Anaerobic digestion is a well-established technology that converts organic waste into clean bioenergy. Biogas and digested substrate (digestate) are the products of anaerobic digestion. The biogas usually contains 55%–65% CH₄, 35%–45% CO₂, 0%–3% N₂, 0%–1% H₂, and 0%–1% H₂S (Milono et al., 1981).

The anaerobic digestion of organic waste is a complex process composed of a series of bio-metabolism steps, which include hydrolysis, acidogenesis, acetogenesis, and methanogenesis, respectively (Figure 3).

The mechanisms reported above are the main ones responsible for the kinetic of reactions, and they are highly dependent on environmental and/or ambient conditions such as temperature, pH, C/N ratio, C/P ratio, particle size, inhibitors, and type of substrate (Mata-Alvarez et al., 2000). The first step of the process is hydrolysis, where enzymes decompose the complex polymeric structures of cellulose, starch and proteins into monomers or oligomers such as glucose, fatty acids, and amino acids. This process is quite fast, but it can be limited by the presence of lignin-rich substrates. Ariunbaatar et al. (2014) report that this step process can be accelerated by introducing specific enzymes.

The second step (acidogenesis) transforms the products of the hydrolytic process into volatile fatty acids (VFAs), alcohols and ketones. This process is fast (30 min, and this causes an accumulation of VFA accumulation in the digester, resulting in digester toxicity if not properly controlled by operational conditions, substrate composition, and microbial population in the anaerobic digestion system (Lukitawesa-Patinvoh et al., 2020). The pH is the operating parameter that affects most the VFA formation, and it was reported that the optimal pH range is 5.5–6.5 (Mao et al., 2015).

The third step (acetogenesis) transforms most products of acidogenesis and some of the long-chain fatty acids from the hydrolysis stage into acetate, CO₂, and H₂ into (CH₃COO-), hydrogen (H₂), and carbon dioxide (CO₂) and the kinetic of reaction is reported to be in a range of 1.5–4 days (Ramos-Suárez et al., 2015). This step is sensible to the presence of H₂ and O_2, and the operating pH has to be in a range between 6–6.2 (Geraldi, 2003; Burton et al., 2014; Stronach et al., 2012).

The fourth step (methanogenesis) plays an essential role in generating methane gas by methanogens. There are two primary mechanisms for methane generation, including acetoclastic (CH3 COOH→CH4+CO2) and hydrogenotrophic methanogenesis (CO2+4H2→CH₄+2H2O). Typically, methanogens are extremely sensitive to pH conditions, the presence of oxygen, and other factors such as free ammonia (FAN), H₂S, and Volatile Fatty Acids (VFAs) (Geraldi, 2003; Mao et al., 2015; Ramos-Suárez et al., 2015; Van et al., 2020).

Most of the literature on anaerobic digestion uses municipal waste and agricultural waste as substrates. However,Xu et al. (2018) reported that the digestion of food waste has more advantages, such as mitigation of climate change, economic benefits and diversion opportunities. For example, Bartocci et al. (2020) reported that by replacing 9,900 tonnes of corn silage with 6,600 tonnes of food waste, it is possible to reduce CO₂ emission by up to 42% of the electricity produced from the biogas plant could be achieved. The benefit of the anaerobic digestion of food waste against agricultural waste creates a strong interest in using fish and algae waste as substrates has been growing since 2018 due to the incredible potential of these sources for the production of biogas through anaerobic digestion.

Fish waste is a complex substrate because the composition of solid and liquid fish processing waste depends on the composition of the fish species used, which in turn depends on the sex, feeding habits, season and health of fish.

These wastes contain high levels of protein (up to 60%), fat (up to 20%) and minerals (calcium and hydroxyapatite from bones and scales), and the high content of fat seems to favour the yielding of the biomethane. An optimised model for anaerobic digestion reported that the production of biomethane can vary from .2 to .9 CH₄ m³/kg VS. added by using different forms of fish waste (Kaspars et al., 2018) (Table 1)

Despite the methane production shown in Table 1, using fish waste as substrate can cause operational problems. Indeed, fish waste releases high levels of ammonia when digested, which inhibits the digestion of substrates (Achinas et al., 2017). High ammonia concentrations can result in the accumulation of VFAs (acetic acid as the main type in the batch tests). To mitigate the negative effect of ammonia formation co-digestion process is a possible technological solution. One of the best ways to co-digest fish waste is with agricultural waste or algae, as reported in Table 2. Nazurally (2018) reported that a general accumulation of VFAs was observed for co-digestion of algae and fish, and this phenomenon was due to the high content of fatty acid. The co-digestion is not the only way to control free ammonia formation and the accumulation of VFAs, but they can also be controlled by the reactor, type and organic loading rate and pH (Shi et al., 2017).

Table 2 reports that the digestion of pure algae produces much less methane when macroalgae are processed alone. The co-digestion of algae and fish waste seems an optimal solution for an environmentally and economically sustainable process. The use of pure macroalgae waste in the bioreactor is not economically sustainable due to several technical issues, such as the seasonal growth associated with different types of macroalgae, the variability of the feedstock and operational costs (Ward et al., 2014; Milledge et al., 2019). In addition, the use of marine waste macroalgae produce several problems, such as difficulty in processing material such as polyphenols, cellulosic fibres and lignin-type components. This results in the reduced biodegradability of the biomass by bacterial processes and thus a limiting digestibility and gas production (Briand and Morand, 1997). Microalgae waste could be more valuable if microalgae species were grown as part of a wastewater treatment process, but this review will not discuss it.

Biohydrogen generated from a renewable and sustainable source is a clean energy carrier since the combustion process leads to water formation, which creates an attractive energy source compared to other renewable sources. Nowadays, 96% of hydrogen (grey) is produced through carbon-intensive processes, where steam reforming of natural gas accounts for 48% of total production capacity, while petroleum fractionation and coal gasification make up 30% and 18% of production capacity, respectively (Franchi et al., 2020). The remaining 4% of hydrogen is considered green because it is produced by renewable sources and water electrolysis. Considering the actual percentage of green hydrogen, the decarbonisation of hydrogen production (grey hydrogen) is a potential paradigm that can be only solved by implementing carbon capture and storage (CCS) integration (blue hydrogen) or considering the use of clean energy sources (green hydrogen). Using clean energy sources is expected to reduce annual carbon dioxide emissions by nearly 440 million tonnes in 2050 (Nicita et al., 2020).

The conversion of waste biomass represents an inexpensive alternative for bio-hydrogen generation because the technologies used do not require high energy consumption compared to water electrolysis. The electricity cost accounts for 80% of the total cost of hydrogen production by electrolysis (Kapdan and Kargi, 2006).

The methods for biohydrogen production can essentially be categorised into two main classes: The thermochemical conversion (pyrolysis and gasification) route relies on high-temperature operations to degrade biomass wastes to produce biohydrogen, where the types and conditions of feedstocks used can heavily influence the outcome of the products.

The biochemical conversion (dark fermentation and photobiological) fermentation route emphasises more on the physical conditions of the medium (living organisms) and the types of catalysts used. In both processes, the hydrogen must be separated by several technologies, such as absorption, adsorption, membrane separation, and cryogenics separation, irrespective of upstream processing routes (Ren and Toniolo, 2018; Jiménez-Llanos et al., 2020).

Studies on several types of biomass waste reported that hydrogen production via the thermochemical pathway seems techno-economic more viable than the biochemical pathways. The thermochemical pathway is proven at a commercial scale, the cost of the catalysts is lower, and the technical challenges are limited compared to the biochemical pathway (e.g., biomass pre-treatment, bioreactor design and restriction to hydrogen), and they are able to achieve overall efficiency in a range between 50%–70% depending on the operating conditions (Dascomb et al., 2013; Kannah et al., 2021).

The gasification process is the most efficient method of hydrogen production from waste products due to the high biomass conversion in the gas phase. Pyrolysis is an ideal method of hydrogen production only at high operating temperatures (over 600°C) and or at low temperatures if the pyrolysis plant is coupled with a steam reforming system able to hydrocrack the bio-oil produced. The thermochemical pathways required biomass waste with a moisture content not higher than 20%. The biomass with moisture content higher than 20%, such as fish or algae, must be dried. The procedure requires an additional power source or an optimised heat exchange system to reduce the moisture contained in the waste before being fed into the system. Thus, using waste with higher moisture will reduce the efficiency of the process and increase the production of hydrogen costs. On the other side, fish waste and algae are inexpensive biomass waste that produces a third-generation of biofuel at low cost. Recent studies on fish waste and algae are easy to process and obtain bio-oil or syngas as precursors in synthetic fuel production. Rowland et al. (2009) successfully demonstrated that fish waste with high moisture content could easily gasify fish waste with dried pellet wood to control the moisture level upstream of the gasifier reactor. The study also demonstrates that adding fish to the pellet wood reduces the High Heating Value (HHV) of the syngas produced, which is still high enough to make this process feasible in rural areas or for local use. Reza et al. (2022) successfully demonstrated that fish waste could easy be gasified into syngas at high temperatures without reducing the moisture content of the biomass. They reported the high syngas production and, thus, hydrogen was achieved operating at a temperature over 600°C. The high temperature favoured the cracking reaction of bio-oils contained in the fish and produced during the pyrolysis process, consequently increasing the fraction of gas produced up to 43% at 600°C. The percentage can be increased by increasing the temperature and ramp rate moving from conventional pyrolysis to a fast pyrolysis process. However, they also reported that using fish waste as feedstock produces a high amount of ash that must be removed during operations, which can affect the operating costs and, consequentially, the biofuel costs. Cao et al. (2020) reported that steam gasification is one of the most efficient processes to produce syngas and hydrogen using biomass with higher moisture content. The process is suitable for large-scale industrial production with a high gasification rate and low ash production. The utilization of a catalyst in this process reduces the operating temperature and favours the gasification process. The introduction of steam increases hydrogen production. However, the process has challenges that must be faced up, such as decreasing the tar contents, optimising the composition of the catalyst to minimise the deactivation, and reducing the energy and material costs. Duman et al. (2014) reported that the feasibility of steam gasification was affirmed by using micro and macroalgae as feedstock. The studies conducted showed that the hydrogen production capacity of macroalgae was much higher than that of microalgae. The maximum hydrogen production depends on the inorganic content of raw materials in macroalgae (18%), which favours gasification and could reach 1,036 ml/g of hydrogen production against 413 ml/g from microalgae.

The studies on biomass gasification reported successful studies on fish and algae waste. The studies report that gasification is a promising technology for hydrogen production and operating parameters such as steam/biomass, pressure temperature and water contents) that drastically affects hydrogen production, as reported in different studies (Han et al., 2013; Iovane et al., 2013; Zhang et al., 2015a).

Thermochemical processes of biomass are a mature and efficient process for converting both dry and wet biomass, but this requires high temperatures and biomass pre-treatment to feed wet biomass. Hence, the scientific community is focusing on developing a biochemical process that has the advantages of using low operating temperatures and microbial to convert wet biomass such as algae and fish waste into biohydrogen. They are still under investigation and have been proven mainly at the lab scale.

Nowadays, thermochemical conversion is the only one mature technology able to produce large amount of biohydrogen. However, this technology can compete with conventional fossil fuel (price below $2.00/kg) only if the cost of the waste biomass has a gate fee that could range anywhere from $50-$75/tonnes and the size of the gasification plant is 150 MW. Above this value the biohydrogen cost can vary between $2.80/kg (Binder et al., 2018; Shahabuddin et al., 2020). Fish and algae waste are low-cost feedstock which could thermochemically converted into low-cost hydrogen.

Biohydrogen production by biological conversion is a promising technology due to the low operating temperature and by-product formation. However, state of the art, this technology has been proven only in a laboratory-scale, and its efficiency strongly depends on the type of technology used. The highest efficiency achieved with the biological conversions is 80% using Direct Bio-Photosynthesis and 42.80% using dark fermentation.

The predicted cost to produce Biohydrogen by biological conversion are calculated by the assumption, and the hydrogen cost can change from $2.13/kg for direct photosynthesis to $7.53/kg for dark fermentation (Forrunque-Ahmed et al., 2021). Although the predicted hydrogen cost productions are comparable to the thermochemical conversion this technology is still not mature to compete for thine biohydrogen production due to technical challenges to face up.

Indeed, the most developed biochemical processes are dark fermentation (DF) (light independent) and photolysis (light dependent). They are still in the demonstration phase due to the low hydrogen production. It will be essential to reduce the reactor cost and investigate different geometry to improve the performance and reduce the production cost. The feedstock cost is also considered an important parameter in lowering the cost of hydrogen. Among all these processes, photo and dark fermentation are essential to biological hydrogen production technologies. Of the two methods mentioned above, DF is the most studied and promising technology for biohydrogen production owing to its higher production rates and treatment capacity for organic wastes. DF can convert several types of substrates, including waste products rich in carbohydrates and fatty acids. At present, the DF process is not mature, and the development at the industrial scale is limited due to the lower hydrogen yield compared to its theoretical maximum yield of 4 mol of H₂ per mole of hexose, as well as the estimated costs associated with the H₂ production. The production cost in a scaled-up system can be minimised by pre-treatment of substrates, enrichment of inoculum and low-cost feedstock. Dark fermentation has been suggested as more practical than the other processes as it does not require external energy to drive the process or a large surface area to capture the necessary light. It can take advantage of existing reactor technologies to utilise organic wastes as feedstock (Han and Shin, 2004; Perera et al., 2010) (Table 3).

Dark fermentation is a complex system where environmental factors and bioreactor operation conditions such as temperature, pH and H₂ partial pressure control metabolic pathways of hydrogen-producing microorganisms. Furthermore, we need to consider other parameters, such as substrate types and their pre-treatment methods, bioreactor configurations, inoculum sources and enrichments that influence biohydrogen production. The complexity of the system requires that different challenges have to be overcome, such as energy balance and COD conversion and improved solid-state fermentation, which has demonstrated higher hydrogen production (Ghimire et al., 2015a, 2015b). This technology is promising to produce biohydrogen, but it still requires a lot of investigations to be economically viable on a large scale. Many researchers have documented that the H₂ production from wastewater, organic waste or biomass by biochemical path has the potential to reduce H₂ production costs (Das and Veziroǧlu, 2001; Chang et al., 2011).

Differently from dark fermentation, photo-fermentation and biophotolisys are two light-driven processes in which carbon sources are converted to biohydrogen using photosynthetic bacteria. A key addition to the whole process is light energy, natural or artificial. The photo-fermentation uses photosynthetic bacteria, mainly called purple non-sulphur bacteria (PNS bacteria). Rare, green bacteria and purple bacteria are used too. The light energy is used to oxidise the carbon source and produce electrons. PNS bacteria (and the other bacteria that can be used) synthesise nitrogenase or hydrogenase enzymes. The methods take place in anaerobic conditions, and nitrogenase under anaerobic conditions use electrons and ATP (Adenosine triphosphate) and produces hydrogen and ADP (Adenosine diphosphate), as described by the Eq. 1 (Rahman et al., 2016). 2 H + 2 e − + A T P → H 2 + 4 A D P + P i (1)

Operating conditions (i.e., pH, temperature and light intensity) depend on the specific bacteria used and the converted feedstock.

Biophotolysis uses microorganisms such as cyanobacteria or microalgae to produce biohydrogen from water. A key factor for this method is sunlight, which is essential for the system to make biohydrogen. Water is not the only reactant that can be used in these processes. Glucose as well as other organic matter, can be used in biophotolytic processes. This method of hydrogen production has been applied in different ways through the years, mostly at the laboratory scale. Biophotolytic processes can be categorised into two main categories: direct biophotolysis and indirect biophotolysis. The general chemical reaction that describes these two processes is given by reaction Eq. 2 (Anto et al., 2020). 2 H 2 O + L i g h t   e n e r g y   → 2 H 2 + O 2 (2)

In direct biophotolysis, the photosystem absorbs the light energy and transports electrons to ferredoxin, reducing the water molecule. The reduced ferredoxin can transport electrons to hydrogenase (biohydrogen-producing enzyme). After that, hydrogenase catalyses the conversion of a proton to biohydrogen, according to the reaction Eq. 3, (Mona et al., 2020). 2 H − + 2 F D − → H 2 + O 2 (3)

The oxygen produced acts as an inhibitor for the hydrogenase enzyme and is a major problem in direct biophotolytic systems.

Indirect biophotolysis is a process that takes place in two stages. In the first stage, the photosynthetic system produces a large amount of biomass to increase the amount of carbohydrates. In the second stage, biomass-rich carbohydrate is utilised as carbon sources. This carbon source plays a similar role to water in direct biophotolysis. This second stage has some similarities with anaerobic fermentation processes. The advantage of indirect photolysis compared to direct photolysis is that oxygen generation is separated from the stage of hydrogen evolution, so oxygen is not inhibiting the H₂ evolution (Rahman et al., 2016; Mona et al., 2020). Table 4 summarises the various biological hydrogen production processes with general technical information involved therein broad classification of microorganisms used with their relative advantages.

Photo fermentation was coupled with dark fermentation to convert residue from anaerobic digestion from fish waste into biohydrogen (Melitos et al., 2021). Ghimire et al. (2015c) reported that connecting the DF-Photo Fermentation reduces the control of operating parameters such as pH and the process impacts on the energy yield reaching 55 MJ/kg volatile solid food waste, adding a synergistic effect to the overall energy recovery during the conversion of food waste.

Biophotolysis is the main use to process hydrogen from water. The process has also been applied to wastewater and improved water quality in the aquaculture system. Malara et al. (2017) use photolysis to inactivate vibrio species (pathogens) in aquaculture.

One of the biggest challenges today is to produce a large amount of sustainable biofuel to decarbonise the heavy transportation sector and achieve the zero-emission target by 2050. Biodiesel and renewable diesel seem to be the more suitable fuel for their physical chemistry properties to replace fossil fuels. However, their economic and environmental sustainability is highly affected mainly by the feedstock type and the oil extraction method, which are responsible for the overall biodiesel and the catalyst used and for production methods and the catalyst. The utilization of edible oils (vegetable oil) is too costly and negatively impacts the environment and society (Sumathi et al., 2008). Utilising non-edible oil plants does not provide enough oil to satisfy the energy demand, and the extraction of the oil from the plant seed is complex (Hamza et al., 2020). Studies have shown that waste feedstock rich in oil, such as fat animal waste (e.g., fish waste) and algae, are potential renewable low-cost fuel sources of interest (Aniokete et al., 2022; Douvartzides et al., 2019; Madeen et al., 2021). Fish waste and algae waste from aquaculture are low-cost feedstock and have demonstrated that they can be easily converted into biofuel with minimal economic and environmental impact (Abomohra et al., 2018; Papargyriou et al., 2019).

Fish and algae waste are two sources rich in oils. The amount of oil contained depends on the type of fish and algae. Depending on the extraction methods, it is possible to extract up to 25 wt% of oils in fish waste, and 10 wt% and 60 wt% of oil from macroalgae and microalgae oils (Zhaohui et al., 2020; Gosch et al., 2012). The oil is extracted using several methods, such as wet rendering, enzymatic hydrolysis, autolysis, dry rendering, solvent extraction, and supercritical fluid extraction (Dumay et al., 2004; Falch et al., 2006; Rai et al., 2010; Ghaly et al., 2013; Jayasinghe et al., 2013; Oliveira et al., 2013; Suseno et al., 2015). While the algae oil is extracted using the Soxhlet apparatus and hexane solvent (Aravind et al., 2020). The studies reported showed that supercritical and microwave-assisted technologies are the most appropriate technologies to extract oil from waste. These technologies demonstrated a high oil recovery efficiency, lower cost, and high environmental sustainability (Zulqarnain et al., 2021). The purified oil can then be converted accordingly to the type of fuel desired. The extracted waste can produce third-generation biodiesel (Fatty Acid Methyl Ester) and renewable diesel (long-chain hydrocarbons). There are two main mature routes for the production of biodiesel and renewable diesel from marine, aquaculture, fish industry and algae, which are transesterification and hydrodeoxygenation (Figure 4).

The transesterification reaction of fish oil or algae oil with low molecular weight alcohols in the presence of a catalyst oil produces Biodiesel (FAME), a biodegradable fuel (Wu and Leung, 2011; Sharmila et al., 2021). The utilization of waste oil makes the production of biodiesel more economically and environmentally sustainable biodiesel (Rajendran et al., 2022). However, the process requires a high energy demand for biodiesel purification and recycling of large volumes of alcohol when it is carried out using homogeneous catalysis (Falch et al., 2006; Tanwar et al., 2013). The recent study focused on using a more sustainable method, replacing the homogenous catalyst with a heterogeneous catalyst based on alkaline Earth oxide (Marinković et al., 2016). Papargyriou et al. (2019) and Mahdavi et al. (2015). The heterogeneous catalysis showed comparable performance compared to the homogenous catalysis, but it has the advantage of reducing energy consumption and operating cost, producing high-purity biodiesel by reusing the catalyst for many cycles (Lee et al., 2014; Knothe and Razon, 2017). Enzymatic catalysts have also been investigated as possible heterogeneous catalysts for the conversion of fish waste oil with success. The utilization of enzymes allows the production of very pure biodiesel, which could further reduce the cost of biodiesel production. However, the enzymatic reactions show a slow reaction rate, and the high cost of the catalysts doesn’t make it a commercial pathway for biodiesel production (Angulo et al., 2020).

Biodiesel production is proven at a commercial scale, and the size of the plant does not affect the price of its sustainability. This means that the plant can also be scaled up accordingly with the amount of feedstocks available locally. The main problem associated with the use of biodiesel is its low oxidation stability, poor cold flow properties and low energy content. This required that biodiesel has to be blended with fossil fuel to improve its properties and can be distributed using existing infrastructure (Knothe and Razon, 2017).

Differently from biodiesel, Renewable diesel is a mixture of long-chain hydrocarbons) with a composition like diesel derived from fossil fuels. This makes green diesel highly stable due to the absence of double bonds in its structure, and more valuable than biodiesel because it has a higher cetane number and a higher heating value than FAME biodiesel and has similar fuel properties to petroleum-diesel. This means that green diesel can replace diesel-derived fossil fuel and can be delivered using existing infrastructure (Miller and Kumar, 2014).

Hydrodeoxygenation (HDO) is a catalytic chemical process that uses hydrogen to decarboxylase the methyl ester for a stable long-chain hydrocarbon with the same characteristics as diesel derived from fossil fuel. HDO of bio-oil generally occurs under high pressure (7 MPa–20 MPa) and high temperature (200°C–400°C) conditions to increase the amount of effective hydrogen in bio-oil and reduce the oxygen content, thereby ameliorating the physical and chemical properties of bio-oil. Oxygen is removed in the form of CO₂ or H₂O, meanwhile, the unsaturated bonds become saturated (Qu et al., 2021).

The hydrodeoxygenation process has the advantages of guaranteeing large conversion, better selectivity, high stability product. However, the process requires highly harsh conditions, a complex equipment system, blockages of the reactor due to the production of high molecular weight wax and deactivation of the catalyst (Qu et al., 2021). The catalysts are the main components of this process because they guide the performance of the process. Transition metals such as Ni, Mo, Co., Zn, Ce, Nb, Fe, and Cu, among which Ni and Mo are generally used as the main active metals (Wang et al., 2015; Hong and Wang, 2017; Yang et al., 2018). The Mo-based catalysts displayed high arene selectivity, while the Ni-based catalysts mainly saturated the aromatic ring in liquid-phase reaction conditions (high hydrogen pressure) (Zhang et al., 2020).

Renewable diesel is produced by the hydro-deoxygenation of vegetable oils, and it is a mature and certified technology at a large scale. However, some gaps still need to be filled, such as selecting the suitable and best catalyst, inexpensive feedstock and studying the mechanism of the reaction. Several studies have been conducted to prove that waste material reduces the production cost, particularly when fat animal waste is used (Toldrá-Reig et al., 2020). However, the hydrodeoxygenation process of algae waste seems not to be economically sustainable due to the highest impact on the water scarcity footprint and the unit production cost (Madeen et al., 2021).

Despite the several challenges and problems related to this technology, the Techno-economic Analysis and Life-Cycle Analysis of Renewable Diesel Fuels Produced with Waste Feedstocks carried out from Longwen et al. (2022) highlight as the use of waste feedstock is still crucial for the production costs and for the Greenhouse gas emissions. However, the main challenge for this technology is the process scalability elated to the viability of large-scale production because distributed waste resources are a well-known feature for any conversion strategy using a waste resource.

Surfactants are amphiphilic compounds, composed of so-called hydrophilic head and lipophilic tail that have the ability to reduce surface and interfacial tensions by accumulating at the interface and forming larger structures called micelles, in the first case, and surfactant liquid crystals, in the second (Kronberg and Lindman, 2014). They fall into four categories (neutral, anionic, cationic, and zwitterionic) depending on the nature of their hydrophilic fragment, the lipophilic fragment remaining structurally similar across all categories of surfactants. Gemini surfactants are a separate category of surfactants constituted of two hydrophilic head groups of any type and a single lipophilic tail group.

Most industrially relevant surfactants have found applications in the detergent, cosmetics, emulsification, and anti-foaming industries (Chang, 2016) and are widely derived from petroleum. In order to meet the ever-increasing demand for surfactants as petroleum reserves run out, biosurfactants have emerged as ideal alternatives. They have the characteristic overall amphiphilic structure of synthetic surfactants but vary at the hydrophilic head group, which can be one of several usual motifs encountered in natural molecules.

These are naturally synthesised by microbial sources such as bacteria (typically Bacillus sp. (Sakr et al., 2021), Pseudomonas sp. (Bhosale et al., 2019)., Botyris cinerea (Abidi et al., 2008), Lactobacilli (Gudiña et al., 2011), and Candida utilis (Ribeiro et al., 2019) from renewable sources and perform equally to better than synthetic surfactants in several areas (e.g., oil spill, oil recovery, wastewater treatment, and pharmaceuticals) (Ng et al., 2022). Apart from being more environmentally friendly than their current counterparts, they also present many desirable features such as being less toxic (Akbari et al., 2018), biodegradable (Vijayakumar and Saravanan, 2015), highly specific and effective at extreme temperature, salinity, and pH conditions (Pacwa-Płociniczak et al., 2011). Certainly, their main selling point is that they can be produced at a lower cost from waste (Sáenz-Marta et al., 2015; Martins and Martins, 2018; Jiménez-Peñalver et al., 2020). However, this is conditional on the isolation of the biosurfactant, as the purification process may still represent the main contribution to the overall production cost (Mukherjee et al., 2006) and identifying suitable organic waste material is crucial for the commercialization of biosurfactant. Additionally, parameters such as carbon and nitrogen sources, trace elements, temperature and pH are other variables which can strongly influence the biosurfactant yield (Patel and Desai, 1997).

Biosurfactant production by bacteria or enzymes has been investigated with all sorts of plant biomass and food industry wastes (Makkar et al., 2011; Mohanty et al., 2021). Alternatively, marine environments host a rich diversity of organisms that naturally produce biosurfactants and emulsifiers (Rahman et al., 2019), some of which are already widely used in the food industry (Liao et al., 2021). In general, biosurfactants from marine environments are derived from a wide range of organisms including, but not limited to, macroalgae, microalgae, bacteria, diatoms, and cyanobacteria (Silva et al., 2012). Algae are the most abundant resource in the ocean and represent a virtually endless source of biosurfactants called polysaccharides (Xu et al., 2017). These long polymers [tens to hundreds of kDa (Usman et al., 2017)] occur as the main structural components of marine algae or as microbial secretions, known as exopolysaccharides (EPS) (Manivasagan and Kim, 2014). Polysaccharides can be extracted with hot water (Zhang et al., 2010; Savage, 2012), although removing unwanted species with organic solvents is more efficient and less costly (Lim et al., 2014). Lately, these methods have been replaced with microwave, ultrasonic, and enzyme-assisted extraction. Microwave-assisted extraction has the advantages of short extraction time, low energy, and low cost (Sousa et al., 2010; Hahn et al., 2012; Kadam et al., 2013). Ultrasonication and enzymes are used to break down the cell walls and release the compounds of interest either by cavitation and diffusion through cell walls or by hydrolysis of some of the cell wall structures (Kadam et al., 2013). All these techniques have proved to have higher efficiency compared with conventional methods (Yuan and Macquarrie, 2015). To a lesser extent, liquid extraction methods have been improved using more environment-friendly solvents such as supercritical fluids (Herrero and Ibáñez, 2015) and ionic liquids (Kadam et al., 2013; Martins et al., 2016). Algal polysaccharides have found widespread applications in the food industry as emulsifiers and thickening and stabilizing agents, cosmetics, biomedical industry due to their anti-cancer, anti-viral, anti-inflammatory and antioxidant activities, and heavy-metal removal among others (Anestopoulos et al., 2020).

Exopolysaccharides (EPS), or extracellular polysaccharides, are complex mixtures of anionic biopolymers consisting primarily of polysaccharides as well as proteins, nucleic acids, lipids, and humic substances (Manivasagan and Kim, 2014). The huge variety of organisms (e.g., macro-/microalgae, bacteria) that produce EPS combined with the sometimes drastically different environment in which they grow has been an endless source of new biopolymers with unique properties. This comes to the price of a challenging isolation and purification of the target compounds, as exopolysaccharides are secreted along with other extracellular polymeric substances (Suresh Kumar et al., 2007; Manivasagan and Kim, 2014). Therefore, EPS may exhibit properties and functions that are actually reflected by the collective extracellular polymeric substances’ characteristics of a mixture of compounds (Xiao and Zheng, 2016). In particular, sulphated polysaccharides are a category of biosurfactants exclusively found in the marine environments with additional properties that structural polysaccharides do not possess, owing to the multiple sulphate functional groups, and that slightly vary depending on the degree of sulphation of the polymers (Silva et al., 2012; Raposo et al., 2013). These are as diverse as joint lubrication and targeted drug delivery.

Another non-negligible potential feedstock for biosurfactant production is fish wastes as sources of both peptide and fatty substrates. With 30%–80% of the fish body weight being discarded during industrial processing operations (Dave and Manuel, 2014), the upcycling of these wastes also contributes to solving environmental and health problems. Zhu et al. (2020) took advantage of the richness of these wastes to synthesize lipopeptides. They first hydrolysed blended fish heads and fish livers to produce peptones that Bacillus subtilis transformed into lipopeptides. The lipopeptides were used in the formulation of a bio-dispersant that exhibited particularly good behaviour to treat oil spills. Similarly, Hu et al. (2021) enzymatically hydrolysed tuna fish red meat to obtain peptones that were converted into biosurfactants by Bacillus subtillis. They found that on a laboratory scale, the surface tension and critical micelle dilution were similar to those obtained by Zhu et al. (2020) but insist that results may vary depending on the fish source. Their process was scaled up to a 100-L pilot-scale, which is to date the most advanced process for the production of biosurfactant from fish materials. The same bacteria Bacillus subtilis was used to prepare lipopeptides in 30% yield from more refined materials, such as fish oil and a culture broth (Saranya et al., 2014). They were then immobilized on nanoporous activated carbon and effectively removed Ca²⁺ (98%) and Cr³⁺ (92%) from water solutions. By using Ustilago maydis FBD12 instead of Bacillus subtillis. Cortes-Sánchez et al. (2011) produced glycolipids from soybean and fish oils. In optimal conditions, the production of glycolipids was higher with fish oil. Moreover, the production of glycolipids from soybean oil was higher when Candida rugosa (lipase) was added, while this revealed detrimental when fish oil was used, suggesting that fish oil may be a better raw material. Finally, Kaskatepe et al. (2015) used Pseudomonas aeruginosa strains to convert fish meal into rhanmopeptides. The three strains tested (ATCC, H1, and SY1) produced biosurfactants at concentrations of 12.3 g/L, 9.3 g/L, and 10.3 g/L respectively, which decreased when the bacteria were exposed to UV light. Kadam and Savant (2019) compared the production of glycolipids from shrimp shell and fish wastes and plant biomass by Pseudomonas stutzeri. Curiously, fish wastes gave by far the worst yield while shrimp shell wastes were the best substrate and could produce 4 g/L–6 g/L of sucrose-based glycolipids under the optimized conditions. When fish wastes were transformed with Corynebacterium spp. CCT, anionic biosurfactants were produced (Martins and Martins, 2018). When compared to sugarcane bagasse, petroleum sludge, and glycerol, both fish wastes and sugarcane bagasse significantly outperformed the last two feedstocks. Despite the more industrially viable production of artificial surfactants, there seems to be a trend according to which biosurfactant production is higher when unrefined feedstocks from sources other petroleum are used. These combinations of sustainable feedstocks and natural microorganisms is economically promising as they valorise cheap wastes that would otherwise create environmental and health problems, without loss of efficiency compared with artificial surfactants.