Back to alkenone sources: molecular profiling of alkenone-producing Isochrysidales in Swiss lakes using targeted DNA amplicon sequencing

Paleoclimate records play a crucial role in improving the performance of climate models by enhancing our mechanistic understanding of climate dynamics and providing an independent framework for evaluating model simulations through model–data comparisons. However, for such comparisons to be successful, reliable climate reconstructions are required, particularly with respect to their seasonal sensitivity. To achieve this, we need robust proxies supported by a solid mechanistic understanding.

In this work, we aim to provide a solid foundation for the use of the lacustrine alkenone paleothermometer in mid-latitude freshwater lakes. Alkenones are temperature-sensitive molecules produced by haptophyte algae from the order Isochrysidales. The alkenone unsaturation degree has been linked to temperature and has been widely used to reconstruct past sea surface temperatures. Alkenones are also present in lakes, although they do not occur in all lakes. In freshwater lakes, alkenone-producing Isochrysidales belong to a phylogenetically distinct group compared to those found in saline lakes and marine environments. Previous work on Swiss lakes has shown that alkenones are relatively common in mid-latitude European lakes, are produced between ice-out and the establishment of lake stratification, and record water temperature, as found in high-latitude lakes. However, this group remains poorly characterized, particularly regarding its life cycle and genetic diversity, which limits our understanding of the lacustrine alkenone proxy in freshwater lakes.

To address these knowledge gaps, we monitored two Swiss lakes, Lake St. Moritz, an alpine lake, and Lake Greifen, a lowland lake, by combining alkenone characterization with DNA sequencing of small subunit (18S), internal transcribed spacers (ITS 1 and ITS2) and large subunit ribosomal RNA (5.8S and 28S) marker genes targeting Isochrysidales. Using this approach, we aim to: (i) refine the identification of the Isochrysidales present in both lakes; (ii) characterize the temporal dynamics of the Isochrysidales community in terms of structure and abundance throughout the bloom period; (iii) identify the life cycle stage during which alkenones are produced; and, (iv) determine the environmental controls on the Isochrysidales bloom timing.