Group 1 phylogeny and alkenone distributions in a freshwater volcanic lake of northeastern China: Implications for paleotemperature reconstructions

Long-chain alkenones (LCAs) produced by Group 1 Isochrysidales from freshwater lakes feature a highly specific profile with almost the same abundances of two C₃₇ tri-unsaturated alkenone isomers. Their unsaturation ratios have been proposed to have great potential for cold-season paleotemperature reconstructions. However, recent study based on the next-generation sequencing has found that there is high level of genetic diversity in Group 1 Isochrysidales, with two main subclades Groups 1a and 1b. It is still unclear whether variable mixtures of different Group 1 subclades can significantly affect the LCA-based paleotemperature proxies in freshwater lake sediment records. Here we investigated LCA distributions and haptophyte-specific 18S rRNA sequences from a sediment core in Tuofengling Tianchi (a freshwater volcanic lake) of northeastern China using a combination of lipid biomarker and next-generation sequencing analyses. We recover 386 amplicon sequence variants (ASVs) from the studied sediment samples, 11 of which are affiliated with the LCA-producing Isochrysidales. Our phylogenetic analysis identifies that all LCA-producing Isochrysidales belong to Group 1 phylotype (Groups 1a and 1b subclades included), with Group 1a being the dominant subclade in most samples. The phylotype is strongly supported by the LCA evidence of two C₃₇ tri-unsaturated alkenone isomers, with RIK₃₇ (ratio of isomeric ketones of C₃₇ chain length) of ∼ 0.57–0.60. Groups 1a and 1b in our sediment core display significant downcore variations in the relative abundances, with Group 1a ranging from 32% to 100%. However, such highly variable mixtures are not a dominant factor in affecting the values of LCA-based on temperature proxies (U₃₇^K, U₃₈^KEt, and R3b) in our sediment core. Our study provides the sedimentary evidence that Group 1 LCAs elude species-mixing effects and highlight the potential importance of Group 1 LCAs from freshwater lakes in quantitatively reconstructing past temperature changes.