Hidden biofilms in a far northern lake and implications for the changing Arctic

Vani Mohit,Connie Lovejoy,Frédéric Bouchard,Alexander Culley,Warwick F Vincent

doi:10.1038/s41522-017-0024-3

Abstract

Shallow lakes are common across the Arctic landscape and their ecosystem productivity is often dominated by benthic, cyanobacterial biofilms. Many of these water bodies freeze to the bottom and are biologically inactive during winter, but full freeze-up is becoming less common with Arctic warming. Here we analyzed the microbiome structure of newly discovered biofilms at the deepest site of a perennially ice-covered High Arctic lake as a model of polar microbial communities that remain unfrozen throughout the year. Biofilms were also sampled from the lake’s shallow moat region that melts out and refreezes to the bottom annually. Using high throughput small subunit ribosomal RNA sequencing, we found more taxonomic richness in Bacteria, Archaea and microbial eukaryotes in the perennially unfrozen biofilms compared to moat communities. The deep communities contained both aerobic and anaerobic taxa including denitrifiers, sulfate reducers, and methanogenic Archaea. The water overlying the deep biofilms was well oxygenated in mid-summer but almost devoid of oxygen in spring, indicating anoxia during winter. Seasonally alternating oxic-anoxic regimes may become increasingly widespread in polar biofilms as fewer lakes and ponds freeze to the bottom, favoring prolonged anaerobic metabolism and greenhouse gas production during winter darkness.

Highlights

Thick biofilms dominated by cyanobacteria occur in marine and freshwater habitats, and are especially well developed in extreme aquatic environments, including in the Arctic[1] and Antarctica.[2]. These biofilms have a complex structural organization that facilitates nutrient recycling,[3] and they often dominate ecosystem biomass[2] and biological production[1, 4] in the shallow lakes and ponds that are a major feature of the Arctic landscape.[5]. Many of these high latitude water bodies freeze to the bottom during winter darkness, recent climate warming has led to reductions in bedfast ice,[6] allowing microbial communities to persist in liquid water throughout the year
Previous studies[1, 7, 8] at WHL have focused on benthic biofilms in the shallow moat of open water that forms in summer (Fig. 1b) and that refreezes to the lake sediment in early winter
It was assumed that benthic communities were restricted to this moat area because earlier ice coring in the central part of the lake had recorded 3.5–4.3 m of ice that extended to the lake bed

Summary

BRIEF COMMUNICATION OPEN

Vani Mohit[1,2,3,4], Alexander Culley[2,3], Connie Lovejoy[1,3,4], Frédéric Bouchard[5] and Warwick F. The increased prevalence of liquid water conditions during winter darkness will likely favor anaerobic processes in the benthic microbiomes of Arctic lakes, and the resultant increases in methane production and denitrification will need to be accounted for in global carbon and nitrogen budgets All of these aspects will require additional studies throughout the circumpolar North to determine whether benthic anoxia is a general feature of lakes without bedfast ice during the winter dark season, and to evaluate whether these environments support a diverse biofilm community of aerobes and anaerobes, as observed here at WHL.

AUTHOR CONTRIBUTIONS

Findings

Open Access This article is licensed under a Creative Commons