Protist diversity in a permanently ice-covered Antarctic lake during the polar night transition

Abstract

The McMurdo Dry Valleys of Antarctica harbor numerous permanently ice-covered lakes, which provide a year-round oasis for microbial life. Microbial eukaryotes in these lakes occupy a variety of trophic levels within the simple aquatic food web ranging from primary producers to tertiary predators. Here, we report the first molecular study to describe the vertical distribution of the eukaryotic community residing in the photic zone of the east lobe (ELB) and west lobe (WLB) of the chemically stratified Lake Bonney. The 18S ribosomal RNA (rRNA) libraries revealed vertically stratified populations dominated by photosynthetic protists, with a cryptophyte dominating shallow populations (ELB-6 m; WLB-10 m), a haptophyte occupying mid-depths (both lobes 13 m) and chlorophytes residing in the deepest layers (ELB-18 and 20 m; WLB-15 and 20 m) of the photic zone. A previously undetected stramenopile occurred throughout the water column of both lobes. Temporal variation in the eukaryotic populations was examined during the transition from Antarctic summer (24-h sunlight) to polar night (complete dark). Protist diversity was similar between the two lobes of Lake Bonney due to exchange between the photic zones of the two basins via a narrow bedrock sill. However, vertical and temporal variation in protist distribution occurred, indicating the influence of the unique water chemistry on the biology of the two dry valley watersheds.

Figure 1

Relative contribution of major eukaryotic groups from Eukarya 18S rRNA partial sequences in the photic zone of Lake Bonney. Lake samples were collected throughout the water column of ELB (black bars) and WLB (gray bars) at various time points between February and April 2008. Data represent relative abundance of phylotypes from 860 clones screened by restriction fragment length polymorphism (RFLP). Unique RFLPs were confirmed by sequencing. Clones were organized into eight eukaryotic supergroups.

Figure 2

Depth distribution of 18S rRNA clones and 18S rRNA abundance in ELB (a) and WLB (b) of Lake Bonney. Samples were collected at four sampling depths throughout the photic zone of both basins various times at various time points between February and April 2008. Data represent the combined set of sequences across all time points. Unique phylotypes were identified by a restriction fragment length polymorphism/sequencing approach. The 18S rRNA transcript levels were quantified using quantitative PCR as described in the methods (n=4). Position of ice-cover is shown (dashed line). Arrows denote the location of the chemocline.

Figure 3

Variation in the distribution of major protist groups in response to the transition from summer to polar winter in Lake Bonney. Samples were collected at three sampling depths from ELB (a, c and e) and WLB (b, d and f) of Lake Bonney at various time points between February and April 2008. Unique phylotypes were identified by a restriction fragment length polymorphism/sequencing approach.