Phylogeny, alkenone profiles and ecology of Isochrysidales subclades in saline lakes: Implications for paleosalinity and paleotemperature reconstructions

Long-chain alkenones (LCAs) produced by phylogenetically classified Groups 1 and 3 Isochrysidales are increasingly used for paleotemperature and/or paleosalinity reconstructions in oligohaline lakes marine environments. However, there considerable difficulties the paleoenvironmental interpretation of LCAs from Group 2 thriving saline lakes. The biggest challenge lies our poor understanding complexity ecological niches individual subclades Here, we perform comprehensive analysis haptophyte-specific 18S rRNA sequences distributions LCAs, long-chain alkenoates (LCEs) surface sediments suspended particulate matter (SPM) 37 northern China. These span a large salinity gradient 0.5 to 308‰. Combined with published genomic data Isochrysidales, phylogenetic reveals three occupying distinct niches: one ice-related bloomer 2i two warm-season bloomers 2w1 2w2. 2i, earliest seasonal bloomer, frequently co-occurs relatively low intermediate waters, whereas 2w2 blooms hypersaline waters. Based on existing data, C39:4 methyl alkenone is chemotaxonomic biomarker 2i. %C37:4 (relative abundance C37:4 total C37 alkenones) values follow order: > 2w1. sediment cores does not directly record past changes, but instead reflects variable contributions production these subclades. This could indirectly partially reflect overall changes some dominated 2w1, can be more complicated other assemblages. For sites, also demonstrate that direct use unsaturation indices (U37K, U37K', U37K'') generally feasible, except cases where alkenone-producing single species/subclade effects circumvented. We propose possible alternative proxies lakes: 1) ratios C41 C42 alkenones, as compounds predominantly limited number species, such Isochrysis nuda (Liao et al., 2020); 2) C38Et/C36OEt ratio (ratio C38 ethyl C36 alkenoates), which appears have similar temperature sensitivity 2w2, no inputs. Our study provides new insights into classifications their ecological/environmental niches, fundamental quantitative rigorous applications LCEs proxies.