Aquatic biomass as sustainable feedstock for biorefineries

The development of biorefineries is a crucial step in the circular economy framework. In biorefineries, research is intensified towards utilizing feedstocks, which do not need arable land or compete with food sources. In this scenario, emerged, submerged and free‐floating aquatic plants are garnering significant attention as potential feedstocks owing to their generation in huge quantities, especially in eutrophic water bodies, similar composition to lignocellulosic biomass with lower lignin content and requirement for only mild pre‐treatments. Therefore, exploring the feasibility of using these aquatic plants for the production of various biocommodities in a biorefinery approach can be of prime importance. In light of this, the current review illustrates the use of some of the major aquatic plants for the production of different biocommodities. The main focus of the study is to shed light on the various biorefinery schemes that could be implemented using these aquatic plants. It also outlines the challenges and prospects of aquatic plant‐based biorefineries. The findings suggest that various biorefinery schemes can be implemented using these aquatic plants and a combination of chemical and biological processes could aid in lowering the cost and achieving better yields. Furthermore, it is also observed that research on large‐scale management and valorization of these aquatic plants also needs to be intensified.


Introduction
I ncreasing demand for resources, the generation of waste, deteriorating ecosystems, ozone depletion and the acidification of oceans are some of the major threats the world is currently facing. To address this environmental crises, the European Union (EU) put in place the 'European Green Deal', a strategy to make the EU sustainable and climate neutral by 2050. 1 Some of the main goals of the European Green Deal are aimed at fully decarbonising Europe's energy supply, tackling CO 2 emissions through carbon capture and storage, creating essential carbon sinks and investing in new carbon-neutral and circular-economy-compatible technologies and systems. 2 The development of technologies which have production and consumption patterns that cause negligible impact on the environment is the main aim of the circular economy framework. The bioeconomy is a concept that is embedded within this framework and seeks the greening of the economy for the generation of novel bio-based products and solutions while minimizing waste. It can be defined as a knowledgebased use of processes and biological resources to produce products within the framework of a sustainable economy. 3 The utilization of renewable biological feedstocks such as organic waste is a key aspect of the bioeconomy. Since waste is utilized as a substrate, the concept of 'cascading use', which encompasses a multi-purpose use of resources to maximize their utilization, holds great promise. One of the ways to apply this cascading approach is the implementation of a biorefinery, which can be considered a prominent method for valorization of renewable feedstocks to valuable, marketable products by adopting principles of environmental and economic sustainability. 4,5 To date, biorefineries have been developed for different kinds of feedstocks such as organic residues, by-products from agriculture and forest industries, sewage sludge and energy crops. Aquatic plants are, however, a majorly neglected sustainable feedstock in these biorefineries. Aquatic plants have a similar composition to terrestrial plants, except for lower amounts of lignin, and hence require milder pre-treatment conditions. They are rich in proteins, starch, sugars and fats and can serve as potential feedstocks in biorefineries. 6 Additionally, they also exhibit higher photosynthetic efficiency, fast growth rates and good biomass productivity. With proper management practices in place, the excess aquatic biomass produced can be used in biorefineries for the production of various biocommodities such as protein sources, fish feed, soil compost, biofertilizer, biofuels and value-added products. Another significant benefit of these aquatic plants is that they do not require arable land for their cultivation and hence can be used in tertiary wastewater treatment systems to remediate pollution through their extensive root structures.
Despite these advantages, the use of aquatic plants for the production of biocommodities in a biorefinery approach is limited. A few reviews have focused on the production of biofuels using different aquatic plants, while other reviews have focused on the use of a specific aquatic plant species for biofuel production. [7][8][9] Different harvesting and pretreatment techniques that have been employed prior to the use of aquatic plants for biocommodities production were also reviewed. 10 However, so far, no detailed reviews on the use of aquatic plants as feedstocks in biorefineries have been carried out. Hence this review sets out to explore the scope of using aquatic plants for the production of different biocommodities and the development of aquatic biomassbased biorefineries. Furthermore, it also elaborates on the probable biorefinery scenarios that can be implemented using these aquatic plants as feedstock. It exclusively focuses on the use of biological methods that are employed in these biorefineries. The review concludes by underlining the challenges that could be anticipated during the development of these biorefineries and provides recommendations and research frameworks that could help overcome these challenges.

Aquatic plants for the production of biocommodities
Aquatic plants are ubiquitous in both temperate and tropical climate regimes. They can be free-floating, submerged and emerged (rooted), grow in different environmental conditions and are abundantly found in water bodies including brackish and polluted waters. 11 Aquatic plants have easy adaptability, require minimal maintenance, and have high expansion capabilities and thus produce huge quantities of biomass when there is high nutrient availability. With an increase in the quantities of waste being discharged into water bodies, aquatic plants thrive efficiently by utilizing these nutrients. The plant biomass is composed of compounds such as carbohydrates, lipids and proteins. 10 Through fermentation, the carbohydrate component can be converted to biogas and biohydrogen (bio-H 2 ), liquid fuels such as bioethanol, biomethanol and biobutanol, and other value-added chemicals and materials ( Table 1). The lipid fractions can be used as a potential biodiesel resource and the protein fraction makes them a suitable protein-rich feed for animals. Over the past few years, research related to the use of aquatic plants for the generation of biofuels and other value-added products has garnered significant attention as observed by the increase in the demand for their biomass. 8 The global harvest of aquatic plants increased from 10.5 million tons in 2000 to 34 million tons in 2018 and is projected to further increase by 20% in 2030. 8 Among the aquatic plants, the widely studied species are water hyacinth (Eichhornia crassipes), water ferns (Salvinia minima), water lettuces (Pistia stratiotes), duckweed (Lemna sp.), hydrilla, frog's bit (Limnobium spongia), mosquito fern (Azolla), water lily (Nymphaea Mexicana), American lotus (Nelumbo lutea) and water spinach (Ipomoea aquatica). 40 These aquatic plants have been extensively used for on-site wastewater treatment approaches and research on their use for the production of bioenergy and value-added products is emerging (Fig. 1). Their ability to grow well on different types of wastewater can be considered an advantage as it reduces the use of freshwater resources while producing different value-added commodities. 41 Biofuel production using aquatic plants

Biogas/biomethane
Plant biomass, especially aquatic biomass, can be considered a potential resource for biofuel production. Conventionally, in order to harvest fuels from aquatic plant biomass, combustion has been widely used. 42 In addition to this, other processes like pyrolysis, incineration, gasification, oxidation, liquefaction, briquetting and anaerobic digestion (AD) have also been employed. 42 Nevertheless, the most direct application of aquatic plants in biofuel production is their use in AD for biomethane production. An extensive amount of literature is available on the use of harvested aquatic plants for the production of biomethane. 43 Although a majority of the research has been directed towards the use of water hyacinth, 44 other aquatic plants such as duckweed, Azolla and water lettuce have been utilized as well. [45][46][47] The most common way to use aquatic plans in AD is to prepare a slurry of the wet biomass. 45 One of the major disadvantages of using aquatic biomass for biogas production is the clogging of reactor components such as feed screws or faster wear and tear of the components. Also the spent biomass generated after operation can result in clogging of the reactor. 48 In order to overcome this challenge, two-stage or multi-stage reactors that aid in the effective handling of the spent biomass were designed. 48 Alternatively, research was carried out on the co-digestion of aquatic plants with other wastes such as manure or sludge to improve the biogas yield. Clementson et al. (2017) evaluated the direct use of E. crassipes as a substrate for biomethane production, wherein the biomass was chopped and mixed with different combinations of manure. Biomethane production was higher in the co-digested water hyacinth and manure in comparison with solely water hyacinth or manure. 45 In another study, water lettuce and waste sludge inoculum were used for biomethane production at different substrate concentrations (30, 40 and 50 g total solids (TS) L −1 ) and digestion temperatures (35, 45, 55 and 65 °C). The highest biogas yield of 321 mL g −1 volatile solids (VS) with 72.5% methane content was achieved at 30 g TS L −1 substrate concentration and 35 °C. 46   examined the AD of Azolla pinnata with cow dung for biogas production. 44 During this study a maximum cumulative biogas production of 3571 L was achieved, of which the methane concentration was 55.62%. 44 In some studies, a pre-treatment step is conducted prior to AD to solubilize the sugars, which were subsequently used for the production of biogas. 47,48 Tonon et al. examined the use of duckweed biomass, harvested from wastewater treatment ponds, for the production of biogas. 49 Three possible pretreatment options (fermentation, drying and alkaline pretreatment) were compared with untreated biomass. A value of 0.25 Nm 3 biogas kg −1 VS fed was obtained with untreated biomass, while the biogas yield was 0.39 Nm 3 biogas kg −1 VS fed when fermentative pre-treatment was employed.
In addition to the production of biomethane, the use of aquatic biomass in AD provides an advantage of recycling the nutrients, as the digestate can be applied as a fertilizer  on agricultural lands. There have been many detailed studies conducted on producing biochar from digestate in particular from AD units, and applying it as a fertilizer to land. 50,51 The liquid digestate has a high level of nitrogen and phosphorous and, therefore, offers the potential to be utilized further in a photobioreactor for the cultivation of algae, which can in turn be used for the production of high-value pharmaceutical agents, pigments and even fuels. Another use for the spent liquid is to utilize it for the cultivation of aquatic plants. 47 Bioethanol Besides biogas, aquatic plants have also been considered as a potential feedstock for bioethanol production due to their high (hemi)cellulose and low lignin content. For the production of ethanol, a pre-treatment step is carried out as well and diastatic yeast is generally used as a biocatalyst for the conversion of the sugars to ethanol (Fig. 2), either via simultaneous saccharification and fermentation (SSF), separate hydrolysis and fermentation (SHF) or via consolidated bioprocessing (CBP). In addition to physical and chemical pre-treatments, enzymatic pre-treatments or a combination of both chemical and biological pre-treatments using fungi have also been carried out, especially for water hyacinth. 52 Higher yields of ethanol can be achieved if the lignin content is low (>60%) prior to fermentation. Therefore, the challenge lies in choosing an appropriate pre-treatment method that can achieve more than 60% reduction in lignin. 52 Additionally, accumulation of a high starch content by the aquatic biomass is another factor that helps in achieving higher titers of bioethanol. 17 Several studies have demonstrated the use of duckweed for bioethanol production due to its low lignin and high starch content. 17 When Landoltia punctata was used as a feedstock, after pretreatment with pectinase, a bioethanol yield of 2.20 g L −1 h −1 was achieved and the ethanol concentration was 30.8 (±0.8) g L −1 . 17 Other researchers have shown that Azolla filculoides can be useful in ethanol production and ethanol yields of 0.09 g.g −1 (equates to 90 g kg −1 ) have been obtained. 9 It has also been estimated that the theoretical ethanol yield from A. filculoides is 9.3 t ha-year −1 [11.7 × 10 3 L ha −1 year −1 , based on the specific volume of ethanol (=0.789 g mL −1 )]. 9 Furthermore, aquatic biomass can achieve comparable yields with other biomass types that are known for ethanol production. For example, Azolla (11.7 × 10 3 L ha-year −1 ) and duckweed (6.3 × 10 3 L ha-year −1 ) have lower ethanol yields than sugarcane (25 × 10 3 L ha-year −1 ), but higher than willow (0.3 × 10 3 L ha-year −1 ) and Miscanthus (2.3 × 10 3 L ha-year). 53,54 The literature nevertheless suggests that bioethanol production can vary based on the composition of the fermentable sugars, like glucose and xylose, present in aquatic weeds. 52 This is in turn dependent on the type of pre-treatment employed for obtaining the fermentable sugars. Further investigation is required on identifying a suitable pre-treatment method to fully utilize the potential of aquatic plants as a feedstock for bioethanol production or any other value-added product synthesis. Furthermore, it is also crucial to identify suitable microbial consortia that can effectively convert the fermentable sugars to ethanol considering the promising yields of ethanol obtained using yeast as an inoculum. 15

Biohydrogen (bio-H 2 )
In comparison with other fuels such as biomethane and biodiesel, bio-H 2 is considered a promising green alternative fuel, owing to its higher energy content (122-142 kJ g −1 ). 55 Several aquatic plants can be employed for the production of bio-H 2 . Pistia stratiotes was used for the production of bio-H 2 and a yield of 2.46 (±0.14) mol-H 2 mol-glucose −1 was obtained. 54 Bio-H 2 production was also carried out using water hyacinth, wherein an initial hydrolysis step was carried out and different parameters such as reaction time (h), acid concentration (H 2 SO 4 ) (v/v) and rotating speed (rpm) were optimized. The maximum bio-H 2 synthesis of 127.7 mmol H 2 . L −1 was obtained at a 7.73 h reaction time, with 264.41 rpm and 1.31% H 2 SO 4 . 56 In another study by Mechery and Sylas (2016), E. crassipes was used as a feedstock and Pseudomonas aeruginosa as a biocatalyst for bio-H 2 production. The aquatic biomass was subjected to acid and alkali pre-treatment at different concentrations and the maximum bio-H 2 production was observed at lower concentrations (2%) of both acid (H 2 SO 4 ) and alkali (NaOH) as opposed to the 4% and 8% acid and alkali. 27 Biodiesel Since the lipid content in aquatic biomass is relatively low compared with other feedstocks such as microalgae, 57 limited studies have been carried out on biodiesel production using aquatic plants. The lipids content in Azolla, Lemna sp., water hyacinth and water lettuce have nevertheless been studied for the production of biodiesel. 58 Duckweed comprises certain important fatty acids such as linoleic acid, palmitic acid, linolenic acid and p-coumaric acid, which can be further used for the production of biodiesel. 53 Although aquatic plants can serve as potential feedstocks for biofuels, their applicability in biodiesel production is still in its infancy and requires considerable research efforts.

Bioelectricity
The rhizosphere of aquatic biomass comprises of various microbes, which take up the root exudates via microbial mineralization processes and provide nutrients for the aquatic plants. 59 Considering this, studies were carried out to harness bioelectricity by designing an ecoelectrogenic engineered system (EES), which contains different aquatic plants in a defined sequence. An electrode assembly was introduced into the system and different circuitry modes (series, parallel and parallel series) were evaluated. The maximum power generation was noticed in the parallel-series connection (9.5 mW m −2 ), which was higher than the individual series and parallel connections (6.5/5 mW m −2 ). 59 In another study by Chiranjeevi et al. (2013), a miniature floating macrophyte ecosystem with water hyacinth and snails (Cyphoma gibbosum) embedded within sediment fuel cells was studied. This system was used for the treatment of domestic sewage and fermented distillery wastewater and simultaneous bioelectricity generation. 60 This further shows that aquatic biomass can be easily integrated with multiple technologies to make the bioremediation process economic and eco-friendly. 61,62 Other applications Numerous studies have been carried out using aquatic plants for soil beneficiation such as vermicomposting, composting or direct application to the soil. 63 Aquatic plants help in recycling nutrients since they take up different types of nutrients present in the water bodies. Taking this as an advantage, studies have been designed to cultivate aquatic plants in metal-laden wastewater and then use them as fertilizers provided toxic thresholds of the metals are not surpassed.  assessed the potential of selenium (Se)-enriched duckweed along with sludge as a soil amendment for increasing the Se content in plants such as green beans (Phaseolus vulgaris). 64 Furthermore, the residual aquatic plant biomass after biogas production is also extensively used as biofertilizers using various processing technologies. 47 Using these aquatic plants as soil amendments not only helps in nourishing the degraded soil, but also helps in the management of the abundant amount of biomass produced by these aquatic plants. Since aquatic plants possess high nutritive values such as high protein content, they can be used as a source of food for fish, animals or humans. 65 Aquatic plants have also been used as substrates for the production of various enzymes such as cellulases and xylanases using various fungal species. 66 Ramirez et al. (2015) utilized water hyacinth and polyester resin for the preparation of a high-performance composite. 67 Due to their high fiber content, aquatic biomass can serve as potential feedstocks for paper making. 67

Aquatic biomass biorefineries
Apart from producing a single biocommodity, coupled systems for wastewater treatment and the generation of a multitude of biocommodities and biofuels using aquatic biomass in a biorefinery approach have also been studied ( Fig. 3; Table 2).

Water hyacinth-based biorefineries
A wastewater-driven biorefinery treating 54 000 L per day of sewage was developed using water hyacinth. 68 After wastewater treatment was achieved, the plants were periodically harvested and were used for the production of VFA and the spent biomass was used as an organic fertilizer by high-rate vermicomposting technology. 68 Water hyacinth was used as a feedstock in a two-or three-stage operation consisting of dark fermentation, biomethanation and microbial fuel cells for maximizing energy recovery. The three-stage process gave an energy recovery of 60% in the form of hydrogen, methane and electricity, with 94% removal of the overall chemical oxygen demand (COD). 70 In another study, water hyacinth was initially subjected to microwave-acid pre-treatment for obtaining liquid hydrolysate containing sugars derived from holocellulosic components of the biomass. This hydrolysate was further subjected to fermentation for the production of microbial lipids and single cell protein using the oleaginous yeast Rhodosporidium toruloides NCIM 3547. 71 After pre-treatment, the inhibitors such as furans were detoxified using the over-liming process and both the detoxified and non-detoxified hydrolysates were used for the production of microbial lipids and single-cell proteins. The results indicated that the reducing sugar concentration was found to be higher in the nondetoxified hydrolysate (65.41 g L −1 ) than the detoxified one (59.18 g L −1 ). In the non-detoxified liquid, the maximum lipid yield of about 0.813 ± 0.041 (g g −1 ) and 53.60 ± 2.68 (g g −1 ) of single cell protein content with 0.038 g L −1 per day of protein productivity was achieved. 71 The increasing demand for bioenergy and other bio-based products presents an opportunity for extending further research on channeling the use of this invasive aquatic plant in biorefinery concepts.

Duckweed based biorefineries
In addition to water hyacinth, duckweed is also a popular feedstock during the development of integrated processes. Duckweed was initially used for the remediation of wastewater and was then sequentially utilized in ethanol fermentation, acidogenic fermentation and methanogenesis for the production of ethanol, carboxylic acids and methane, respectively, in order to enhance the carbon conversion efficiency. The biomass was finally dried and used as a fertilizer. 76 In another study the starch content of Spirodela polyrhiza was enhanced by growing it under nutrientstarvation conditions and the high starch-containing biomass was subsequently acid-pre-treated and used for the production of ethanol using Saccharomyces cerevisiae QG1 MK788210 strain. 77 Additionally, the fermentation vinasse obtained after ethanol fermentation was further anaerobically digested for the production of biogas. 76 Another study demonstrated the use of duckweed to purify and recover nutrients from an effluent obtained after struvite precipitation and ammonia stripping from the liquid fraction of anaerobic digestate. 78 The duckweed was subsequently used for the production of high-quality feedstuff for animals. 78 This approach not only uses resources in a circular approach but also allows for the effective recovery of minerals from these effluents. Duckweed was pre-treated by autoclaving at 121 °C for 20 min. The pre-treated biomass was subsequently subjected to enzymatic pre-treatment using enzymes, αamylase and amyloglucosidase, after which simultaneous saccharification and fermentation were conducted using dry yeast, while succinate was produced using Actinobacillus succinogenes. 80 In another study, duckweed obtained from a local pond was pre-treated using 1% H 2 SO 4 , 1% NaOH and water and pre-treated hydrolysate was used as a feedstock for bio-H 2 production through dark fermentation. The fermentative effluent rich in VFA was subsequently used for the production of lipids using Chlorella sacchrarophila. 75 A maximum bio-H 2 production of 169.30 mL g −1 dry weight was achieved, while the microalgal biomass and the lipid production were about 2.8 and 33 times higher with respect to the autotrophic growth. 75

Azolla-based biorefineries
Studies are also emerging on the use of Azolla as a feedstock in biorefineries. Dohaei et al. (2020) developed a biorefinery wherein Azolla filiculoides was used for the extraction of phenolics, proteins and lipids. After the extraction, the residues obtained were further used in biogas production. 81 Deoiled A. pinnata biomass was used both as an electrode  material as well as a substrate in a microbial fuel cell for bioelectricity generation. 82 Another biorefinery model using A. pinnata was developed, wherein several unit operations such as acidogenesis, photosynthesis, hydrolysis and pyrolysis were sequentially integrated. 82 Acidogenesis was carried out using distillery effluent for the production of bio-H 2 production. The effluents rich in VFA were used for the cultivation of A. pinnata for the production of carbohydrates, proteins and lipids. The lipids were then extracted from the Azolla biomass and the deoiled biomass was harvested and pre-treated using mild acid-hydrolysis. The hydrolysate was used again as a substrate in acidogenesis, thus developing a closed-loop operation. 83

Pistia-based biorefineries
Bio-H 2 production was investigated via photofermentation using P. stratiotes hydrolyzed effluent. 84 Pistia stratiotes effluent was obtained by hydrolysis and acidogenesis using ruminal fluid as inoculum. The effluent was stripped of ammonium and was then subjected to photofermentation using two strains of Rhodopseudomonas palustris: a natural strain (R. palustris 42OL) and a mutant strain with a low ammonium sensitivity (R. palustris CGA676). 85 Both strains depicted higher bio-H 2 production in nitrogenstripped substrate and it was higher for R. palustris 42OL than for R. palustris CGA676 (1224 and 720 mL L −1 , respectively). A pilot-scale biorefinery with different modules was evaluated, wherein in the first module polluted water from an urban river was treated using a phytofiltration lagoon containing P. stratiotes. In the second module, the harvested biomass was used for biogas production which was in turn carried out in two phases. Phase one consisted of VFA production and phase two was operated for biogas production. 85 Growing P. stratiotes biomass in a phytofiltration lagoon is thus feasible within a biorefinery framework, providing biomass throughout the year that can be used for the generation of biocommodities, while also treating polluted water.

Gas-phase biorefineries using aquatic biomass
Although several studies have focused on the use of aquatic plants for biogas production via AD, the cost of biogas production is mostly still higher than that of coal. 86 For example, in the case of E. crassipes it was reported that the production cost of biogas was 1.9 times higher in comparison with production from coal. 86 Therefore, to reduce the production cost the biogas generated could  be used as a substrate for methanol production via the methylotrophic platform (see the section "Gasificationsyngas fermentation"). Alternatively, CO 2 could be separated using various techniques such as pressure swing adsorption, membrane separation, physical or chemical CO 2 absorption and cryogenic separation and can then be further used for the production of bio-based chemicals or for the growth of microalgae. 87 Furthermore, thermochemical conversion methods including gasification and pyrolysis are widely utilized for the production of gaseous products and solid residues (biochar) from aquatic plants. Similar to the use of biogas from AD, the gaseous products obtained in gasification (syngasa mixture of CO, CO 2 and H 2 ) can also be used in syngas fermentation for the production of other value-added products. 88 Therefore, in addition to the direct use of the wet biomass in fermentation processes, cascading the biotechnical use of the gaseous products obtained via either AD or gasification for the generation of biocommodities could make these processes economically feasible (Fig. 4).

AD-methanol platform
The use of aquatic plants can be further extended towards the methylotrophic platform for the production of methanol from the CH 4 generated in the AD process. Methanol is a valued liquid chemical, which can be directly used as a fuel source, or be converted to other products. This can be carried out by a thermochemical process, but is costly as it is operated at high temperatures (200-900°C) and requires expensive metal catalysts. Alternatively, the biological conversion of biogas to methanol can be carried out using methanotrophs, 89 which are aerobic bacteria that utilize the enzyme methane monooxygenase (MMO), followed by oxidation to formaldehyde by pyrroloquinoline quinone (PQQ)containing methanol dehydrogenase (MDH). [89][90][91] Methanotrophs isolated from AD units can be used to convert biogas to methanol. 88 Methanol production was achieved using several methanol dehydrogenase inhibitors and formate as an electron donor. This process achieved a maximum methanol concentration of 0.43 ± 0.00 g L −1 , a gas conversion time of 48 h and a CH 4 to methanol conversion of 25.5 ± 1.1% when using biogas as the substrate and a growth medium containing 50 mmol L −1 phosphate and 80 mmol L −1 formate. 88 However, there are no studies to date that focus on the ability of methylotrophs to convert methane to methanol using aquatic biomass as the feedstock for methane formation. This is an area that needs to be examined further, as it shows great potential for the production of biofuels and valued chemicals.

Gasification-syngas fermentation
Gasification coupled with syngas fermentation is a hybrid process, in which a hybrid thermochemical-biochemical pathway of gasification and fermentation delivers biofuels, like ethanol, butanol and other valued chemicals. 92 One way in which this is done is by acetogenic bacteria, which are well well known for the fixation of CO and CO 2 through the Wood-Ljungdahl pathway. 92 Numerous acetogens have been described, with Acetobacterium woodii and Clostridium autoethanogenum serving as model organisms. 93 The production of acetic acid and ethanol from syngas, i.e. a mixture of CO, H 2 and CO 2 , follows a sequenced set of biochemical reactions as described by Köpke et al. 94 Briefly, energy and carbon from syngas (CO and CO 2 ) are used to produce acetyl-CoA. CO 2 is converted to a methyl group (tetrahydrofolate cycle) through a set of reactions that use one adenosine triphosphate (ATP) and three reducing equivalents of hydrogen (2H + + 2e − derived from CO or H 2 ). Acetic acid can be released from the cell into the bulk liquid or reduced through acetaldehyde to ethanol, consuming another two reducing equivalents. 94 A hybrid gasification-syngas fermentation platform can produce more bioethanol utilizing all biomass components compared with the biochemical conversion technology. 90 In particular, syngas fermentation operates at lower temperatures and pressure, in turn avoiding the need for a pre-treatment process. When using a feedstock, first gasification of biomass occurs, following the fermentation of CO and H 2 into ethanol. This could give a theoretical yield of up to 138 million L of anhydrous ethanol per year. Hybrid gasification/fermentation studies show that the production of anhydrous ethanol using 1200 tons per day (wet basis) of switch grass was feasible in producing 3.07 tons of ethanol per dry ton of biomass. The results revealed a potential production of about 1.4 million m 3 of anhydrous ethanol per yeara higher yield than other biochemical platforms. 90 Syngas production by gasification of aquatic plants has been studied. Aquatic biomass can achieve high conversion to gas on a carbon base compared with woody biomass in gasification. 93,94 This is an indication that gasification of wet aquatic biomass is possible; however further work is needed to determine ethanol production rates with a hybrid gasification/ethanol model. In addition, the yields of syngas need to be optimized since different aquatic plants grown under different environmental conditions can give variable CO yields.

Challenges and future recommendations
Aquatic plant biomass has a high growth rate, while also addressing the need for treating toxic compounds from water supplies. Although aquatic plants have been extensively used for the phytoremediation of metal-polluted waters, there are only a few studies that have reported their further use after treatment. In order to use aquatic biomass for the generation of biofuels or bioenergy, it is important to understand the factors that balance the achieved energy output against the cost incurred during harvesting. Harvesting the biomass is a tedious task and appropriate harvesting methods need to be implemented and developed. Additionally, since aquatic plants might be invasive, their overgrowth needs to be controlled by periodic harvesting. It is also important to consider that post harvesting, problems associated with their storage and degradation owing to the high moisture content might arise, which need to be addressed by developing costeffective dewatering processes.
A frequently encountered hurdle is their collection from point sources and transporting the aquatic plants, and the need to maintain a continuous supply could increase production costs. This challenge can be overcome by installing processing systems closer to the growth and harvesting sites. Furthermore, designing appropriate pre-treatment methods for maximum value addition and process intensification are also two important areas that need consideration while developing aquatic plant-based biorefineries. In order to maximize the growth rate and yield of the desired product, the use of local plant strains rather than exogenous species can be beneficial. 40 Despite some of the disadvantages mentioned above, the fact that many aquatic plants are rich in micronutrients opens up opportunities to enhance the economic attractiveness of these plants. For example in the case of biogas production, undersupply of certain essential nutrients such as nickel, molybdenum, cobalt and Se could lead to suboptimal biogas or bio-H 2 yields. 43 Numerous studies have reported the positive effects of supplying these nutrients to the process to enhance the biogas or bio-H 2 yield. 43 Therefore, the use of aquatic plants that are enriched with these micronutrients could aid in reducing the external addition of these micro nutrients and further extend the application of aquatic biomass specifically in mono-fermentation reactors. However, if wastewaters laden with these micronutrients are used as biofertilizers or for biofortification, it is important to evaluate the presence of these pollutants and their toxic levels and design appropriate methods to lower the toxicity and ensure that toxic thresholds of the metals are not surpassed. In addition, risk assessment studies can further help to reach the full potential of using aquatic plants for nutrient and metal recovery.
Life cycle analysis (LCA) analysis needs to be carried out to obtain a realistic impact assessment of the biorefinery processes. In order to achieve this the first step would be to gather primary data from various simulation tools and vendors to determine the scale of the biorefinery. Sensitivity analysis on the carbon content of aquatic plants and the yield of the products must also be considered in order to improve the economics of the biorefineries. Furthermore, it is also important to take into account the impact of these biorefineries on economies in terms of processing costs and optimization of the supply chain.

Conclusions
This study reviewed the valorization of different aquatic plants for the production of biocommodities using various strategies and biorefinery schemes. Although they have been used to produce biofuels and other bioproducts, aquatic plants are currently an underutilized feedstock in biorefineries and it is, therefore, critical to examine their use within downstream and upstream operations or processes in the context of a circular economy. Although the process schemes followed for the biorefining of aquatic biomass are similar to other biomass crops, owing to their lower lignin contents, milder pre-treatment methods and process conditions can be used. Another major advantage of aquatic plants in comparison with terrestrial plants is that they do not require arable land for their cultivation. The use of aquatic biomass in this manner addresses some of the concerns of energy crops such as intensive labour costs, the use of fertilizers, the maintenance of crops and the prevention of crop diseases. Owing to the current emphasis on steering away from non-renewable fossil fuels, reducing greenhouse gas and carbon emissions, aquatic biomass can be considered as a potential alternative biomass source for greener biofuel production and generation of valuable by-products. This, in turn, lowers carbon emissions, benefits the environment, and helps in achieving the EU Green Deal.

Amulya Kotamraju
Dr Amulya Kotamraju is currently working as a post-doctoral researcher at the University of Galway's IETSBIO 3 group and her research focuses on the development of a duckweed biorefinery for selenium bioremediation and synthesis of biofuels and value-added products. She also works on wastewater treatment and the recovery of value-added products, biopolymers and biochemicals.

Sinead Morris
Dr. Sinead Morris is currently working as assistant lecturer in biochemistry and molecular biology at Southeast Technological University, Carlow Campus. Her research areas include process optimization for the production of potable alcohol, grain whiskey, the effects of agronomic practices and grain composition on alcohol yields, and alternative biomass for the production of alcohol. She formerly worked as a biofuels researcher at the University of Galway's IETSBIO 3 group, focusing on the development of second-generation biorefinery technologies.

Piet Lens
Professor Piet Lens is an established professor of New Energy Technologies at University of Galway. He is funded by the SFI Research Professorship Innovative Energy Technologies for Biofuels, Bioenergy and a Sustainable Irish Bioeconomy (IETSBIO 3 ), which aims to carry out an ambitious research program on the development for new energy production processes from organic wastes in support of providing a self-sufficient Irish energy sector. His research focuses on biofilms, sulfur biotechnology, metal speciation, bioavailability and removal, natural treatment systems, anaerobic wastewater and waste gas treatment for resource recovery and reuse. Besides innovative research, he is also a leader in education and capacity-building, organizing numerous study days, conferences, summer schools and short courses.