Lipid biomarker production by marine phytoplankton under different nutrient and temperature regimes

Abstract

The ocean’s biological pump strongly influences atmospheric CO2 and regulates Earth’s climate. Determining the contribution of different phytoplankton groups to the biological pump over geological timescales remains an important, yet elusive, goal in biogeochemistry and organic geochemistry. Towards that end, source-specific lipid biomarkers can be used, but this approach requires the quantification of the biomarker-to-carbon ratio in different phytoplankton species, and under differing environmental conditions. We investigated responses of brassicasterol, dinosterol and C37 alkenones to three temperatures (15, 20 and 25 °C) and three N:P supply ratios (10:1, 24:1 and 63:1 mol mol−1) in three diatoms, three dinoflagellates and one coccolithophore, in laboratory experiments. Brassicasterol was produced by one diatom species, three dinoflagellates and the coccolithophore Emiliania huxleyi, while dinosterol and C37 alkenones were produced by dinoflagellates and E. huxleyi, respectively. Overall, carbon-normalized contents of lipid biomarkers varied by about a factor of three over the wide ranges of temperature and N:P supply ratios, in all species. Within the factor of three, brassicasterol was highest under the balanced N:P condition in diatoms, but under N and P deficiency in dinoflagellates. Brassicasterol in E. huxleyi was highest at lower temperatures. Dinosterol in dinoflagellates and C37 alkenones in E. huxleyi varied with temperature and N:P supply ratios, but not systematically. Compared to those in our experiments, smaller ranges are expected of N:P ratios and temperature and hence carbon-normalized biomarker contents at individual locations over time. Thus, our results imply that lipid biomarkers can be used to estimate taxon-specific carbon fluxes through time.