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. The fate of diazotroph(N2 fixers) derived carbon (C) and nitrogen (N) and theircontribution to vertical export of C and N in the western tropical SouthPacific Ocean was studied during OUTPACE (Oligotrophy to UlTra-oligotrophyPACific Experiment). Our specific objective during OUTPACE was to determinewhether autocatalytic programmed cell death (PCD), occurring in somediazotrophs, is an important mechanism affecting diazotroph mortality and afactor regulating the vertical flux of organic matter and,thus, the fate of the blooms. Wesampled at three long duration (LD) stations of 5 days each (LDA, LDB andLDC) where drifting sediment traps were deployed at 150, 325 and 500 mdepth. LDA and LDB were characterized by high chlorophyll a (Chl a)concentrations (0.2–0.6 µg L−1) and dominated by densebiomass of the filamentous cyanobacterium Trichodesmium as well as UCYN-B and diatom–diazotrophassociations (Rhizosolenia with Richelia-detected bymicroscopy and het-1 nifH copies). Station LDC was located at anultra-oligotrophic area of the South Pacific gyre with extremely low Chl aconcentration (∼ 0.02 µg L−1) with limited biomass ofdiazotrophs predominantly the unicellular UCYN-B. Our measurements of biomassfrom LDA and LDB yielded high activities of caspase-like and metacaspaseproteases that are indicative of PCD in Trichodesmium and otherphytoplankton. Metacaspase activity, reported here for the first time fromoceanic populations, was highest at the surface of both LDA and LDB, where wealso obtained high concentrations of transparent exopolymeric particles(TEP). TEP were negatively correlated with dissolved inorganic phosphorus andpositively coupled to both the dissolved and particulate organic carbonpools. Our results reflect the increase in TEP production under nutrientstress and its role as a source of sticky carbon facilitating aggregation andrapid vertical sinking. Evidence for bloom decline was observed at both LDAand LDB. However, the physiological status and rates of decline of the bloomsdiffered between the stations, influencing the amount of accumulateddiazotrophic organic matter and mass flux observed in the traps during ourexperimental time frame. At LDA sediment traps contained the greatest exportof particulate matter and significant numbers of both intact and decayingTrichodesmium, UCYN-B and het-1 compared to LDB where the bloomdecline began only 2 days prior to leaving the station and to LDC where noevidence for bloom or bloom decline was seen. Substantiating previousfindings from laboratory cultures linking PCD to carbon export inTrichodesmium, our results from OUTPACE indicate that nutrientlimitation may induce PCD in high biomass blooms such as displayed byTrichodesmium ordiatom–diazotroph associations. Furthermore, PCD combined with high TEPproduction will tend to facilitate cellular aggregation and bloom terminationand will expedite vertical flux to depth.

Programmed cell death in diazotrophs and the fate of organic matter in the western tropical South Pacific Ocean during the OUTPACE cruise