Nitrate is reduced by heterotrophic bacteria but not transferred to Prochlorococcus in non-axenic cultures

Abstract

The ability to assimilate nitrate in non-axenic isolates of Prochlorococcus spp. was addressed in this work, particularly in three low-irradiance adapted strains originating from ocean depths with measurable nitrate concentrations. None of the studied strains was able to use nitrate as the sole nitrogen source. Nitrate reductase (NR; EC 1.6.6.2) activity was, however, detected using the methyl viologen/dithionite assay in crude extracts from all studied Prochlorococcus strains. Characterization of this activity unambiguously demonstrated its enzymatic origin. We observed that NR activity did not decrease in vivo under darkness. Attempts to detect the narB gene (coding for NR in other cyanobacteria) by PCR with primers designed on the basis of the specific codon usage in Prochlorococcus were unsuccessful. However, when primers were designed considering the codon frequencies typical of other bacteria, we could amplify different fragments of nas genes, coding for bacterial assimilatory NRs. Similar amplification products were obtained using colonies of contaminant bacteria from Prochlorococcus cultures as PCR template. Furthermore, NR activity was found in cultures of these contaminants, demonstrating the non-cyanobacterial origin of the enzyme. These results strongly suggest that the studied strains of Prochlorococcus lack NR, in spite of inhabiting environments with nitrate as the main nitrogen source. In addition, they indicate that the nitrite produced by heterotrophic bacteria is not transferred to Prochlorococcus for growth, thus discarding a trophic nitrogen chain between heterotrophic bacteria and Prochlorococcus in the studied cultures.

Prochlorococcus contributes to new production in the Sargasso Sea deep chlorophyll maximum

“…[9] Although reduced nitrogen substrates accounted for the majority of the measured nitrogen uptake, these deep populations of Prochlorococcus do assimilate a significant fraction of NO 3 À , $5 -10% of the total (Figure 2a). Indeed, a time course experiment of nitrogen uptake showed there was no time lag for NO 3 À uptake (Figure 2b) suggesting that there is no 'trophic processing' of 15 NO 3 À that would make the labeled N available for assimilation through a reduced N pathway [Lopez-Lozano et al, 2002]; an important observation for relating NO 3 À uptake to new production. This observation of NO 3 À uptake is in direct contrast to conclusions drawn from cultured isolates, and has the potential to drastically alter the biogeochemical role of Prochlorococcus in the oceans.…”

Glutamine synthetase from the marine cyanobacteria Prochlorococcus spp.: characterization, phylogeny and response to nutrient limitation

“…Because a main feature of the natural habitat of Prochlorococcus is the limitation in nutrients (Partensky et al, 1999a), the occurrence of mechanisms to optimize nutrient uptake and/or utilization by Prochlorococcus has been postulated (Partensky et al, 1999a;Scanlan and West, 2002). Our group has observed some striking traits concerning the in vivo regulation of GS in the strain PCC 9511 and the inability of different Prochlorococcus strains to assimilate nitrate López-Lozano et al, 2002). In this study, we further explored the possible evolutive modifications of glutamine synthetase from Prochlorococcus.…”

“…The higher similarity of GS from Prochlorococcus MIT9313 to that of Synechococcus WH7803 or WH8102 than to Prochlorococcus MED4 or SS120 could be an additional indication of the process that, presumably, induced the appearance of Prochlorococcus from ancient marine Synechococcus-like organisms. In this model, low-light adapted Prochlorococcus strains (such as MIT9313; genome size of 2.4Mbp) evolved from marine Synechococcus (such as WH8102; genome size of 2.72 Mbp) by losing some unnessential genes Strehl et al, 1999;Rippka et al, 2000;López-Lozano et al, 2002;Moore et al, 2002;Ting et al, 2002), leading to a progressive compaction of the genome (to strains like LL-adapted SS120-1.75 Mbp-, and eventually HL-adapted MED4-1.66 Mbp), that has been proposed as one of the advantages of Prochlorococcus to adapt to very oligotrophic marine conditions (Strehl et al, 1999). This model would explain the higher similarity between GS from Prochlorococcus SS120 and MED4 than SS120 and MIT9313 (both LL-adapted strains) observed in Fig.…”

Nitrogen starvation induces extensive changes in the redox proteome of Prochlorococcus sp. strain SS120

“…(Lindahl and Kieselbach, 2009). Taking into account that Prochlorococcus SS120 (as all other cultured strains thus far) is incapable of nitrate utilization (López-Lozano et al, 2002;García-Fernández et al, 2004), we focused our comparison to the N depletion vs ammonium repletion conditions described by Wegener and coworkers. Hereafter we will summarize the comparisons for the three cases for which data were provided (expressed as log 2 ratios in the supplemental table 6 of the manuscript by Wegener and coworkers) under boh N depletion and ammonium repletion:…”