2. Coastal Marine Biorefinery Systems
The concept of biorefineries for the production of liquid fuels is not new, as the processing of different biomass feedstocks into bioethanol is a common practice in many countries such as Brazil and the USA. However, such industries consume huge amounts of freshwater. It was estimated that the production of 1 L of bioethanol requires 5–10 L of water during the fermentation process alone, with the total water footprint ranging between 1388 to 9812 L when taking into account conventional routes of biomass production
[15][16]. Given the major concerns regarding freshwater shortages, this production method is unsustainable, and a coastal marine biorefinery could provide a solution. This is a conceptual refinery system that relies on the use of marine components (seawater, yeast, and seaweeds) to produce biofuels and other valuable products. In the case of bioethanol, seaweeds are used as a feedstock, seawater as a growth medium and marine yeast for marine fermentation (
Figure 1). Though research has been done on each individual element, “coastal”, “marine”, and “biorefinery”, a single system combining all three elements has not yet been considered
[14].
Figure 1. Visual diagram of a coastal seaweed marine biorefinery. Marine components include seawater, seaweed, and marine yeast (orange arrow). After processing, a number of outputs (blue arrows) are obtained. High-value chemicals (HVC) are extracted during the pre-treatment step, while bioethanol and by-products (e.g., plant fertilizer and salted animal feed) are produced during or after the fermentation. Produced CO
2 is captured and stored in seawater or utilised to promote the growth of seaweed
[14].
All major components of the coastal marine biorefineries offer environmental and economic advantages compared to their conventional counterparts. As will be detailed later in this re
vise
warch, macroalgal biomass is an ideal substrate for bioethanol production. As macroalgae generally grow in marine environments, they do not require freshwater or arable land and therefore do not compete with food production. The successful use of seaweed as a substrate in a biorefinery has been investigated in several studies
[17][18][19][20][21]. Though often titled “marine biorefineries”, these papers focused solely on the marine nature of the feedstock where, in reality, the majority of the processes still rely on the use of freshwater and conventional terrestrial yeast strains. The potential use of seawater and marine microorganisms for marine fermentation has nonetheless been demonstrated. Isolated marine yeast strains, such as
Saccharomyces cerevisiae AZ65, have shown a high capability of producing bioethanol from glucose and molasses in seawater media
[15][22]. These strains also produce more bioethanol from glucose compared to terrestrial yeasts and are more tolerant to fermentation inhibitors
[22][23]. Seawater’s mineral content further eliminates the need for mineral nutrients and enables the production of sea salts and salted animal feed as co-products. The process of marine fermentation can also yield high-quality distilled water
[24]. Furthermore, the high-salt environment may reduce the chances of microbial contamination within a bioreactor
[14].
The importance of coastal locations must also be highlighted. As both the substrate and media are marine sourced, the establishment of marine biorefineries along coastal regions would decrease transportation costs. Furthermore, coastal sites are easy water access points for arid and semi-arid areas. From an economic perspective, coastal locations enable rural regeneration, providing jobs to former coastal industrial sites that have historically been hard to maintain
[25].
Though coastal marine seaweed biorefineries can operate on a stand-alone basis, there is the potential to pair such a system with other biorefineries and energy outlets to maximise the valuable outputs and minimise the cost. Such emerging combined systems are called coastal integrated marine biorefineries (CIMB). As can be seen in
Figure 2, integrated biorefineries may combine both seaweed and microalgal refineries, with certain by-products of each serving as inputs for the other. CO
2 and spent seaweed hydrolysate may be used as organic and inorganic carbon substrates for the growth of microalgae
[26][27]. Each fraction of microalgae has a certain potential industrial application. Lipids can be extracted and processed into biodiesel, proteins sold as livestock feed, and carbohydrates fermented into bioethanol. Aside from the primary metabolites, microalgae contain a host of HVCs, many of which have commercial uses. These include unsaturated long-chain fatty acids, shown to have a number of health benefits, as well as pigments, such as chlorophylls, phycocyanin, and carotenoids
[28][29]. As both biorefineries require electricity, this can be provided from sustainable sources, such as wind, solar, or wave energy. Conversely, coastal integrated marine biorefineries may also serve as a means of energy storage for renewable electricity. For example, peaks in solar electricity production generally surpass simultaneous energy demands. Excess electricity is often lost as there are currently no means of storage. Coastal integrated marine biorefineries enable the storage of excess renewable electricity by using it to produce biofuels. This integrated biorefinery system would lead to the production of less waste from each individual system and an increase in HVC and co-product yields
[14].
Figure 2. Coastal Integrated Marine Biorefinery (CIMB) system. Marine components (seawater, marine biomass, and marine microorganisms) serve as inputs for the biorefineries. Different biological and biochemical conversion processes are performed (blue box). Biofuels and high-value chemicals are obtained (purple arrows and ovals). Renewable energy sources (green box/arrow) are integrated with the biological system to improve efficiency
[14].
3. Seaweed
Unlike terrestrial plants, algae do not require freshwater or agricultural land, two rapidly depleting world resources
[30]. Furthermore, they can serve as a means for bioremediation as they have been shown to eliminate heavy metals and other contaminants in wastewater such as microplastic
[31][32]. Macroalgae are also capable of removing high concentrations of nitrogen and phosphorous from coastal waters
[33][34]. Therefore, seaweeds have been discussed as a potential feedstock for bioethanol and biogas production coupled with heavy metal removal
[35]. The main groups of marine macroalgae include species of the phyla Rhodophyta (red), Chlorophyta (green), and Phaeophyta (brown), which are differentiated mainly based on their pigmentation. Seaweeds have a wide variation in biochemical composition and, therefore, have many applications in HVC production
[14].
4. Evaluation of CO2 Removal and CCS by Seaweeds
Over the course of the 21st century, an estimated 100–1000 gigatons (Gt) of CO
2 need to be removed from the atmosphere to mitigate the impacts of global warming expected by the end of this century
[36]. In recent years, planting trees has gained traction, with certain campaigns amassing millions of dollars in funds
[37]. Such efforts highlight the widespread interest in finding tangible solutions to the current crisis. However, afforestation is mostly suited to tropical regions, where fast plant growth is possible
[38]. Furthermore, tree monoculture and planting in arid regions have been shown to have a negative environmental impact, increasing water scarcity and the creation of “Green Deserts”
[39][40]. Other carbon capture and sequestration (CCS) technologies are therefore necessary to combat climate change.
Seaweeds have been discussed as an alternative carbon sink to terrestrial biomass. Seaweeds do not use freshwater or arable land. Furthermore, as the ocean covers more than 70% of the Earth’s surface, the available area for seaweed growth is much larger than what can be grown on land. Most importantly, the biomass productivity of seaweeds is much higher than that of terrestrial plants. Whereas the carbon productivity of second-generation lignocellulosic crops is less than 1 kg Carbon (C) m
−2 year
−1, seaweed productivity ranges between 1 and 3.4 kg C m
−2 year
−1, depending on the species
[41].
Seaweed carbon sequestration is part of blue carbon sequestration, referring to the removal of atmospheric CO
2 by marine ecosystems through the accumulation and sequestration of carbon by marine organisms. Blue carbon sequestration accounts for around 55 to 71% of all biological carbon sequestration on the planet
[42][43]. Wild seaweeds (naturally grown seaweed) have already been shown to be an effective means of carbon removal, permanently sequestering on average 0.634 Gt CO
2 per year, mainly through deep sea biomass exportation or coastal sediment burial
[44]. A study by Johnston et al. determined how much time is required to sequester 100 Gt of CO
2 by growing seaweed biomass in large seaweed farms based on the available marine area. To achieve that, three scenarios (A, B, and C) have been explored based on the total cultivation area. Scenario A accounts for 5.7 M km
2 which is the inshore coastal area that is suitable for seaweed cultivation. Scenario B accounts for 100 M km
2 which is the total ocean area that could be used for seaweed farming. Scenario C accounts for 47 M km
2, which is the ecologically available ocean area for seaweed farming. In order to determine the total seaweed biomass production from each scenario, the average productivity of wild (naturally grown) seaweed and the productivity of the highly productive cultivated seaweed species,
M. pyrifera and
Ulva sp., were used in the calculation. The biomass production in each scenario determines the number of years required to sequester 100 Gt of CO
2 [14].
Scenario A represents the total time and biomass needed to remove 100 Gt of CO
2 when growing seaweeds on inshore coastal surface areas. Using an area of 5.7 million km
2, a total removal of excess CO
2 can be achieved in less than 11.28 years based on average wild seaweed NPP. When selecting highly productive species, the time goes down to 5.65 years using
Ulva sp. and 3.64 years using
M. pyrifera [14].
In Scenario B, the surface area for seaweed farming can be expanded to include the total inshore and offshore ocean surface area that can be theoretically used for seaweed farming. Based on
Ulva sp., a theoretical growth area of around 100 million km
2 can be farmed with 0.838 kg C m
−2 yr
−1 NPP to remove 100 Gt of CO
2 in just 116.8 days. Total yearly CO
2 removal using
Ulva sp. farms in this scenario is 17 times higher than that of growth limited to inshore coastal sites
[14].
Though
Ulva sp. can be theoretically grown over 100 M km
2 of the ocean when only considering certain factors such as temperature, light, depth, and pH, the model does not consider all ecological limits on seaweed growth. A better model taking into account these constraints estimates that the ocean surface area ecologically available for seaweed farms is in fact 48 million km
2. Therefore, in Scenario C, a total target CO
2 removal, considering NPP of wild seaweed,
M. pyrifera, and
Ulva sp. would take 1.34, 0.43, and 0.67 years, respectively. Across all seaweed classes, the yearly rate of CO
2 removal was 8.42 times greater for seaweed farms compared to inshore coastal areas
[14].
5. Future Perspectives
According to the Intergovernmental Panel on Climate Change (IPCC) and the European Commission, a number of targets must be met in order for the planet to not reach a stage of irreversible climate change. Firstly, human activity must be carbon neutral by 2050
[45]. Second, a minimum of 100 GtCO
2 must be removed from the atmosphere using carbon dioxide removal (CDR) strategies by 2100
[3]. Both objectives aim to maintain global temperature increase to below 2 °C.
OurThe results show total sequestration of 100 GtCO
2 could take less than 12 years, based on wild seaweed cultivated in inshore coastal sites alone. This is already considerably shorter than the time scale left until the 2100 deadline. However, this period can be significantly reduced if highly productive seaweed species are selected for farming. Therefore, seaweed cultivation is an efficient means of atmospheric carbon dioxide removal
[14].
Scenario B of seaweed farms was limited to
Ulva sp. as the used model was specifically designed for that genus of
Chlorophyta. Though other types of macroalgae can generally grow within the same niche as
Ulva, growth over 100 million km
2, around 10% of the total ocean area, is restricted to
Ulva sp.
[46]. However, Both the scale at which seaweed may be grown and, consequently, the efficient carbon capture and sequestration power of seaweed. Indeed, seaweed farms have the potential to offset total carbon emissions from entire industrial sectors. Seaweed farming on 3.8% of the West Coast Exclusive Economic Zones could offset carbon emissions for the entire Californian land farming sector. Moreover, only an estimated 474 km
2 of seaweed farms are required to completely offset the entire global seafood aquaculture industry
[47].
However, the
Ulva sp. growth model does not take into account the ecological constraints of all seaweed species. A broader more accurate model estimates that 48 million km
2 of ocean surface could be used for seaweed farming
[47]. Under these conditions, 100 GtCO
2 removal could be achieved in under a year when farming
M. pyrifera and
Ulva sp. Furthermore, as the yearly productivity of both species exceeds the minimum CDR target, there is the potential to go beyond the IPCC’s requirements. Further CO
2 removal would contribute to “negative carbon” emissions. This could not only completely limit global warming but also reverse the 1.3 °C temperature rise that has already occurred
[48]. Indeed, the removal of the IPCC’s upper limit, 1000 GtCO
2, would undo 20 years of global GHG emissions
[36][49].
Only the available surface area for seaweed growth was explored in this analysis. However, improvements in seaweed farming conditions could enhance seaweed productivity and would thus shorten the time needed for CO
2 removal and/or a decrease in necessary surface area. There is, however, a lack of research on seaweed farming conditions and their direct impact on seaweed NPP. Certain studies have focussed on the various factors that influence biomass production but not NPP
[50][51]. Aside from temperature, the most limiting parameter is the rate of photosynthesis, itself limited by multiple physiological processes
[52]. A study by Golberg and Liberzon showed that the use of an external mixing system, one that would cycle seaweed culture plots, enabling optimised light exposure, could increase total energy gain by two orders of magnitude
[53]. However, practical technologies based on this principle have yet to be developed.
Though seaweed farming could be a means of carbon capture, a number of studies have highlighted the economic and environmental costs of such a strategy
[47][54]. There can be some debate on the feasibility of
our three scenarios. For example, despite Scenario C being ecologically possible, biomass transportation from distant offshore seaweed farms to marine biorefineries becomes a major challenge, leading to increased production costs. Indeed, until more advances are made in transportation technologies, seaweed farming will be restricted to areas close to the coast
[55]. Another major issue is the environmental consequences of seaweed farming. This includes concerns regarding the release of artificial and organic materials into the environment, as well as the noise disturbance to marine wildlife
[56]. However, seaweed farming has also been shown to have a number of ecological benefits. Macroalgae mitigate ocean acidification whilst replenishing oxygen supplies by removing CO
2 and producing O
2 through photosynthesis
[54]. This is particularly important in hypoxic environments, resulting from the eutrophication of water bodies. Seaweed can further help to bioremediate nutrients and metals from agricultural and urban runoffs
[57]. Aside from biochemical impacts, seaweed farms can also serve as a means of wave attenuation, providing protection from extreme weather phenomena
[54].
To further explore the economic potential of seaweed, the mass and value of the bioethanol and HVC that could be produced from the biomass were calculated. In 2018, worldwide oil consumption was estimated at around 4622 million tonnes (Mtoe)
[58][59]. According to
ourthe results, bioethanol production from coastal sites could generate around 6310 Mtoe of bioethanol. This value more than exceeds planetary oil requirements. In fact, total seafarm bioethanol production, estimated at 53,200 Mtoe, greatly exceeds the 2018 global energy demand of 13,864.9 Mtoe
[58]. It is worth noting that, in the study by Johnston et al., bioethanol estimates were based on production using fresh water and genetically modified
E. coli [14][60]. Further research is needed for bioethanol production on such a scale within a coastal marine biorefinery, using seawater and marine yeast. Nonetheless, the volumes of bioethanol that could be produced using carbon capture seaweed could meet worldwide energy demands and replace the petrol industry, a main driver of CO
2 emissions. Climate protection policies could also lead to the expansion of the bioethanol market. The Renewable Fuels Standard (RFS) mandates the blending of 36 billion gallons of renewable fuels by 2022, of which only 42% can be corn-based ethanol
[61]. The remaining gap can be filled by seaweed bioethanol, a market only set to grow in the coming years. Global bioethanol production is projected to rise by 14%, with the biofuels market set to reach USD 246.52 billion by 2024, at a compound growth rate of 4.92%
[62][63].
Turning to HVC, like all seaweed species, the chemical composition of
M. pyrifera varies greatly depending on environmental conditions
[64][65][66]. Specific values and parameters chosen for
ourthe calculation came down to the quality of the study conducted by Johnston et al.
[14][67]. Considerable amounts of phlorotannin can be extracted. Many phlorotannins have commercial value as they have been shown to have anti-oxidant, anti-diabetic, radioprotective, hepatoprotective, and anti-inflammatory activity
[68]. Indeed, phlorotannin is the most valuable compound (USD ~70/kg) that can be extracted from
M. pyrifera [69]. However, given the low production yields, they tend to generate the least revenue. Optimisation of extraction procedures could increase the total volume and revenue of the product. However, given the generally low phlorotannin content of seaweed, there is a ceiling limit
[70].
The most interesting seaweed HVC are alginate and mannitol as both sugars have multi-billion-dollar valuations. Alginate is the most abundant of the extractable HVC and is also the most lucrative while mannitol is the second most profitable. This is in contrast to bioethanol, which, despite having the highest production volumes, is the lowest-grossing product. Given their abundance, polysaccharides are the most cost-effective HVC for future investments. Furthermore, the market for algal sugars is set to expand in the coming years. Alginate is finding increasing pharmaceutical and biomedical applications while mannitol, a low-calorie sweetener, is facing increasing demand in a health-concerned population
[71][72].
Though large quantities of proteins can be extracted from
M. pyrifera, as most have not been characterised and, therefore, they currently have no commercial applications; however, they may have tremendous potential, especially as animal feed. Brown seaweeds also contain fucoidan, a sugar with interesting properties and commercial value. However, due to a lack of efficient extraction procedures, no values could be estimated for fucoidan from
M. pyrifera [73][74].
In tTh
is article, the e compound estimations were based on the individual extraction values of each chemical. This means that the extraction process was optimised for a single compound. For the simultaneous extraction of all HVC and the production of bioethanol, the design of a downstream process is necessary. However, in such downstream production processes, product yields and valuations may decrease as the extraction procedures are not tailored to the individual chemicals. Furthermore, the product valuations in the study conducted by Johnston et al. are based on current market prices. With an influx of HVC on the market, the increased supply may exceed the demand, leading to an overall drop in price. However, a supply increase and price fall make a product more accessible, thus opening its use to further markets. The average price of each compound is also based on its bulk sale value. Laboratory-grade chemicals sell for a higher price but, in turn, entail higher purification costs. Nonetheless, seaweed biorefineries represent a potential multi-billion-dollar business that could potentially aid in the removal of excess CO
2 and help combat climate change
[14].