Group 2i Isochrysidales produce characteristic alkenones reflecting sea ice distribution

Karen Jiaxi Wang,Yongsong Huang,Sian Liao,Markus Majaneva,Patricia Cabedo-Sanz,Simon T Belt,Timothy D Herbert,Tyler R Kartzinel,Joseph Novak,Nora Richter

doi:10.1038/s41467-020-20187-z

Abstract

Alkenones are biomarkers produced solely by algae in the order Isochrysidales that have been used to reconstruct sea surface temperature (SST) since the 1980s. However, alkenone-based SST reconstructions in the northern high latitude oceans show significant bias towards warmer temperatures in core-tops, diverge from other SST proxies in down core records, and are often accompanied by anomalously high relative abundance of the C37 tetra-unsaturated methyl alkenone (%C37:4). Elevated %C37:4 is widely interpreted as an indicator of low sea surface salinity from polar water masses, but its biological source has thus far remained elusive. Here we identify a lineage of Isochrysidales that is responsible for elevated C37:4 methyl alkenone in the northern high latitude oceans through next-generation sequencing and lab-culture experiments. This Isochrysidales lineage co-occurs widely with sea ice in marine environments and is distinct from other known marine alkenone-producers, namely Emiliania huxleyi and Gephyrocapsa oceanica. More importantly, the %C37:4 in seawater filtered particulate organic matter and surface sediments is significantly correlated with annual mean sea ice concentrations. In sediment cores from the Svalbard region, the %C37:4 concentration aligns with the Greenland temperature record and other qualitative regional sea ice records spanning the past 14 kyrs, reflecting sea ice concentrations quantitatively. Our findings imply that %C37:4 is a powerful proxy for reconstructing sea ice conditions in the high latitude oceans on thousand- and, potentially, on million-year timescales.

Highlights

Alkenones are biomarkers produced solely by algae in the order Isochrysidales that have been used to reconstruct sea surface temperature (SST) since the 1980s
They are a class of C35–42 methyl and ethyl ketones with two to four double bonds produced exclusively by the algae in the order Isochrysidales[1,4,5,6,7,8]: in ocean settings Emiliania huxleyi and the closely related Gephyrocapsa oceanica have been considered as the exclusive producers of di-unsaturated (C37:2) and tri-unsaturated (C37:3) methyl alkenones[1,4,5]
Variable warm bias in U3K70-based SST reconstructions is often accompanied by the occurrence of tetra-unsaturated alkenone (C37:4), which is usually absent in mid-to-low latitude oceans and E. huxleyi cultures even under exceedingly low growth temperature[8]

Summary

Introduction

Alkenones are biomarkers produced solely by algae in the order Isochrysidales that have been used to reconstruct sea surface temperature (SST) since the 1980s. We used next-generation sequencing (NGS) to identify Isochrysidales in water column, sea ice, and sediment samples in northern high latitude marine environments. We sequenced the V4 region of the 18S rRNA gene from three surface sediment samples containing high %C37:4 (59–68%; Supplementary Fig. 2, Supplementary Data 1) that were collected from Victoria Strait in the Canadian Arctic Archipelago where the sea ice regime is mainly governed by first-year ice. Four Isochrysidales amplicon sequence variants (ASVs) were recovered from those surface sediments (Supplementary Table 2).

Results

Conclusion