High abundance and diversity of flagellates under ice cover in Lake Baikal revealed by microscopy and metabarcoding

Abstract

Lakes located in the north temperate zone may be covered with ice for long periods. Under the ice, a habitat is created, different from the open water period, which is characterized by low temperature and reduced light due to ice and snow cover. We investigated phytoplankton in sub-ice communities (SI) at the ice-water interface and the 0–25 m under-ice water column communities (UW) in the pelagic zone of Lake Baikal. Community structure was assessed using microscopy and metabarcoding of fragments of 18S rRNA gene. Flagellate diversity included 34 taxa from 64 microalgae identified by microscopy and 48 operational taxonomic units (OTUs) from 56 identified by metabarcoding. In conditions of complete snow-covered ice, high diversity and abundance of flagellates (up to 14 million cells L⁻¹) were observed, with an advantage due to their mobility and different feeding modes. Both microscopy and metabarcoding data show that the taxonomic structure of SI and UW is different. SI communities of Lake Baikal showed mass development of a mixed group of nanoflagellates, which consisted of “Spumella- and Chlamydomonas-like flagellates”. SI communities were less taxonomically diverse, suggesting that available resource gradients on the ice bottom are more tightly constrained, and taxa have to be ice bottom specialists to survive there. The relative spatial heterogeneity of SI communities is reflective of greater homogenization of habitat parameters in the winter water column and more dynamic conditions on the ice bottom. The data obtained expand the understanding of the diversity and abundance of organisms in under-ice habitats in north temperate latitudes.