Abstract
Abstract. The fate of diazotroph (N2 fixers) derived carbon (C) and nitrogen (N) and their contribution to vertical export of C and N in the western tropical South Pacific Ocean was studied during OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment). Our specific objective during OUTPACE was to determine whether autocatalytic programmed cell death (PCD), occurring in some diazotrophs, is an important mechanism affecting diazotroph mortality and a factor regulating the vertical flux of organic matter and, thus, the fate of the blooms. We sampled at three long duration (LD) stations of 5 days each (LDA, LDB and LDC) where drifting sediment traps were deployed at 150, 325 and 500 m depth. LDA and LDB were characterized by high chlorophyll a (Chl a) concentrations (0.2–0.6 µg L−1) and dominated by dense biomass of the filamentous cyanobacterium Trichodesmium as well as UCYN-B and diatom–diazotroph associations (Rhizosolenia with Richelia-detected by microscopy and het-1 nifH copies). Station LDC was located at an ultra-oligotrophic area of the South Pacific gyre with extremely low Chl a concentration (∼ 0.02 µg L−1) with limited biomass of diazotrophs predominantly the unicellular UCYN-B. Our measurements of biomass from LDA and LDB yielded high activities of caspase-like and metacaspase proteases that are indicative of PCD in Trichodesmium and other phytoplankton. Metacaspase activity, reported here for the first time from oceanic populations, was highest at the surface of both LDA and LDB, where we also obtained high concentrations of transparent exopolymeric particles (TEP). TEP were negatively correlated with dissolved inorganic phosphorus and positively coupled to both the dissolved and particulate organic carbon pools. Our results reflect the increase in TEP production under nutrient stress and its role as a source of sticky carbon facilitating aggregation and rapid vertical sinking. Evidence for bloom decline was observed at both LDA and LDB. However, the physiological status and rates of decline of the blooms differed between the stations, influencing the amount of accumulated diazotrophic organic matter and mass flux observed in the traps during our experimental time frame. At LDA sediment traps contained the greatest export of particulate matter and significant numbers of both intact and decaying Trichodesmium, UCYN-B and het-1 compared to LDB where the bloom decline began only 2 days prior to leaving the station and to LDC where no evidence for bloom or bloom decline was seen. Substantiating previous findings from laboratory cultures linking PCD to carbon export in Trichodesmium, our results from OUTPACE indicate that nutrient limitation may induce PCD in high biomass blooms such as displayed by Trichodesmium or diatom–diazotroph associations. Furthermore, PCD combined with high TEP production will tend to facilitate cellular aggregation and bloom termination and will expedite vertical flux to depth.
Highlights
The efficiency of the biological pump, essential in the transfer and sequestration of carbon to the deep ocean, depends on the balance between growth and death
Diazotrophic communities comprised of unicellular cyanobacteria lineages (UCYN-A, B and C), diatom– diazotroph associations (DDA) such as Richelia associated with Rhizosolenia and diverse heterotrophic bacteria such as alpha and γ -proteobacteria are responsible for these rates of N2 fixation
When looking for biomass at depth, the deep chlorophyll maximum (DCM) recorded at ∼ 80 m depth was characterized by chlorophyll a (Chl a) concentrations increasing from 0.3 to 0.5 μg L−1 between days 3 and 5, respectively (Fig. 1d)
Summary
The efficiency of the biological pump, essential in the transfer and sequestration of carbon to the deep ocean, depends on the balance between growth (production) and death. N2 fixing (diazotrophic) prokaryotic organisms are important contributors to the biological pump and their ability to fix atmospheric N2 confers an inherent advantage in the nitrogen-limited surface waters of many oceanic regions. The most conspicuous of all diazotrophs, and predominating in terms of biomass, is the filamentous bloomforming cyanobacterium Trichodesmium forming massive surface blooms that supply ∼ 60–80 Tg N yr−1 of the 100– 200 Tg N yr−1 of the estimated marine N2 fixation (Capone et al, 1997; Carpenter et al, 2004; Westberry and Siegel, 2006) with a large fraction fixed in the WTSP (Dupouy et al, 2000, 2011; Barboza Tenório et al, 2018) that may, based on NanoSIMS cell-specific measurements, contribute up to ∼ 80 % of bulk N2 fixation rates in the WTSP (Bonnet et al, 2018)
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