Forming the primary nitrite maximum: Nitrifiers or phytoplankton?

Abstract

As intermediary in a number of key biological processes, the dynamics of oceanic NO 2 2 concentrations have historically been used as an indicator of the balance between oxidative and reductive pathways in the marine nitrogen cycle. As appreciation of the role of NO 2 2 in the marine nitrogen cycle grew through the 1960s and 1970s, and data sets from different ocean basins became available, a common feature was observed in stratified water columns: a peak in NO 2 2 concentrations at the base of the euphotic zone, with near zero concentrations both shallower and deeper. These concentrations are significant; they commonly range between 10 and 400 nmol L 21 but as high as 4,500 nmol L 21 . This peak in NO 2 2 concentration is termed the primary nitrite maximum (PNM). Since the 1960s, the mechanisms sustaining the ubiquitous PNM have remained uncertain, with available data supporting either bacterial nitrification or NO 2 2 release by phytoplankton. Simple box models have reproduced the PNM feature with nitrification as the source of NO 2 2 , whereas others have succeeded solely with phytoplankton. Conclusive identification of the mechanism(s) maintaining the PNM in the world's oceans has yet to be achieved, but the preponderance of data supports phytoplankton excretion, with nitrification likely playing only a supporting role. Furthermore, there are a number of potentially important inconsistencies in the role of nitrification between culture studies and field observations. Biological-physical interactions are likely also important in controlling PNM formation and maintenance.

Physiology and evolution of nitrate acquisition in Prochlorococcus

“…Although focA is also similar to formate transporters, evidence implicates its role in nitrite uptake in Prochlorococcus; for example, the gene is located near other nitrite assimilation genes (Figure 3), it is upregulated under nitrogen stress (Tolonen et al, 2006) and it is absent from Prochlorococcus that cannot grow on nitrite (Moore et al, 2002;Coleman and Chisholm, 2007;Kettler et al, 2007) (Supplementary Figure S5). As PAC1 possesses both a nitrite transporter (focA) and the dual-function nitrate/nitrite transporter (napA), it is possible that focA provides some advantage to LL adapted cells that are often maximally abundant near the nitrite maxima in the oceans (Scanlan and West, 2002;Lomas and Lipschultz, 2006). LL adapted cells that possess the dual-function nitrite/nitrate transporter may benefit from having an additional transporter for nitrite.…”

Episodic meteorological and nutrient-load events as drivers of coastal planktonic ecosystem dynamics: a time-series analysis

“…Nitrite is excreted from cells under growthlimiting conditions (e.g. when light is low; Collos 1998), because its further reduction to ammonia needs energy, and presumably because of its toxicity (Lomas & Lipschultz 2006). This co-variation is evidence that N is not the dominant limiting element in Blanes Bay, as in N-limiting conditions nitrite is expected to be significantly taken up by phytoplankton (Collos 1998).…”

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

“…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]. If this is true, then the NO 3 À produced by the wintertime nitrification of the NO 2 À released in the primary nitrite maximum during summer should be considered allochthonous and not recycled, since the N was not assimilated into biomass.…”