Genomic identification of the long-chain alkenone producer in freshwater Lake Toyoni, Japan: implications for temperature reconstructions

Julien Plancq,Jill L Mccoll,James A Bendle,Osamu Seki,Jillian M Couto,Andrew C.G Henderson,Youhei Yamashita,Kimitaka Kawamura,Jaime L Toney

doi:10.1016/j.orggeochem.2018.09.011

Abstract

Identifying the lacustrine haptophyte species that produce long-chain alkenones (LCAs) is essential prior to down-core temperature reconstructions. Here, we investigated the identity of LCA-producing species from Lake Toyoni, Japan using 18S ribosomal DNA (rDNA) and organic geochemical analyses. The rDNA analyses identified eighteen operational taxonomic units (OTUs) of which only one fell within the haptophyte phylotype. This haptophyte belongs to the Group I phylotype, as supported by the LCA distribution found in surface and down-core sediments, and is closely related to a haptophyte found in Lake BrayaSø (Greenland). The inferred temperature using the Greenland calibration is very close to the Lake Toyoni surface temperature recorded during the spring/early summer season, when the LCA-producing haptophyte is likely to bloom. We therefore suggest that the temperature calibration from the Lake BrayaSø, Greenland is a suitable calibration for down-core temperature reconstructions at Lake Toyoni.

Highlights

Long-chain alkenones (LCAs) are a class of C35–C42 unsaturated ketones that are produced by three distinct phylogenetic groups of haptophyte algae within the Isochrysidales order (e.g., Theroux et al, 2010)
Eighteen operational taxonomic units (OTUs) were identified in Lake Toyoni, of which only OTU7 fell within the haptophyte phylotype (Fig. 2 and Supplementary Fig. S2), and was 96% similar to a haptophyte found in Lake BrayaSø (Greenland) based on a BLAST analysis (Table 2 and Supplementary Table S1)
The second closest BLAST hit for OTU7 was a haptophyte species from the freshwater Lake Esch-sur-Sûre in Luxembourg (Supplementary Table S1)

Summary

Introduction

Long-chain alkenones (LCAs) are a class of C35–C42 unsaturated ketones that are produced by three distinct phylogenetic groups of haptophyte algae within the Isochrysidales order (e.g., Theroux et al, 2010). Since the degree of unsaturation of C37 LCAs varies as a function of environmental temperature (Brassell et al, 1986; Prahl and Wakeham, 1987), LCAs have been used as temperature proxies based on the alkenone unsaturation indices UK37 and UK370 (e.g., Brassell et al, 1986; Prahl and Wakeham, 1987). The presence of multiple species within and among lakes has complicated the development of LCA-based proxies for temperature reconstructions since different species may produce different temperature calibrations (Theroux et al, 2010; Randlett et al, 2014). Identifying the LCA-producing haptophyte species present prior to lacustrine down-core temperature reconstructions is essential for selecting the most appropriate calibration

Methods

Results

Discussion

Conclusion