Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet

Abstract

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.

Figure 1

Aerial views of surface ice inland from the Leverett Glacier terminus in West Greenland: (a) algal blooms at station T7, 34 km inland, 5 August 2010; (b) purple brown hue of algal bloom on surface ice at station T7; (c) surface clumping of ice algae; (d) true colour NASA MODIS Terra image acquired 17 August 2010 showing the presence of a dark band and location of sampling stations. Distances from the deglaciated Leverett terminus along the transect were T5=7 km, T6= 17 km, T7=34 km, T8=51 km and T9=79 km, S1 was 2 km from the ice margin at Point 660.

Figure 2

Autotrophs from surface ice of the Greenland ice sheet at station S1: (a) filament of orange-sheathed cyanobacterium; (b) cells of Cylindrocystis brebissonii, stained with Lugol's iodine resulting in detachment of the protoplast from the cell wall (c) live cell of Mesotaenium cf. berggrenii. (d) live filament of Ancylonema nordenskiödii. Scale bar=10 μm.

Figure 3

Cryoconite hole structure and cyroconite composition. (a) Surface ice at station S1 showing cryoconite holes with cryoconite granules in the bottom of the holes. The adjacent surface ice has pigmented areas colonised primarily by algae. (b) Cryoconite granule located on surface of ice sheet with orange-sheathed filamentous cyanobacteria with bases of filaments anchored in a central darker amorphous organic matrix. Apices of filaments extend out from matrix to obtain light for photosynthesis; (c) confocal microscopy image of part of the central matrix revealing the presence of additional autofluorescent filaments of cyanobacteria (red) embedded in the matrix. Concanavalin A-conjugate stains extracellular polymeric substances, (green), revealing a complex network of filaments and extracellular polymeric substance. Surrounding some filaments, a layer of extracellular polymeric substance is visible adjacent to the sheath suggesting extracellular polymeric substance secretion from filaments.

Figure 4

Primary and secondary production at the glacial surface in four nominal categories of ice pigmentation. Samples were incubated in situ at station S1, 2 km inland from the ice margin: LAC, low algal coverage; MAC, medium algal coverage; DAC, dense algal coverage. All rates are recorded as mean±s.e.m., n=3.

Figure 5

Rapid light response curves and derived parameters for photosynthetic microbial assemblages sampled at the station S1 and T5–T9. (a) The relationship between rETR and PAR showing no evidence of saturation, (n=23). The arrow shows an inflexion point in the light curve at ∼1200 μmol PAR m⁻² s⁻¹, where rETR was 230 relative units, ≈40% lower than the value of rETR_max derived by iterative curve fitting, (b) measurements of the physiological downregulation parameter for non-photochemical quenching. The calculated values showed saturation at ∼800 μmol PAR m⁻² s⁻¹, just before the light level at which the inflexion point was observed. (c) Only a 20±2% reduction in the maximum fluorescence yield (F_m′) was measured by the end of the light curve. All values are mean±s.e.m.

Figure 6

Microspectrophotometry and albedo measurements. (a) Representative absorbance spectra determined using image microspectrophotometry of intact filaments and cells of algae, cyanobacteria and cleaned mineral particles from the surface ice at station S1. (b) Albedo measurements from three categories of ice on the GrIS: CI (clean ice with no visible surface debris, n=3); DCC (dense cryoconite coverage, n=14); DAC (dense algal coverage, n=20). All values are mean±s.e.m. The albedo in algal covered (DAC) and cryoconite covered (DCC) areas were significantly lower than the albedo in areas of clean ice (CI), (***P<0.001).