Seasonal changes in the D  /  H ratio of fatty acids of pelagic microorganisms in the coastal North Sea

Abstract. Culture studies of microorganisms have shown that the hydrogen isotopic composition of fatty acids depends on their metabolism, but there are only few environmental studies available to confirm this observation. Here we studied the seasonal variability of the deuterium-to-hydrogen (D / H) ratio of fatty acids in the coastal Dutch North Sea and compared this with the diversity of the phyto- and bacterioplankton. Over the year, the stable hydrogen isotopic fractionation factor ε between fatty acids and water (ε_lipid/water) ranged between −172 and −237 ‰, the algal-derived polyunsaturated fatty acid nC20:5 generally being the most D-depleted (−177 to −235 ‰) and nC18:0 the least D-depleted fatty acid (−172 to −210 ‰). The in general highly D-depleted nC20:5 is in agreement with culture studies, which indicates that photoautotrophic microorganisms produce fatty acids which are significantly depleted in D relative to water. The ε_lipid/water of all fatty acids showed a transient shift towards increased fractionation during the spring phytoplankton bloom, indicated by increasing chlorophyll a concentrations and relative abundance of the nC20:5 polyunsaturated fatty acids, suggesting increased contributions of photoautotrophy. Time periods with decreased fractionation (less negative ε_lipid/water values) can potentially be explained by an increased contribution of heterotrophy to the fatty acid pool. Our results show that the hydrogen isotopic composition of fatty acids is a promising tool to assess the community metabolism of coastal plankton potentially in combination with the isotopic analysis of more specific biomarker lipids.<