Effects of iron and dissolved N:P stoichiometry on the uptake of bicarbonate, nitrate, and amino acids by a Ross Sea microbial community

The Southern Ocean is seasonally one of the most productive regions of the ocean. Phytoplankton growth in the Southern Ocean is typically limited by iron (Fe), and Fe input is predicted to increase over the next century, which will likely enhance primary productivity. Dissolved concentrations of nitrogen (N) and phosphorus (P) may also change in the future due to both physical and biological drivers, translating to a difference in the stoichiometric N:P ratio of the water column. Increased Fe availability coupled with altered N:P ratios could impact rates of primary production and cause nutrient utilization by microorganisms to change. A natural microbial community was collected from the Ross Sea, amended with Fe, and grown under a range of N:P ratios for nine days to determine how Fe addition in combination with different N:P ratios impact uptake rates of carbon and N. Stable isotope tracer techniques were used to measure uptake rates of bicarbonate, nitrate, and amino acids by two size fractions (0.7-5.0 μm and >5.0 μm) of microorganisms. We found that the physiology of the larger microorganisms was less sensitive to changes in dissolved N:P stoichiometry than smaller microorganisms. Absolute uptake rates of nitrate by smaller microorganisms increased when dissolved N:P ratios were elevated above ambient levels, while uptake rates of bicarbonate initially increased with moderate elevations in N:P and then decreased. When dissolved N:P ratios were lowered, uptake rates of nitrate were not affected, however, uptake rates of bicarbonate decreased. The addition of Fe and the subsequent changes in dissolved N:P ratios in the Southern Ocean will impact the rates at which nitrate and bicarbonate are cycled by different microbial size fractions. This may alter the current biogeochemical balance between smaller and larger microorganisms.