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 (Ͻ20ЊC), high NO fluxes (Ͼ25 Ϫ 3 M-N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO uptake kinetics and direct short-term temperature manipulation studies Ϫ 3 on field diatom-dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO uptake by a Rhizoselenia-dominated population did not appear to saturate even at concen- of experimental temperature (ambient Ϯ 9ЊC). Over the temperature range of 7-25ЊC, absolute uptake rates of NO ( NO3Ϫ ) decreased an average of 46% with increasing temperature from 7 to 25ЊC (nine individual experiments),while NH4ϩ and 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 in excess of nutritional requirements, the reductionof 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 uptake increases proportionately with themagnitude 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 by diatom-dominatedassemblages, other observations of nonsaturating NO kinetics in field populations, and the apparent ''preference''for NO by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO uptake.

Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models

“…Then, the overestimated NO 3 may be caused by insufficient phytoplankton nutrient uptake (i.e., too low a rate for Arctic waters) since NPP was overall underestimated by most of the models. Phytoplankton nitrate uptake is strongly dependent on both temperature (i.e., less at colder temperatures) [Lomas and Glibert, 1999] and irradiance (i.e., photoinhibited at higher levels) [Hu and Smith, 1998] and becomes more dependent on their combined effect in the simultaneously cold and dark Arctic waters during the transition to spring conditions [Tremblay et al, 2006]. It is likely that several of the models have globally relevant phytoplankton physiological parameter values that are too high and may thus not correspond to the lower and narrower ranges required by Arctic (or polar) phytoplankton.…”

Vertical stability and the annual dynamics of nutrients and chlorophyll fluorescence in the coastal, southeast Beaufort Sea

“…New production in spring inferred from nutrient draw-down, submitted to Marine Ecology Progress Series, 2008.). On the vertical, the association between the nitrite maximum, the SCM and the nitracline (Figure 6) strongly suggests that the NO 2 À was released by phytoplankton, possibly because irradiance at the SCM was not always sufficient to drive the complete reduction of NO 3 À or because shade-adapted algae used NO 3 À reduction as an electron sink when transiently exposed to higher light intensities [Lomas and Glibert, 1999]. Phytoplankton are hypothesized to be the principal cause of formation and maintenance of the primary nitrite maximum in other stratified oceans (see references given by Lomas and Lipschultz [2006]), which is even more likely in the Beaufort Sea where the maximum is shallow enough for light to inhibit NH 4 + oxidizers during summer [e.g., Guerrero and Jones, 1996].…”

Metatranscriptomes reveal functional variation in diatom communities from the Antarctic Peninsula

“…The assimilation of nitrate by sea ice diatoms may help in controlling cellular energy balance under photorespiratory conditions (Lomas and Glibert, 1999). Photorespiration in WKI is suggested by abundant photoprotective LHCSR proteins, reduced photosynthetic pathway enrichment (Figure 4) and by enrichment of mitochondrial glycine decarboxylase (GDCT), a key enzyme and marker for photorespiration (Douce et al, 2001).…”