Changes in long chain alkenone distributions and Isochrysidales groups along the Baltic Sea salinity gradient

Abstract

Isochrysidales species of the phylum Haptophyta are the exclusive producers of C37 to C42 long chain alkyl ketones, also called long chain alkenones (LCAs). While LCA distributions are known to vary with temperature and salinity, it is difficult to tease apart the direct effects of environmental parameters vs changes in the LCA-producing organisms. The Baltic Sea surface salinity gradient, which ranges from oligohaline (0.5–5 g/kg) to polyhaline (18–30 g/kg), represents a unique opportunity to study the relationships between salinity changes, species distribution and LCA biomarkers in a single ecosystem. LCA biomarkers revealed the presence of the three known Isochrysidales groups (Groups I, II and III) in Baltic Sea surface sediments, and the presence of Groups I and II were further confirmed with DNA sequencing. Group III Isochrysidales were present in the mixoeuhaline Skagerrak based on LCA signature alone. Groups I and II Isochrysidales were found for the first time in the Baltic Sea using a combination of LCAs and DNA biomarkers, solving an eighteen-year long mystery of Baltic Sea LCA-producing haptophyte identity. Group II Isochrysidales, which have a large salinity tolerance range, were spread over the Skagerrak and the complete Baltic Sea, but were characteristic for the central Baltic Sea. Oligohaline Group I Isochrysidales were representative for the northern Baltic Sea. However, evidence of Group I Isochrysidales in the central and southern Baltic Sea suggests a possible transport by surface currents since this group is typically confined to oligohaline conditions. Testing the recently developed ratio of isomeric C37 ketones (RIK37) against the Baltic Sea surface salinity gradient revealed a significant positive correlation. This may represent a salinity proxy reflecting the amount of Group I Isochrysidales relative to Group II Isochrysidales in oligohaline environments. The present study elucidates for the first time the identity and the spatial distribution of LCA producers thriving in a large and stable brackish environment.