The stakes are considerable. Glacier ice algae belong to the Zygnematophyceae, a lineage of Streptophyte green algae distinct from snow and sea ice algae, and they thrive in the top centimeters of surface ice across the Arctic, the Alps, the Himalayas and Antarctica. Along the western margin of the Greenland Ice Sheet, summer blooms reach densities of around ten thousand cells per milliliter and cover between four and ten percent of the bare-ice area during the ablation season. Both Ancylonema species contain secondary phenolic pigments that absorb solar energy and lower the albedo of the ice surface. Previous work estimated that the albedo reduction associated with these algae contributed up to ten to thirteen percent of the total runoff in southwest Greenland during the summer of 2017, and in the Kangerlussuaq region roughly fifty-eight percent of the ice surface was found to be covered by the organisms.

Because the melting of the Greenland Ice Sheet is the primary cryospheric contributor to global sea-level rise, estimated at 13.7 plus or minus 1.1 millimeters between 1972 and 2018, understanding the biology of the algae that darken its surface has become an urgent scientific priority. Yet the algae are non-motile, meaning they cannot escape the environmental swings of the ice surface. They must endure photosynthetically active radiation that can exceed 1700 micromoles of photons per square meter per second under the constant light of the Arctic summer, alongside elevated ultraviolet radiation. The pH of the waters where they live has been measured anywhere between 3.6 and 8.6, and temperatures fluctuate daily between roughly 1.5 and 6 degrees Celsius at the field site during summer.

To probe how these stressors shape algal fitness, the researchers collected approximately one hundred kilograms of surface ice from a site 617 meters above sea level, 7.5 kilometers from the southern Greenland ice sheet margin, between 13 July and 8 August 2021. The ice was melted slowly in a tent under low light at 5 to 10 degrees Celsius over 72 hours, and composite samples were then distributed into sterile bags for a series of controlled in situ incubations. For each of four environmental parameters, pH, salinity, light and temperature, the team created a five-point gradient, with one unmodified bag per replicate serving as a control representing the prevailing conditions at the study site, where the surface ice pH was 6.7 plus or minus 0.2.

The pH gradient spanned 3.58 to 9.21, achieved by amending samples with sodium hydroxide or hydrochloric acid. Salinity was raised from ambient levels to 5, 15, 35 and 50 parts per thousand by adding sodium chloride. Light intensity was manipulated by wrapping bags in solar control films of different optical performance, producing samples exposed to 89, 75, 47, 32 and 14 percent of ambient irradiance, with the 75 percent treatment also excluding ultraviolet radiation. Five replicate bags per point in the pH, salinity and light gradients were incubated directly on the bare ice surface under natural light for 48 hours. For the temperature experiment, triplicate bags were placed in manually heated water baths at 5, 10, 15, 20 and 25 degrees Celsius, but because maintaining the baths was difficult, those incubations ran for only 4 hours under low light inside a tent.

The team assessed the photophysiology of the microbial community using pulse-amplitude-modulation, or PAM, fluorometry, a nondestructive technique that measures chlorophyll fluorescence in vivo to examine photosystem II. Samples were dark-adapted for 30 minutes before rapid light curve assessments consisting of nine 20-second steps of incrementally increasing actinic light intensities ranging from 33 to 1306 micromoles of photons, each followed by a saturating light pulse. From these measurements the researchers derived the maximum quantum efficiency of photosystem II, expressed as the Fv/Fm ratio, and the relative maximum electron transport rate, a proxy for the throughput of the photosynthetic electron transport chain. A reduced Fv/Fm ratio is frequently employed as an indicator of cellular stress, since it can signify damage to photosystem II reaction centers.

The results revealed a clear hierarchy of stressors. Light emerged as the primary driver of algal photophysiology. When irradiance was reduced to 32 percent of ambient conditions, the Fv/Fm ratio increased from 0.48 plus or minus 0.05 to 0.58 plus or minus 0.03, and the relative maximum electron transport rate rose from 0.17 plus or minus 0.01 to 0.25 plus or minus 0.03, both statistically significant changes. Excluding ultraviolet light alone had no effect on either parameter. In other words, the algae performed measurably better when the incoming solar radiation was dimmed, a finding consistent with the hypothesis that the production of the purpurogallin pigment under high natural light is energetically expensive for the cell, and that managing reactive oxygen species, which can damage DNA and proteins, diverts resources away from photosynthesis.

Salinity, by contrast, proved to be a genuine weakness. Even a modest 5 parts per thousand increase above ambient conditions caused the Fv/Fm ratio to fall from 0.41 plus or minus 0.03 to 0.29 plus or minus 0.02, and a 35 parts per thousand increase drove it down to 0.06 plus or minus 0.02, with the electron transport rate collapsing to 0.02 plus or minus 0.02. Elevated salt concentrations are a known abiotic stressor in green microalgae, where reactive oxygen species interfere with photosynthesis by damaging photosystem I and weakening photosystem II, and transcriptomic data from the model green alga Chlamydomonas reinhardtii have shown that downregulation of photosystem I light-harvesting complex genes reduces photosynthetic performance. For algae that could conceivably be washed toward the ocean as ice melts, this sensitivity to osmotic change may constrain their distribution.

Perhaps the most striking findings concern what did not matter. The photophysiology of the community remained largely stable across the pH gradient, with Fv/Fm values ranging from 0.29 plus or minus 0.03 to 0.37 plus or minus 0.06 between pH 4.7 and 9.21, and no significant differences in light utilization efficiency. Only the most acidic treatment, pH 3.5, produced a lower Fv/Fm ratio of 0.11 plus or minus 0.02 relative to the control. Temperature elevations showed no discernible impact either, with Fv/Fm values ranging from 0.63 plus or minus 0.03 to 0.68 plus or minus 0.03 across the 5 to 25 degree Celsius range. Given that the maximum air temperature recorded at the field camp during the summer of 2021 was just 6.21 degrees Celsius, the algae were still actively photosynthesizing at temperatures four times higher than anything they experience during a Greenland summer. This suggests that many polar microalgae, including glacier ice algae, may be psychrotolerant rather than truly psychrophilic, an adaptation that is advantageous for non-motile organisms living in a dynamic environment that can change rapidly over short periods.

The study also documented the toll of natural light exposure. Samples measured immediately after the melting process, which had spent 72 hours under low light in a tent, showed Fv/Fm ratios of 0.66 to 0.72, close to the ideal range of 0.7 to 0.8 reported for non-stressed green microalgae and plants. After 48 hours of incubation under in situ light conditions, control samples dropped to ratios between 0.37 and 0.48, corresponding well with values reported in the literature for naturally occurring glacier algal communities in Greenland. The authors note that their rapid light curve assessments were insufficient to fully saturate photosystem II reaction centers, preventing derivation of some parameters, and that the temperature experiments ran for only 4 hours, so more pronounced differences could emerge over longer durations. Future work with freshly cultured algae could explore interactive effects, varying light and temperature simultaneously to better simulate the combined impact of a changing Arctic climate on the organisms that help decide the fate of the ice sheet.

Subject of Research: Photophysiological responses of glacier ice algae to light, pH, salinity and temperature stress on the Greenland Ice Sheet

Article Title: Photophysiological response of glacier ice algae to abiotic stressors

Article References: Jensen, M. B., Turpin-Jelfs, T., Tranter, M., Benning, L. G., & Anesio, A. M. (2024). Photophysiological response of glacier ice algae to abiotic stressors. Frontiers in Geochemistry, 2, Article 1436488. https://doi.org/10.3389/fgeoc.2024.1436488

Image Credits: AI Generated

DOI: 10.3389/fgeoc.2024.1436488

Keywords: glacier ice algae, Greenland Ice Sheet, Ancylonema, photophysiology, PAM fluorometry, ice darkening, albedo, psychrotolerance, salinity stress, Arctic, climate change, sea-level rise