High planktonic diversity in mountain lakes contains similar contributions of autotrophic, heterotrophic and parasitic eukaryotic life forms

Abstract

A rich eukaryotic planktonic community exists in high-mountain lakes despite the diluted, oligotrophic and cold, harsh prevailing conditions. Attempts of an overarching appraisal have been traditionally hampered by observational limitations of small, colorless, and soft eukaryotes. We aimed to uncover the regional eukaryotic biodiversity of a mountain lakes district to obtain general conclusions on diversity patterns, dominance, geographic diversification, and food-web players common to oligotrophic worldwide distributed freshwater systems. An unprecedented survey of 227 high-altitude lakes comprising large environmental gradients was carried out using Illumina massive tag sequencing of the 18S rRNA gene. We observed a large Chrysophyceae dominance in richness, abundance and novelty, and unveiled an unexpected richness in heterotrophic phagotrophs and parasites. In particular, Cercozoa and Chytridiomycota showed diversity features similar to the dominant autotrophic groups. The prominent beta-dispersion shown by parasites suggests highly specific interactions and a relevant role in food webs. Interestingly, the freshwater Pyrenean metacommunity contained more diverse specific populations than its closest marine oligotrophic equivalent, with consistently higher beta-diversity. The relevance of unseen groups opens new perspectives for the better understanding of planktonic food webs. Mountain lakes, with remarkable environmental idiosyncrasies, may be suitable environments for the genetic diversification of microscopic eukaryotic life forms.

Fungi in aquatic ecosystems

“…In these high flow velocities systems, most of the microbial activity takes place in microbial biofilms 186 and thus we can expect that fungi are mainly situated in biofilms, able to degrade the terrestrial and algal OM. In contrast, high-altitude alpine lakes comprise an unexpected richness in Chytridiomycota and thus presumably a large proportion of parasites of phytoplankton 187 . These examples suggest a close coupling between autotrophs and fungi and an important role of fungal parasites for foodweb structure in these oligotrophic and cold lentic systems.…”

Tracing the Origin of Planktonic Protists in an Ancient Lake

“…Metabarcoding is highly relevant for pico/nano-sized (~0.8-5 µm, as defined in [3]) unicellular eukaryotes, as they are too small to show unambiguous morphological differences and are difficult to culture [3,4]. The key ecological role of these minute eukaryotes was shown in both oceans [3,5] and lakes [6,7]. Yet, we are still beginning to understand their diversity, evolution and dispersal.…”

Tracing the Origin of Planktonic Protists in an Ancient Lake

“…Only one fraction of the plankton (0.45-8 µm), collected during one week from a relatively small depth range (1-50 m), contained between~800 and~1400 OTUs (at 97% and 99% similarity, respectively). For comparison, there were between 82 to 715 OTUs (at 97% similarity, 0.22-50 µm size-fraction, Illumina sequencing) per lake in one of the latest freshwater-protist metabarcoding studies including 227 Pyrenean mountain lakes [7]. Given its long history and the fact that Lake Baikal could have served as glacial refugia during the last glaciations [59], protists species may have accumulated in the lake more than in other younger waterbodies, thus partially explaining their higher richness.…”

“…Metabarcoding is highly relevant for pico/nano-sized (~0.8-5 µm, as defined in [3]) unicellular eukaryotes as they are too small to show unambiguous morphological differences and are difficult to culture [3,4]. The key ecological role of these minute eukaryotes has been shown in both oceans [3,5] and lakes [6,7]. Yet, we are still beginning to understand their diversity, evolution, and dispersal.…”