Benthic microbial mat adaptations to nutrient scarcity and nutrient cycling dynamics in oligotrophic, perennially ice-covered Lake Vanda and Lake Fryxell in the McMurdo Dry Valleys, Antarctica

The primary productivity of all ecosystems is linked to biogeochemical cycles and is often limited by nutrient availability. These links and interactions result in feedback loops, where changes in biogeochemical cycling dynamics and ecosystem dynamics can profoundly influence each other. In lacustrine environments, the availability of either nitrogen or phosphorus can be limiting, as well as other environmental factors such as light availability. The McMurdo Dry Valleys are the largest ice-free region in Antarctica. They receive very little precipitation, experience freezing temperatures, and frequent freeze-thaw cycles. With very little liquid water, habitat connectivity is limited, and thus, terrestrial biomass. While there are seasonal glacial meltwater streams and rivers, liquid water only persists year-round in the perennially ice-covered lakes. Fed by glacial meltwater and many occupying closed basins, these lakes exhibit interesting physical and chemical properties. The permanent ice covers offer protection from the extreme terrestrial environmental conditions, allowing the lakes to remain mostly isolated from the atmosphere, resulting in meromictic and salinity stratified water columns. Most of the lakes are home to thick, photosynthetic, benthic microbial mats. In some lakes, the primary productivity of the benthic communities is greater than the planktonic communities.Perennially ice-covered lakes Vanda and Fryxell are found in Wright and Taylor Valley, respectively. The geology and exposure age of the valleys differ, and due to the slow rates of weathering, nutrient (nitrogen and phosphorus) availability varies. The longest river in the MDVs is the Onyx River, which feeds Lake Vanda with multiple sources and sinks of nitrogen and phosphorus along it. Lake Fryxell is fed directly by the Canada and Commonwealth glaciers and several smaller ephemeral meltwater streams. Lake Vanda’s water column has a severe orthophosphate deficiency, whereas Lake Fryxell’s water column is limited in both nitrogen and phosphorus. These limitations have been characterized for the planktonic communities; however, very little research has been conducted on constraints of primary productivity by nutrients in the benthic mat communities. The benthic microbial mats accrue biomass on annual scales as laminations that can vary in community composition. Despite the nutrient-depleted water columns, microbial mats grow along both varying nutrient and light regimes in both lakes. Metagenomic surveys of the benthic communities show a metabolic adaptation to efficiently scavenge nitrogen and phosphorus from their environment and tightly recycle nutrients within the mat through a suite of organic degradation and synthesis gene pathways, the synthesis and hydrolysis of nitrogen and phosphorus storage compounds as availability varies either seasonally or within their environment. The microbial communities are likely not limited by nutrients due to these advantageous adaptations; instead, they act as a nutrient sink in their environment and are limited by other factors.

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