Using Urea at the Ends of the Earth.

In the Arctic, productivity is limited in the winter by short days and high sea ice coverage. This may allow the build up of nitrates, sustaining later phytoplankton blooms. Another source of nitrogen is urea, shown to be influential during blooms. Most work on the importance of urea has been done at lower latitudes, thus the significance of it at polar regions is unknown. Also unknown is the extent to which bacteria and phytoplankton compete for urea. Biogeochemical cycles in the arctic may be disrupted by climate change but little research has been done on how these could alter, especially in regard to major cycles, carbon and nitrogen. Another understudied area is dark carbon fixation (DCF) and the way in which organisms use it in life strategies. DCF may be used it as a way to balance metabolic processes when there is limited nutrient availability. To address all of these unknowns Connelly et al hypothesised that Bacteria and Archaea using ammonia, nitrates and urea were unable to compete against phytoplankton blooms during the summer months. They also hypothesise that Bacteria and Archaea will use dark carbon fixation during the winter months.

Samples were taken from 8m in the summer and 1m in the winter, both from the water column about 2.5km offshore of Barrow, Alaska. To asses if the microbes fix carbon, dependent of light, ¹³C Stable Isotope Probing (SIP) of bicarbonate was used to see if they incorporated the carbon into their DNA. SIP was also used with ¹⁵N to see assimilation of nitrogen.

In accordance with previous research, similar microbial communities between the two seasons were found. Archaea were far less abundant in the summer than the winter, constituting 1.8% and 11.8% respectively. This was the largest change in abundance, although there was a 2.8 fold increase in Bacteria abundance in the winter. This suggests Bacteria and Archaea are a part of a stable Arctic community. For bacteria, there were higher abundances in the summer of Cyanobacteria and Verrucomicrobia, whereas winter yielded mostly SAR11 and Oceanospirillum. Greater species richness overall was found during the winter months, supporting the idea they compete with phytoplankton.

Assimilation of ¹⁵NH4+ was only in the summer, conversely ¹⁵N urea was only in the winter; suggesting a shift in the sources of nitrogen during the seasons. One limitation acknowledged was the size of filter used, most cells in the winter were smaller than the mesh size, thus it is possible there was an underestimation of urea. There was no evidence for ¹⁵NO3- uptake, contrary to other findings, although this was suggested to be a last chance source of Nitrogen. This study added onto previous research by showing that dark carbon fixation is possible in-situ but future studies should look at if there is a difference in the contribution of dark carbon fixation by heterotrophic, mixotrophic and chemoautotrophic microbes.

In my opinion, the authors have not fully provided conclusive answers to their hypotheses. Also, I think this study was not necessarily essential; it added very little to the existing knowledge, based on ample similar studies given in the discussion. Although the fact this study supports other research helps to provide a more conclusive understanding of diversity changes, even if in only a limited geographical area.

Connelly, T. L., Baer, S. E., Cooper, J. T., Bronk, D. A., & Wawrik, B. (2014). Urea uptake and carbon fixation by marine pelagic bacteria and archaea during the Arctic summer and winter seasons. Applied and environmental microbiology, 80(19), 6013-6022.