Biosynthesis of Fatty Acids and Stable Carbon Isotope Fractionation

Understanding the lipid biochemistry and carbon isotope fractionation of the deep-sea piezophilic bacteria is of paramount importance because the deep-sea piezophilic biosphere is a critical component of the Earth’s biogeochemical cycle. Piezophilic bacteria are ultimately responsible for nutrient cycling, geochemical speciation, and the chemistry of the ocean. However, our current theories and practice in marine microbial biogeochemistry is based on models and parameters that are derived from studies on surface warm temperature and low pressure microorganisms. This is an important impediment to marine biogeochemistry. There is an urgent need, therefore, to characterize the lipid biochemistry and carbon isotope fractionation of piezophilic bacteria.

We also determined the stable carbon isotopic composition (Figure 1) of individual fatty acids of two hyperpiezophilic bacteria Shewanella benthica DB21MT-2 and Moritella yayanosii DB21MT-5 (Fang et al., 2002). The two piezophilic strains had rather different PLFA profiles. S. benthica DB21MT-2 contained high concentrations of odd-carbon numbered fatty acids (i15:0, i- and a17:1) as well as 20:5w3 (10.8%), whereas M. yayanosii DB21MT5 contained a16:0 and 22:6w3 (10.7%). The variations of the d13C values between fatty acids of these piezophiles were nearly 8‰ (DB21MT-5) and 14‰ (DB21MT-2). Despite the fact that the two bacterial strains were grown on the same medium and under the same temperature/pressure conditions, DB21MT-2 showed a systematic enrichment of 13C in fatty acids compared to strain DB21MT-5 on a molecule-to-molecule basis (Figure 1). For both extremely piezophilic bacteria, the polyunsaturated fatty acids (20:5 and 22:6) exhibited the most depleted d13C values. All fatty acids except the odd-carbon-numbered fatty acids from DB21MT-2 were depleted in 13C relative to bacterial growth substrate (marine broth 2216, Figure 1). We concluded that the same type of microorganisms could have rather different d13C under the same growth conditions, and that sedimentary fatty acids with distinct d13C values do not necessarily have to originate from different organisms. Interpretation of d13C values of sedimentary biomarkers may be complicated by such large variations in d13C of fatty acids given that they are biosynthesized by the same type of microorganisms grown under identical conditions.

Isotopic composition of bacterial growth media (marine agar 2216, average of three measurements) and individual fatty acids isolated from two extremely piezophilic bacteria DB21MT5 (diamonds) and DB21MT2 (circles).