Photoacclimation by Arctic cryoconite phototrophs

The distribution and drivers of microbial pigments in the cryoconite of four Tibetan glaciers

Microbial pigments play a significant role in glacier albedo reduction, thereby contributing to accelerated glacier retreat. The Tibetan Plateau has experienced rapid glacier retreat in recent decades due to global warming, yet there is limited understanding of microbial pigment distribution in the region. Here, we investigated the pigment concentration and composition in cryoconite from four glaciers. Our results showed that chlorophylls were the dominant pigments in Palong No. 4 (PL) and Jiemayangzong (JMYZ) glaciers located in the south of the Tibetan Plateau, while carotenoids were dominant in Qiangyong (QY) and Tanggula (TGL) glaciers located in the central region. Additionally, the chlorophyll b to chlorophyll a ratio, which is an indicator of the algae-to-cyanobacteria ratio, was higher in PL and JMYZ compared to QY and TGL. By using Random Forest Regression and Structural Equation Modelling, we determined that the concentrations of chlorophyll a, chlorophyll b, and carotenoids were associated with autotrophic bacteria relative abundance, climatic factors, and a combination of bacterial and climatic factors, respectively. This study is the first to describe the distribution of microbial pigments in cryoconite from Tibetan glaciers, providing additional support on the influence of algal pigment on glacier retreat.

Is Oxygenation Related to the Decomposition of Organic Matter in Cryoconite Holes?

Cryoconite is a sediment occurring on glacier surfaces worldwide which reduces ice albedo and concentrates glacier surface meltwater into small reservoirs called cryoconite holes. It consists of mineral and biogenic matter, including active microorganisms. This study presents an experimental insight into the influence of sediment oxygenation on the cryoconite ability to produce and decomposition of organic matter. Samples were collected from five glaciers in the Arctic and the European mainland. Cryoconite from three glaciers was incubated in stagnant and mechanically mixed conditions to imitate inter-hole water–sediment mixing by meltwater occurring on glaciers in Northern Hemisphere, and its effect on oxygen profiles and organic matter content. Moreover, we investigated short-term changes of oxygen conditions in cryoconite from four glaciers in illuminated and dark conditions. An anaerobic zone was present or approaching zero oxygen in all illuminated cryoconite samples, varying in depth depending on the origin of cryoconite: from 1500 µm from Steindalsbreen (Scandinavian Peninsula) and Forni Glacier (The Alps) to 3100 µm from Russell Glacier and Longyearbreen (Arctic) after incubation. Organic matter content varied between glaciers from 6.11% on Longyearbreen to 16.36% on Russell Glacier. The mixed sediment from the Forni Glacier had less organic matter than stagnant, the sediment from Longyearbreen followed this trend, but the difference was not statistically significant, while the sediment from Ebenferner did not differ between groups. Our results have implications for the understanding of biogeochemical processes on glacier surfaces, the adaptation of organisms to changing physical conditions due to abrupt sediment mixing, but also on the estimation of productivity of supraglacial systems.

Physiological Capabilities of Cryoconite Hole Microorganisms

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.

Time-dependent upregulation of electron transport with concomitant induction of regulated excitation dissipation in Haslea diatoms

Photoacclimation by strains of Haslea "blue" diatom species H. ostrearia and H. silbo sp. nov. ined. was investigated with rapid light curves and induction-recovery curves using fast repetition rate fluorescence. Cultures were grown to exponential phase under 50 µmol m^-2 s^-1 photosynthetic available radiation (PAR) and then exposed to non-sequential rapid light curves where, once electron transport rate (ETR) had reached saturation, light intensity was decreased and then further increased prior to returning to near growth light intensity. The non-sequential rapid light curve revealed that ETR was not proportional to the instantaneously applied light intensity, due to rapid photoacclimation. Changes in the effective absorption cross sections for open PSII reaction centres (σ_PSII') or reaction centre connectivity (ρ) did not account for the observed increases in ETR under extended high light. σ_PSII' in fact decreased as a function of a time-dependent induction of regulated excitation dissipation Y(NPQ), once cells were at or above a PAR coinciding with saturation of ETR. Instead, the observed increases in ETR under extended high light were explained by an increase in the rate of PSII reopening, i.e. Q_A^- oxidation. This acceleration of electron transport was strictly light dependent and relaxed within seconds after a return to low light or darkness. The time-dependent nature of ETR upregulation and regulated NPQ induction was verified using induction-recovery curves. Our findings show a time-dependent induction of excitation dissipation, in parallel with very rapid photoacclimation of electron transport, which combine to make ETR independent of short-term changes in PAR. This supports a selective advantage for these diatoms when exposed to fluctuating light in their environment.

The sources of high airborne radioactivity in cryoconite holes from the Caucasus (Georgia)

Cryoconite granules are mixtures of mineral particles, organic substances and organisms on the surface of glaciers where they decrease the ice albedo and are responsible for formation of water-filled holes. The contaminants are effectively trapped in the cryoconite granules and stay there for many years. This study evaluates the contamination level of artificial and natural radionuclides in cryoconite holes from Adishi glacier (Georgia) and identifies the sources of contamination based on activity or mass ratios among artificial radionuclides. Results revealed high activity concentrations of fallout radionuclides reaching 4900 Bq/kg, 2.5 Bq/kg, 107 Bq/kg and 68 Bq/kg for ¹³⁷Cs, ²³⁸Pu, ^239+240Pu and ²⁴¹Am, respectively. The main source of Pu is global fallout, but the low ²⁴⁰Pu/²³⁹Pu atomic ratios also indicated local tropospheric source of ²³⁹Pu, probably from the Kapustin Yar nuclear test site. Also, high activity ratios of ²⁴¹Am/^239+240Pu could originate from Kapustin Yar. The natural radionuclides originate from the surrounding rocks and were measured to control the environmental processes. ²¹⁰Pb in cryoconite granules comes predominantly from the atmospheric deposition, and its activity concentrations reach high values up to 12000 Bq/kg.