Oil content and quality in tropical seaweeds

The potential of seaweeds as feedstock for oil-based bioproducts was investigated, and the results support seaweeds as a biomass source for oil-based bioproducts. Seaweeds have not traditionally been perceived as a suitable feedstock for oil-based bioproducts because of their low content of lipids and fatty acids. In contrast to this perception the first major outcome of this thesis was the provision of new benchmark species of seaweed for oil-based bioproducts, selected because of their high oil contents combined with high proportions of omega-3 polyunsaturated fatty acids (PUFA(n-3)), which are the target fatty acids for high value products in health and nutrition. The three key species of seaweed identified were Spatoglossum macrodontum, Dictyota bartayresii and Derbesia tenuissima with high total lipid contents (~ 12 % dry weight (dw)) and high total fatty acid contents (4 – 8 % dw). These species also had a high proportion of PUFA(n-3) which were ~ 20 % of total fatty acids (TFA) in S. macrodontum and D. bartayresii and over 30 % of TFA in D. tenuissima. The second major outcome of this thesis was then the identification and quantification of natural variability of fatty acids within the key species of seaweed which can be exploited for further improvements in the content and composition of fatty acids. For S. macrodontum, the content of TFA (55 – 83 mg g⁻¹ dw) and the proportion of PUFA(n-3) (16 – 25 % of TFA) varied substantially (~ 50 %) on a temporal scale. For D. bartayresii, the content of TFA (45 – 55 mg g⁻¹ dw) varied slightly (~ 20%) and the proportion of PUFA(n-3) (16 – 24 % of TFA) varied substantially (~ 50 %) on a temporal scale. There was also spatial variation for D. bartayresii which was ~ 50 % for the content of TFA (36 – 54 mg g⁻¹ dw) but less than 10 % for the proportion of PUFA(n-3) (18 – 20 % of TFA). The third major outcome of this thesis was then the demonstration that environmental parameters are drivers for fatty acid variability in these species. The first line of evidence was from seasonal field-based studies on S. macrodontum and D. bartayresii which showed a higher proportion of PUFA(n-3) in winter when water temperature and light availability were at their annual minimum (~ 40 – 50 % higher in winter). In a second line of experimental evidence for D. tenuissima, colder water temperature was identified as the major driver (explained ~ 40 % of the total variability) to improve the proportion of PUFA(n-3) by ~ 20 % in this species. In a similar manner, high light intensity reduced the quality of the biomass by increasing saturation. The fourth major outcome of this thesis was the identification of a relationship between biotic parameters (plant size and life cycle stage) and fatty acids. In D. bartayresii, plants with a larger thallus length had significantly higher contents of TFA and slightly higher proportions of PUFA(n-3) and in S. macrodontum older plants in their "decline phase" had a more saturated fatty acid profile than younger plants in their "growth phase". Both the environmentally and biotic driven variability in fatty acids can be exploited through culture and harvest strategies to improve the fatty acid content and quality in these feedstocks. The fifth major outcome of this thesis was for the first time the provision of evidence for the genotypic variability of fatty acids within species of seaweed which is the basis for selective breeding to improve the yield of target fatty acids. First, there was substantial spatial variability (~ 40 – 60 %) in the content of fatty acids between the sampling locations of D. bartayresii, suggesting genotypic differences between populations. The second line of evidence was from experimental data on isolates of D. tenuissima where the content of TFA ranged from 34 to 55 mg g⁻¹ dw and 49 % of the variation was genotypic (between isolates). The proportion of PUFA(n-3) ranged from 31 to 46 % of TFA with a strong interactive effect of genotype and water temperature. In two isolates, the proportion of PUFA(n-3) increased by 20 % under cultivation at low temperature while in a third isolate temperature had no effect. Increases in PUFA(n-3) occurred with a stable content of TFA and high growth rates, leading to net increases in PUFA(n-3) productivity in two isolates. And last, the sixth major outcome of this thesis was the identification of fatty acid variability within individual plants. The content of TFA and to a lesser degree in the composition of fatty acids varied substantially within plants of S. macrodontum (TFA: 21 – 106 mg g⁻¹ dw) and D. bartayresii (TFA: 40 – 57 mg g⁻¹ dw) with a higher content of TFA and a higher proportion of PUFA(n-3) in the upper sections compared to the base. Overall, this thesis provides the basic framework on which to develop strategies for the domestication of seaweeds for the production of oil-based bioproducts in a similar manner to the past improvements in the oil yield of terrestrial oil crops and also microalgae. The most important domestication steps identified in this thesis were species selection and permanent improvements in the content and composition of fatty acids through selective breeding with D. tenuissima being the prime target species for this process.