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. Our 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 our 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 our 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 this article, the 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].