The under‐ice microbiome of seasonally frozen lakes

Abstract

Compared to the well-studied open water of the ''growing'' season, under-ice conditions in lakes are characterized by low and rather constant temperature, slow water movements, limited light availability, and reduced exchange with the surrounding landscape. These conditions interact with ice-cover duration to shape microbial processes in temperate lakes and ultimately influence the phenology of community and ecosystem processes. We review the current knowledge on microorganisms in seasonally frozen lakes. Specifically, we highlight how under-ice conditions alter lake physics and the ways that this can affect the distribution and metabolism of auto-and heterotrophic microorganisms. We identify functional traits that we hypothesize are important for understanding under-ice dynamics and discuss how these traits influence species interactions. As ice coverage duration has already been seen to reduce as air temperatures have warmed, the dynamics of the underice microbiome are important for understanding and predicting the dynamics and functioning of seasonally frozen lakes in the near future.The quality of freshwater, which is tightly tied to many essential ecosystem services, is influenced in large part by the activities of microbial communities. In inland waters, as in other ecosystems, microorganisms are at the hub of most biogeochemical processes and largely control ecosystem functioning via their metabolic activities. However, attempts to describe the taxonomy and ecology of the freshwater microbiome mainly involve samples collected during the icefree season and, hence, largely neglect microbial communities and their activities during the ice-covered period. Knowledge about the year-round ecological and biogeochemical traits of typical freshwater microorganisms is required to understand when and where certain microorganisms will appear and how they will influence other organisms or biogeochemical processes and, hence, water quality. Together, this information will improve our ability to predict and model freshwater ecosystem and biogeochemical dynamics and to determine their role in the landscapes in the face of environmental change.A large number of lakes, particularly those situated at high altitude and the numerous high-latitude lakes in the temperate and boreal climate zones, are seasonally covered by ice for more than 40% of the year (Walsh et al. 1998). Despite this, surprisingly little is known about the ecology, diversity, and metabolism of microorganisms that reside under the ice cover in such lakes (Salonen et al. 2009). The traditional view is that ecosystems subjected to low temperatures are ''on hold'' and that cellular adaptations for survival at low temperatures control the composition of the winter microbial community, which awaits environmental conditions more conducive for growth. This traditional concept fails to recognize that the winter season affects the ecology and metabolic features of freshwater microorganisms, as well as their involvement in food webs and global biogeochemical c...

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