Microbial biogeochemistry and phosphorus limitation in cryoconite holes on glaciers across the Taylor Valley, McMurdo Dry Valleys, Antarctica

Cryoconite holes host active microbial communities despite their extreme physical conditions. In the McMurdo Dry Valleys of Antarctica, these perennially cold, mini-ecosystems form ice lids that can persist for many years thereby isolating the cryoconite from nutrient and carbon inputs. Despite much recent work on cryoconite holes in Antarctica, little is known about nutrient dynamics and limitations in these ice-enclosed ecosystems. We used multiple biogeochemical approaches, including stable isotope signatures (δ15N and δ13C), nutrients concentrations (C, N, P), and enzyme activities, to evaluate what nutrients are likely limiting to biological activity in cryoconite hole sediments on Taylor, Canada, and Commonwealth glaciers in Taylor Valley, one of the McMurdo Dry Valleys. Nutrient concentrations (C, N, and P) varied in accordance with previous studies showing that the most inland of the three glaciers (Taylor Glacier) is the most oligotrophic. C-to-N ratios of Canada and Commonwealth cryoconite-hole sediments were close to the global mean for biologically-active sediments and soils, whereas Taylor Glacier cryoconite deviated from the global mean and were similar to the high C:N ratios seen in Taylor Valley soils. C and N stable isotope signatures on Commonwealth and Canada glaciers are congruent with values for efficient C and N fixation by nostocalean cyanobacteria, combined with higher levels of denitrification on Canada Glacier. In contrast, stable isotope signatures on the more oligotrophic Taylor Glacier are reflective of atmospheric deposition of N and C, or N inputs from nearby soils. Enzyme stoichiometric approaches further support extreme nutrient limitation on Taylor Glacier and indicate that P is the ultimate limiting nutrient across all three glaciers. Extremely high DIN-to-phosphate ratios also indicate P limitation across all three glaciers with Commonwealth Glacier being less severely P-limited than the other two glaciers. At a broader scale, this work provides a comprehensive framework for understanding how biogeochemical cycling of C, N and P vary across nutrient and climatic gradients in the cryobiosphere, and point towards the need for experimental work to test the relative controls of climate, microbes, and nutrients on biogeochemistry of cryoconite holes and other ecosystems of the cryosphere.