Phylogenetic diversity and dominant ecological traits of freshwater Antarctic Chrysophyceae

Previous studies conducted in summer in the lakes at Hope Bay (Antarctic Peninsula) between 1991 and 2007 showed a large numerical contribution of flagellated Chrysophyceae to the phytoplankton communities, particularly in the oligotrophic lakes, as evidenced by light microscopy observations and molecular fingerprinting. Given the ecological relevance of this group in these Antarctic microbial foodwebs, we carried out further molecular analyses (clone libraries and 18S Illumina high throughput sequencing) to characterize their phylogenetic diversity. The results of this study significantly increased the retrieved Chrysophyceae biodiversity. Clone libraries in two selected lakes (one oligotrophic and one mesotrophic) yielded 12 different chrysophycean OTUs, whereas 81 Swarm OTUs were recovered from six lakes using Illumina HiSeq. With the combination of both methods, we observed sequences of all the chrysophyte known clades, although most of the diversity belonged to Clade D, a group comprising mixotrophic and heterotrophic species. The percentage of reads for this clade in Illumina HiSeq ranged from 30% to 96% of the total Chrysophyceae reads. Based on experiments and observations, we also describe the main ecological traits of this group: the dominant taxa were small pigmented flagellates, well adapted to survive in oligotrophic systems, sometimes abundant under ice-cover subjected to low light intensities, and that have phagotrophic behavior. The used combination of methods allowed us to characterize the biodiversity and ecology of the Chrysophyceae, the dominant phytoplankton group in the oligotrophic lakes of this Maritime Antarctic region.