Temperature regulation of nitrate uptake: A novel hypothesis about nitrate uptake and reduction in cool‐water diatoms

Abstract

Marine diatoms generally form large blooms during periods of cool temperature (<20°C), high NO 3 − fluxes (>25 μM‐N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO 3 − uptake kinetics and direct short‐term temperature manipulation studies on field diatom‐dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO 3 − uptake by a Rhizoseleni ‐dominated population did not appear to saturate even at concentrations as high as 180 µM‐N. We observed contrasting patterns of NO 2 − , NH + , and urea utilization as a function of experimental temperature (ambient ± 9°C). Over the temperature range of 7–25°C, absolute uptake rates of NO 3 − (ρ NO3− ) decreased an average of 46% with increasing temperature from 7 to 25°C (nine individual experiments), while ρ NH4+ and ρ UREA increased with increasing temperature by an average of 179 and 86% (eight individual experiments), respectively. Based on these observations and the nature of the physical environment, we hypothesize that these diatom‐dominated populations were taking up NO 3 − in excess of nutritional requirements, the reduction of which may serve as a sink for electrons during transient periods of imbalance between light energy harvesting and utilization. We suggest that the increase in non‐nutritional NO 3 − uptake increases proportionately with the magnitude of the imbalance between light energy harvesting and imbalance. This hypothesis reconciles previous observations of low C:N uptake ratios, high release rates of dissolved organic nitrogen or NO 2 − by diatom‐dominated assemblages, other observations of nonsaturating NO 3 − kinetics in field populations, and the apparent "preference" for NO 3 − by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative relationship between NO 3 − uptake and short‐term temperature shifts, have important ecological implications. The hypothesized ability of these diatom‐dominated populations to better modulate the flow of photosynthetic electron energy, via NO 3 − reduction, in variable environments may provide a competitive advantage to diatoms and could potentially explain why diatoms frequently dominate in regions of cool temperature, high NO 3 − flux, and turbulent mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO 3 − uptake. Finally, if a significant fraction of NO 3 − uptake is regulated by non‐nutritional mechanisms in the cell, and if some fraction of nitrogen reduced by this mechanism is subsequently released in the form of NO 2 − , NH + , or dissolved organic nitrogen (DON), then estimates of new production based solely on NO 3 − uptake could be seriously biased.