Taxon-specific hydrogen isotope signals in cultures and mesocosms facilitate ecosystem and hydroclimate reconstruction

Phytoplankton play a key role in biogeochemical cycles, impacting atmospheric and aquatic chemistry, food webs, and water quality. However, it remains challenging to reconstruct changes in algal community composition throughout the geologic past, as existing proxies are suitable only for a subset of taxa and/or influenced by degradation. Here, we investigate if compound-specific hydrogen isotope ratios (δ2H values) of common algal lipids can serve as (paleo)ecological indicators. First, we grew 20 species of algae – representing cyanobacteria, diatoms, dinoflagellates, green algae, and cryptomonads – in batch cultures under identical conditions and measured δ2H values of their lipids. Despite identical source water δ2H values, lipid δ2H values ranged from -455 ‰ to -52 ‰, and clustered according to taxonomic groups and chemical compound classes. In particular, green algae synthesized fatty acids with higher δ2H values than other taxa, cyanobacteria synthesized phytol with relatively low δ2H values, and diatoms synthesized sterols with higher δ2H values than other eukaryotes. Second, we assessed how changes in algal community composition can affect net δ2H values of common algal lipids in 20 experimental outdoor ponds, which were manipulated via nutrient loading, and the addition of macrophytes and mussels. High algal biomass in the ponds, which was mainly caused by cyanobacterial and green algal blooms, was associated with higher δ2H values for generic fatty acids, relatively stable δ2H values for phytol and the dinoflagellate biomarker dinostanol, and lower δ2H values for the more cosmopolitan sterol stigmasterol. These results are consistent with expectations from our culture-based analyses, suggesting that measuring δ2H values of multiple lipids from sediment and calculating 2H-offsets between them can resolve changes in algal community composition from changes in source water isotopes. With an appropriate availability of sedimentary lipids, this approach could permit the reconstruction of both taxonomic variability and hydroclimate from diverse sedimentary systems.

Methods
20 species of phytoplankton were cultured under identical conditions in batch cultures. Biomass was filtered from cultured water. Mesocosm experiments were conducted in large-volume experimental ponds. Algal biomass was filtered from the water at three time points from each of the 20 experimental ponds. From both sample sets, lipids were extracted and purified, derivatized (methylated or acetylated) and compound-specific hydrogen isotope ratios of fatty acids, phytol, and sterols were measured by gas chromatography - isotope ratio mass spectrometry (GC-IRMS). Resulted lipid δ2H values are corrected for hydrogen added during derivatization. The isotopic composition of filtered water was determined by cavity ring-down spectroscopy.