Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet.

Meltwaters on the surface of glaciers have been identified as hot spots for microbial activity. Records indicate that cyanobacteria and green algae dominate the autotrophic assemblages found in the benthic debris in cryoconite holes. Diatoms are commonly recorded in lentic and lotic ecosystems within polar habitats and, in line with the ubiquity principle for microbial communities, potentially, diatoms should be frequently found in the cryoconite of supraglacial environments. In this study, we cultured debris from cryoconite material collected in Svalbard and Greenland, to promote the growth of diatoms. Diatom generic richness varied between 12 and 17 between sites and was ∼5-fold higher than previously reported. Cryoconite supported aerophytic, halophytic, epipelic and bryophilic diatoms, suggesting multiple origins of colonizing cells. Twenty-seven genera were cultured from material that had been frozen (–20°C) for >1 year, indicating their long-term cryotolerance. The diatom flora composition was similar to that recorded in relatively acidic arctic lakes of low conductivity, and bore similarities at the generic level to those from terrestrial/semi-terrestrial moss communities from both the Arctic and Antarctic. As glaciers retreat, the diatom cells residing in cryoconite have the potential to act as seeding agents for a variety of terrestrial and aquatic habitats in proglacial regions and beyond.

Phytobenthos, major primary producers in freshwater, may be periodically exposed to herbicides through runoff, spray drift, and leaching, but experimental work on their response to herbicides is limited. Outdoor flumes were used to measure the recovery of intact phytobenthic biofilms, following a short-term pulse to a glyphosate-based herbicide (GBH) and chlorotoluron (CLT), singly or as a mixture (GBHC). Two days following the removal of the herbicides, significant declines in biomass and rates of areal gross maximum photosynthesis were recorded in GBH and GBHC-treated flumes. Herbicide pulses negatively impacted the biomass of phytobenthos, measured as chlorophyll a, which in turn may have alleviated resource limitation, evidenced by significant increases in biomass-specific rates of gross maximum photosynthesis. After 4.5 days, biomass concentrations were similar in controls and treatments, indicating biofilm recovery in the GBH and GBHC biofilms, though the proportion of green algae relative to diatoms was significantly higher in GBH and GBHC-impacted biofilms. Significant increases were recorded in the Trophic Diatom Index (TDI), indicating benthic diatom development along different trajectories, following the herbicide pulse. Collectively, these results indicate greater insights into herbicide impacts on phytobenthos may be gained by working with the whole phytobenthic community.