Physiological Capabilities of Cryoconite Hole Microorganisms

Abstract

Cryoconite holes are miniature freshwater aquatic ecosystems that harbor a relatively diverse microbial community. This microbial community can withstand the extreme conditions of the supraglacial environment, including fluctuating temperatures, extreme and varying geochemical conditions and limited nutrients. We analyzed the physiological capabilities of microbial isolates from cryoconite holes from Antarctica, Greenland, and Svalbard in selected environmental conditions: extreme pH, salinity, freeze-thaw and limited carbon sources, to identify their physiological limits. The results suggest that heterotrophic microorganisms in cryoconite holes are well adapted to fast-changing environmental conditions, by surviving multiple freeze-thaw cycles, a wide range of salinity and pH conditions and scavenging a variety of organic substrates. Under oxic and anoxic conditions, the communities grew well in temperatures up to 30°C, although in anoxic conditions the community was more successful at colder temperatures (0.2°C). The most abundant cultivable microorganisms were facultative anaerobic bacteria and yeasts. They grew in salinities up to 10% and in pH ranging from 4 to 10.5 (Antarctica), 2.5 to 10 (Svalbard), and 3 to 10 (Greenland). Their growth was sustained on at least 58 single carbon sources and there was no decrease in viability for some isolates after up to 100 consecutive freeze-thaw cycles. The elevated viability of the anaerobic community in the lowest temperatures indicates they might be key players in winter conditions or in early melt seasons, when the oxygen is potentially depleted due to limited flow of meltwater. Consequently, facultative anaerobic heterotrophs are likely important players in the reactivation of the community after the polar night. This detailed physiological investigation shows that despite inhabiting a freshwater environment, cryoconite microorganisms are able to withstand conditions not typically encountered in freshwater environments (namely high salinities or extreme pH), making them physiologically more similar to arid soil communities. The results also point to a possible resilience of the most abundant microorganisms of cryoconite holes in the face of rapid change regardless of the location.

Keywords: cryoconite; cultivation; extreme conditions; freeze-thaw; microbial physiology.

FIGURE 1

MPN counts of aerobic (oxic) microbial community of cryoconite holes in freshwater medium. Microbial growth was measured by MPN counts as the (average of 3 different cryoconite holes sediments for each location after 71 days of incubation. Samples incubated at 30°C dried out after 30 days, but they are included on the graph for comparison (marked with stripes).

FIGURE 2

MPN counts of anaerobic (anoxic) microbial community of cryoconite holes in fermenter medium. Microbial growth was measured by MPN counts as the average of 3 different cryoconite holes sediments for each location after 71 days of incubation.

FIGURE 3

Boxplots showing the median pH tolerance of all microbial isolates from cryoconite holes from Greenland, Svalbard, and Antarctica. The pH of all incubations with positive growth after 30 days (n = 43) was noted, and compared between the locations. The red line depicts the median pH, the box envelops an interquartile range and the whiskers mark the 97th centile. There were no outliers.

FIGURE 4

Survival of cryoconite isolates subjected to multiple freeze-thaw cycles as measured by MPN technique. Error bars depicts 95% confidence levels calculated for MPNs. (A) Greenland and Svalbard yeasts, (B) Antarctic bacteria, (C) Svalbard bacteria, (D) Greenland bacteria.