Diazotroph derived nitrogen supports diatom growth in the South West Pacific: A quantitative study using nanoSIMS

Abstract

International audienceNitrogen is essential for life but is often a major limiting nutrient for growth in the ocean. Biological dinitrogen fixation is a major source of new nitrogen to surface waters and promotes marine productivity. Yet the fate of diazotroph-derived nitrogen (DDN) in marine ecosystems has been poorly studied, and its transfer to auto- and heterotrophic plankton has not been measured. Here, we use high-resolution nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with N-15(2) isotopic labelling and flow cytometry cell sorting to examine the DDN transfer to specific groups of natural phytoplankton and bacteria during three diazotroph blooms dominated by the cyanobacterium Trichodesmium spp. in the South West Pacific. During these experiments, 13%+/- 2% to 48%+/- 5% of the fixed N-15(2) was released into the dissolved pool and 6%+/- 1% to 8%+/- 2% of this DDN was transferred to non-diazotrophic plankton after 48 h. The primary beneficiaries of this DDN were diatoms (45%+/- 4% to 61%+/- 38%) and bacteria (22%+/- 27% to 38%+/- 12%), followed by pico-phytoplankton (3%+/- 1% to 21%+/- 14%). The DDN was quickly converted to non-diazotrophic plankton biomass, in particular that of diatoms, which increased in abundance by a factor of 1.4-15 over the course of the three experiments. The single-cell approach we used enabled quantification of the actual transfer of DDN to specific groups of autotrophic and heterotrophic plankton in the surface ocean, revealing a previously unseen level of complexity in the pathways that occur between N-2 fixation and the eventual export of DDN from the photic zone

Effect of light on N<sub>2</sub> fixation and net nitrogen release of <i>Trichodesmium</i> in a field study

“…6). Our fractions are consistent with previous reports by Bonnet et al (2016a), in which 6 % ± 1 % of DD 15 N was transferred to non-diazotrophic plankton in naturally occurring Trichodesmium blooms. Our fractions are slightly lower than the DD 15 N transfer (∼ 12 %) by Berthelot et al (2016), who inoculated Trichodesmium erythraeum into natural surface oligotrophic seawater.…”

“…5). The overall range agrees well with previous field studies (Glibert and Bronk, 1994;Mulholland et al, 2006;Bonnet et al, 2016a;Konno et al, 2010;Benavides et al, 2013;Berthelot et al, 2015). Our data revealed that the fraction of DDN release to gross NF increased as light decreased (all p values < 0.05).…”

“…The reproducibility for δ 15 N TDN measurements was better than 0.5 ‰. The DDN released to the dissolved pool was calculated following the equation proposed by Bonnet et al (2016a).…”

“…Conversely, diatom and dinoflagellate blooms have often been observed following Trichodesmium blooms, suggesting that DDN potentially supported nondiazotrophic phytoplankton growth (Devassy et al, 1978;Lenes et al, 2001). By using nanometer-scale secondary ion mass spectrometry, Bonnet et al (2016a) recently showed that the DDN is quickly (1-3 days) transferred to surrounding plankton, predominantly diatoms and bacteria, during Trichodesmium blooms. A mesocosm experiment performed in the western tropical South Pacific (VAHINE) revealed an incommensurately high contribution of NF to export production (> 50 %; Knapp et al, 2016) during a bloom of UCYN-C bloom.…”

Diazotrophs: Overlooked Key Players within the Coral Symbiosis and Tropical Reef Ecosystems?

“…For example, DDN contribution to the primary production of Red Sea coral rocks varied from 2% in winter to more than 27% in summer, considering the maximal net primary production measured at midday . Moreover, DDN cannot only contribute to support the growth of primary producers, but also the growth of secondary producers via fixed nitrogen release from diazotroph cells (Mulholland et al, 2004;Berthelot et al, 2015;Bonnet et al, 2016), grazing (Williams and Carpenter, 1997), or via diazotroph biomass recycling. The estimations of the annual average of the total contribution of DDN to reef nitrogen cycle thus vary from 150 to ca.…”