Laboratory investigation of inorganic carbon uptake by cryoconite debris from Werenskioldbreen, Svalbard

Abstract

Laboratory experiments were undertaken to determine the inorganic carbon uptake rate and the interactions between photosynthesis and water chemistry, particularly pH and nutrient concentrations, for cryoconite debris from Werenskioldbreen, a well‐researched Svalbard glacier. Microorganisms in cryoconite debris took up inorganic carbon at rates between 0.6 and 15 μg C L−1 h−1 and fixed it as organic carbon. Cyanobacterial photosynthesis (75–93%) was the main process responsible for inorganic carbon fixation, while heterotrophic uptake (6–15%) only accounted for a minor part. The microbes in cryoconite debris were active shortly after melt and fixed carbon as long as there were favorable conditions. They were not truly psychrophilic: their physiological optimum temperature was higher than is prevalent in cryoconite holes. The pH was also a factor affecting photosynthesis in the cryoconite slurry. The highest dissolved inorganic carbon (DIC) uptake rates per liter of slurry occurred at pH ∼7, and there was a significant correlation between the initial pH and DIC fixation on a per cell basis, showing increasing DIC uptake rates when pH increased from ∼5.5 to 9. Inorganic carbon fixation resulted in an increased pH in solution. However, the microbes were able to photosynthesize in a wide range of pH from ∼4 to ∼10. The average C:N:P molar ratios in solution were ∼350:75:1. Unlike nitrogen, phosphorus concentrations decreased with increasing carbon uptake, and when the rate approached ∼15 μg C L−1 h−1, all available dissolved phosphorus was utilized within 6 h. Hence phosphorus is probably biolimiting in this system.