Temperature regulation of nitrate uptake: A novel hypothesis about nitrate uptake and reduction in cool-water diatoms
Limnol. Oceanogr., 44(3), 1999, 556-572 | DOI: 10.4319/lo.1999.44.3.0556
ABSTRACT: Marine diatoms generally form large blooms during periods of cool temperature (<20°C), high NO3- fluxes (>25 uM-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- (rNO32) decreased an average of 46% with increasing temperature from 7 to 25°C (nine individual experiments), while rNH+ and rUREA 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.