Next generation sequencing to identify lacustrine haptophytes in the canadian prairies: significance for temperature proxy applications

Long-chain alkenones (LCAs) are lipids produced by a restricted number species of haptophyte algae of the Isochrysidales Order, and which have a great potential for palaeotemperature reconstructions in lacustrine settings. However, it is first essential to identify the LCA-producing haptophyte species present to select the most appropriate calibration of the LCA-based temperature proxy U37K. In this study, we use next generation sequencing (NGS) targeting the haptophyte 18S rRNA gene in five lakes with distinct LCA profiles from the Canadian Prairies to determine the LCA-producer(s). In total 374 operational taxonomic units (OTUs) were identified across the sample studied, of which 234 fell within the Phylum Haptophyta. When considering the most represented OTUs, three were assigned as LCA-producers, one of which was included in the Group I of LCA-producing haptophytes and two in the Group II of LCA-producing haptophytes. Even though the OTU from the Group I haptophytes produced a highly specific LCA profile, one OTU from Group II haptophytes produced different LCA profiles. These results, compared with previous studies, allow us to discuss the importance of genomic identification of LCA-producing species and to determine which calibrations of the U37K index are the most appropriate for temperature reconstructions in the Canadian Prairies. Our results also suggest that the temperature dependency is likely consistent within different taxa from Group I and Group II haptophytes, but that other environmental parameters influence the temperature calibration.

长链烯酮（Long-chain alkenones, LCAs）是由等鞭金藻目（Isochrysidales）下有限类群的定鞭藻（haptophyte algae）所产生的脂质类物质，在湖相环境的古温度重建中具备极高应用潜力。然而，需首先明确研究区域内产LCAs的定鞭藻物种，方能为基于LCAs的温度代用指标U37K（U37K index）选择最适配的校准方案。本研究针对加拿大草原区五个具有差异化LCAs特征的湖泊，以定鞭藻18S核糖体RNA基因（18S rRNA gene）为靶标开展下一代测序（next generation sequencing, NGS），以此明确产LCAs的藻类物种。本次研究共鉴定得到374个操作分类单元（operational taxonomic units, OTUs），其中234个隶属于定鞭藻门（Phylum Haptophyta）。针对丰度较高的OTUs进行分析后发现，其中3个被归类为产LCAs物种：1个归入产LCAs定鞭藻类群I（Group I of LCA-producing haptophytes），另外2个归入产LCAs定鞭藻类群II（Group II of LCA-producing haptophytes）。尽管类群I的定鞭藻OTU所对应的LCAs特征具有高度特异性，但类群II中的某一OTU却可产生多种不同的LCAs特征。将本研究结果与既往研究对比后，我们得以探讨产LCAs物种的基因组鉴定的重要性，并明确针对加拿大草原区湖相古温度重建，哪一版本的U37K指数校准方案最为适配。此外，本研究结果还表明，类群I与类群II定鞭藻的不同类群间，其温度依赖性可能保持一致，但其他环境参数会对温度校准结果产生影响。