The distinct microbial ecology and biogeochemistry of rotten sea ice on the Arctic shelf

Abstract:

Rotten ice is ice at an advanced stage of melt and is characterized by large, interconnected pore spaces, uniform temperatures, and low brine content. As the Arctic warms, this ice type may become increasingly prevalent. It is also fragile and difficult to work on, inhibiting its study, and therefore our understanding of how this ice type may influence the biogeochemistry of the present and future Arctic. In July 2015, our research team collected rotten ice samples from floes off the coast of Utqiaġvik, Alaska in order to characterize its physical and chemical properties and resident microbial community. Samples came from two different ice floes: one with low, the other with high sediment load. We also collected and analyzed earlier-season (May, June) landfast ice from the same region for comparison. Physical properties have been previously reported (Frantz, et al., 2019). To characterize biogeochemistry, we measured suspended particulate matter (SPM), dissolved organic carbon (DOC), particular organic carbon (POC), particulate nitrogen (PN), and particulate extracellular polysaccharide substances (pEPS). The microbial community was characterized by epifluorescence microscopy, microscopic classification of phytoplankton, chlorophyll measurements, and Illumina sequencing of 16S and 18S SSU RNA gene amplicons from DNA and RNA. Our results establish rotten ice as a physically and chemically distinct microbial habitat from earlier-season ice, with microbial communities that, while distinct in the two different sampled floes, were also distinct from the communities in earlier-season ice. We also observed increases in the amount of particulate carbon and nitrogen present in rotten vs. earlier-season ice. The melting of rotten ice could therefore contribute significantly to Arctic shelf carbon and nitrogen cycling, and therefore to Arctic biogeochemistry more generally.