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 NO3− 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 NO3− 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 NO3− 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 NO2−, 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 NO3− (ρNO3−) decreased an average of 46% with increasing temperature from 7 to 25°C (nine individual experiments), while ρNH4+ and ρUREA 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 NO3− 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 NO3− 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 NO2− by diatom‐dominated assemblages, other observations of nonsaturating NO3− kinetics in field populations, and the apparent "preference" for NO3− by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative relationship between NO3− 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 NO3− 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 NO3− flux, and turbulent mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO3− uptake. Finally, if a significant fraction of NO3− 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 NO2−, NH+, or dissolved organic nitrogen (DON), then estimates of new production based solely on NO3− uptake could be seriously biased.