Trichodesmium and other planktonic cyanobacteria in New Caledonian waters (SW tropical Pacific) during an El Niño episode

Abstract

Contributions of filamentous and picoplanktonic cyanobacteria to the 26 phytoplankton community structure were examined in New Caledonian waters during the 2001-27 2003 El Niño period at three stations. Morphometric characteristics of diazotrophic filamentous 28 cyanobacteria are given as well as the seasonal and inter-annual variations of their surface areas 29 and integrated abundances. Trichodesmium tenue and T. thiebautii were the dominant species 30 followed by T. erythraeum, altogether accounting for more than 51-80 % of the biomass of the 31 free-living filamentous cyanobacteria. Katagnymene spp. accounted for a smaller percentage 32 (<13.8 % at ocean stations, <3.6 % in the lagoon). R. intracellularis biomass was relatively small 33 (<1 % of total surface area and volume of Trichodesmium trichomes) with the highest 34 2 ! concentration observed in summer (735 trichomes l -1 ). Colonies of unidentified cyanobacteria 35 composed of spherical cells accounted on average for <1 % of the Trichodesmium biomass, with 36 values exceeding 4000 cells l -1

Response to reviewer #1

“…4a, Table 2). The TEP concentrations from this study correspond with values and trends reported from other marine environments (Engel, 2004;Bar-Zeev et al, 2009) and specifically with TEP concentrations measured from the New Caledonian lagoon .…”

“…In addition to exported Trichodesmium and Rhizosolenia-Richelia associations, the small unicellular UCYN-B (< 4 µm) was also found in the sediment traps, including the deeper (500 m) traps. UCYN-B is often associated with larger phytoplankton such as the diatom Climacodium frauenfeldianum (Bench et al, 2013) or in colonial phenotypes (> 10 µm fraction) as has been observed in the northern tropical Pacific (ALOHA) . Sedimenting UCYN-B was detected during the VAHINE mesocosm experiment in the New Caledonian lagoon in shallow (15 m) sediment traps and were also highly abundant in a floating sediment trap deployed at 75 m for 24 h in the North Pacific Subtropical Gyre (Sohm et al, 2011).…”

“…Limited availability of Fe and P induces PCD in Trichodesmium (Berman-Frank et al, 2004;Bar-Zeev et al, 2013). At LDA and LDB, Fe concentrations at the time of sampling were relatively high (> 0.5 nM), possibly due to island effects .…”

“…Sinking velocities of diatoms embedded in aggregates are generally fast (50-200 m d −1 ) (Asper, 1987;Alldredge, 1998) compared with those of individually sinking cells (1 −10 m d −1 ) (Culver and Smith, 1989), allowing aggregated particles to sink out of the photic zone to depth. Assuming a sinking rate of Trichodesmium-based aggregates of 150-200 m d −1 (Bar-Zeev et al, 2013), we would need to shift the time frame by 1 day to see whether PCD measured from the surface waters is coupled with changes in organic matter reflected in the 150 m sediment traps. Thus, at LDA, examining metacaspase activities from the surface with mass flux and particulate matter obtained 24 h later yielded a significant positive correlation between these two parameters (Fig.…”

“…Diazotrophic communities comprised of unicellular cyanobacteria lineages (UCYN-A, B and C), diatomdiazotroph associations (DDA) such as Richelia associated with Rhizosolenia and diverse heterotrophic bacteria such as alpha and γ -proteobacteria are responsible for these rates of N 2 fixation. 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 N 2 fixation (Capone et al, 1997;Westberry and Siegel, 2006) with a large fraction fixed in the WTSP (Dupouy et al, 2000(Dupouy et al, , 2011Barboza Tenório et al, 2018) that may, based on NanoSIMS cell-specific measurements, contribute up to ∼ 80 % of bulk N 2 fixation rates in the WTSP .…”