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Here, we present the first study to investigate the seasonal importance of amino acid-nitrogen (N) to Arctic near shore microbial communities. We measured primary productivity and the uptake of ammonium, nitrate, urea, and amino acids in two size fractions (> 3 μm and approximately 0.7-3 μm), as well as ammonium regeneration and nitrification using 15N and 13C tracer approaches in the near-shore waters of the Chukchi Sea, during January, April, and August for two consecutive years. At discrete depths, nitrate comprised 46-78% of the total dissolved N pool during January and April but only 2-6% during August. Dissolved organic N (DON) concentrations increased between January and August though the carbon (C) : N (mol : mol) of the DON pool declined. Of the substrates tested, amino acids supported the bulk of both N and C nutrition in both size fractions during January and April (ice-covered). Urea generally had the lowest uptake rate under ice-covered conditions; uptake of urea-C was only detectable in August. Though previous Arctic studies focused largely on nitrate, we found nitrate uptake was generally lower than other substrates tested. The sharp decline in nitrate concentration between April and August, however, indicates a drawdown of nitrate during that period. Rates of ammonium uptake were highest in August, when it was the dominant N substrate used. During all sample periods, rates of ammonium regeneration were sufficient to supply ammonium demand. Rates of nitrification varied between sample periods, however, with much higher rates seen in January and April. [ABSTRACT FROM AUTHOR]

The coastal Chukchi Sea has a particularly productive landfast ice ecosystem but is currently lacking direct measurements of nitrogen (N) cycling within the ice. Using stable isotopic tracers, we measured uptake and regeneration of ammonium and nitrate, along with primary and secondary production at three depths within landfast sea ice in the Chukchi Sea near Barrow, Alaska, during April 2011 and January 2012. These are the first data of N uptake and regeneration rates in landfast Arctic sea ice during spring and winter. Both inorganic and organic nutrient concentrations in the ice were generally higher than the water column, with the exception of phosphate, which may have limited production in certain sections of ice. Primary production at all ice depths was higher than the water column during April, but below the detection limit in January. Bacterial production in the bottom ice (0-10 cm from the ice-water interface) was consistently higher than the water column across seasons. Absolute uptake of ammonium was highest in the bottom ice and higher than absolute uptake of nitrate at all depth horizons except in the upper ice (30-40 cm from ice-water interface) during January. While N uptake rates in the ice were higher than the water column, nitrification rates were lower. Regeneration of ammonium and nitrate far exceeded uptake within the ice. The magnitude of N uptake and regeneration rates in landfast ice highlights the importance of the biological sea ice community to the region. [ABSTRACT FROM AUTHOR]

Biogeochemical rate processes in the Arctic are not currently well constrained, and there is very limited information on how rates may change as the region warms. Here we present data on the sensitivity of ammonium (NH4+) uptake and nitrification rates to short-term warming. Samples were collected from the Chukchi Sea off the coast of Barrow, Alaska, during winter, spring, and summer and incubated for 24 h in the dark with additions of 15NH4+ at −1.5, 6, 13, and 20°C. Rates of NH4+ uptake and nitrification were measured in conjunction with bacterial production. In all seasons, NH4+ uptake rates were highest at temperatures similar to current summertime conditions but dropped off with increased warming, indicative of psychrophilic (i.e., cold-loving) microbial communities. In contrast, nitrification rates were less sensitive to temperature and were higher in winter and spring compared to summer. These findings suggest that as the Arctic coastal ecosystem continues to warm, NH4+ assimilation may become increasingly important, relative to nitrification, although the magnitude of NH4+ assimilation would be still be lower than nitrification. [ABSTRACT FROM AUTHOR]

How Arctic climate change might translate into alterations of biogeochemical cycles of carbon (C) and nitrogen (N) with respect to inorganic and organic N utilization is not well understood. This study combined 15N uptake rate measurements for ammonium, nitrate, and urea with 15N- and 13C-based DNA stable-isotope probing (SIP). The objective was to identify active bacterial and archeal plankton and their role in N and C uptake during the Arctic summer and winter seasons. We hypothesized that bacteria and archaea would successfully compete for nitrate and urea during the Arctic winter but not during the summer, when phytoplankton dominate the uptake of these nitrogen sources. Samples were collected at a coastal station near Barrow, AK, during August and January. During both seasons, ammonium uptake rates were greater than those for nitrate or urea, and nitrate uptake rates remained lower than those for ammonium or urea. SIP experiments indicated a strong seasonal shift of bacterial and archaeal N utilization from ammonium during the summer to urea during the winter but did not support a similar seasonal pattern of nitrate utilization. Analysis of 16S rRNA gene sequences obtained from each SIP fraction implicated marine group I Crenarchaeota (MGIC) as well as Betaproteobacteria, Firmicutes, SARI 1, and SAR324 in N uptake from urea during the winter. Similarly, 13C SIP data suggested dark carbon fixation for MGIC, as well as for several proteobacterial lineages and the Firmicutes. These data are consistent with urea-fueled nitrification by polar archaea and bacteria, which may be advantageous under dark conditions. [ABSTRACT FROM AUTHOR]