spread to a higher nutritional level through the food chains, resulting in a significant burden on marine ecology.

Internal challenges
Seaweed production and processing technology does not match its increasing demand. At present, the cultivation of seaweed is mainly developed in some underdeveloped areas. Only a small number of countries such as China, have realized industrialized seedling raising, large-scale offshore cultivation, and mechanized harvesting, forming an industrial chain from product processing to sales. However, most countries mainly focus on basic cultivation, the automation of large-scale harvesting and processing are not high. It results in the unguaranteed quality, low efficiency, high employment cost, and processing waste. Most seaweed processed products are mainly used as edible and industrial raw materials. In recent years, large amounts of bioactive compounds from seaweed have been studied. However, the valueadded products is generally low because of the lack of advanced techniques for extraction and purification (Pérez et al. 2016).

Response measures
The large-scale human activities at sea should be restricted, while the implement staggered peak aquaculture of algae and timely issue relevant water quality protection policies should be encouraged. Algae with high temperature resistance and disease resistance need to be cultivated. The advantages of algae should be used to solve problems and realize a virtuous cycle. For example, macroalgae can absorb carbon, nitrogen, and phosphorus in seawater through photosynthesis. The absorption of CO 2 can play the ability of marine carbon fixation. Algae can reduce nitrogen and phosphorus in the enrichment of waters by inorganic nutrients. Ulva ohnoi has been proved to be an ideal target species for bioremediation activities at land-based aquaculture facilities in eastern Australia (Lawton et al. 2013). Seaweed has also been used to treat anaerobic digestion piggery effluent (Nwoba et al. 2016). In the integrated multi-trophic aquaculture (Chopin et al. 2012), the waste nutrients released or excreted into the water in the aquaculture system can be used by algae as the source of nutrients, which can achieve the goal of recirculating aquaculture system and regulating water quality.

Conclusions
The development of seaweed farming is growing year by year. The seaweed farming technologies are constantly updated, and seaweed processed products are emerging. Therefore, seaweed plays a huge role in the economic, Page 23 of 30 Zhang et al. Food Production, Processing and Nutrition (2022) 4:23 social, and ecological fields. It is rich in a variety of biological active substances as drug sources, cosmetics, and agricultural regulators, which have been largely developed. It is necessary to strengthen investment in seaweed farming and processing technologies and develop high value-added products with the integrated multi-trophic aquaculture, which have great market potential and need in-depth exploration.