Microbial community structure in the western tropical South Pacific

Abstract

Abstract. Oligotrophic regions play a central role in global biogeochemical cycles, with microbial communities in these areas representing an important term in global carbon budgets. While the general structure of microbial communities has been well documented in the global ocean, some remote regions such as the western tropical South Pacific (WTSP) remain fundamentally unexplored. Moreover, the biotic and abiotic factors constraining microbial abundances and distribution remain not well resolved. In this study, we quantified the spatial (vertical and horizontal) distribution of major microbial plankton groups along a transect through the WTSP during the austral summer of 2015, capturing important autotrophic and heterotrophic assemblages including cytometrically determined abundances of non-pigmented protists (also called flagellates). Using environmental parameters (e.g., nutrients and light availability) as well as statistical analyses, we estimated the role of bottom-up and top-down controls in constraining the structure of the WTSP microbial communities in biogeochemically distinct regions. At the most general level, we found a "typical tropical structure", characterized by a shallow mixed layer, a clear deep chlorophyll maximum at all sampling sites, and a deep nitracline. Prochlorococcus was especially abundant along the transect, accounting for 68 ± 10.6 % of depth-integrated phytoplankton biomass. Despite their relatively low abundances, picophytoeukaryotes (PPE) accounted for up to 26 ± 11.6 % of depth-integrated phytoplankton biomass, while Synechococcus accounted for only 6 ± 6.9 %. Our results show that the microbial community structure of the WTSP is typical of highly stratified regions, and underline the significant contribution to total biomass by PPE populations. Strong relationships between N 2 fixation rates and plankton abundances demonstrate the central role of N 2 fixation in regulating ecosystem processes in the WTSP, while comparative analyses of abundance data suggest microbial community structure to be increasingly regulated by bottom-up processes under nutrient limitation, possibly in response to shifts in abundances of high nucleic acid bacteria (HNA).

Seasonal variability and vertical distribution of autotrophic and heterotrophic picoplankton in the Central Red Sea

“…Surface waters showed persistently high stratification, limiting the availability of DIN and DIP, which resulted in low phytoplankton biomass for most of the year (Figs. 1F and 2) and a clear dominance of small cells consistent with previous work (Bock et al, 2018;Van den Engh et al, 2017;Wei et al, 2019). Accordingly, DOC concentrations did not exceed 95 µmol L −1 .…”

“…A recent study in the Red Sea assessing 16S rRNA gene sequences, more sensitive than flow cytometry, was able to find Prochlorococcus below 200 m, though with low numbers (Shibl et al, 2016). In other studies conducted in tropical waters, Prochlorococcus and Synechococcus were, however, detected by flow cytometry at depths of 150 to 200 m (Bock et al, 2018;Partensky, Hess & Vaulot, 1999;Van den Engh et al, 2017). The flow cytometer used in this study has a high sensitivity to detect small cells (<1 µm) and viruses (Monier et al, 2017).…”

Peer Review #3 of "Seasonal variability and vertical distribution of autotrophic and heterotrophic picoplankton in the Central Red Sea (v0.1)"

“…Surface waters showed persistently high stratification, limiting the availability of DIN and DIP, which resulted in low phytoplankton biomass for most of the year (Fig. 1F and 2) and a clear dominance of small cells consistent with previous work (Bock et al 2018;van den Engh et al 2017;Wei et al 2019). Accordingly, DOC concentrations did not exceed 95 µmol L -1 .…”

“…A recent study in the Red Sea assessing 16S rRNA gene sequences, more sensitive than flow cytometry, was able to find Prochlorococcus below 200 m, though with low numbers (Shibl et al 2016). In other studies conducted in tropical waters, Prochlorococcus and Synechococcus were, however, detected by flow cytometry at depths of 150 to 200 m (Bock et al 2018;Partensky et al 1999;van den Engh et al 2017). The flow cytometer used in this study has a high sensitivity to detect small cells (<1 µm) and viruses (Monier et al 2017).…”

Diazotrophic &lt;i&gt;Trichodesmium&lt;/i&gt; impact on UV–Vis radiance and pigment composition in the western tropical South Pacific

“…Filters were preserved (at −80 • C) until analysis in the laboratory (IRD French Polynesia). For the determination of picoplanktonic population abundances (Bock et al, 2018), water samples were fixed with paraformaldehyde (final concentration of 0.2 %) immediately after sampling, flash frozen in liquid nitrogen, and stored in liquid N 2 in Nunc ® cryogenic vials until analysis, and abundances at 5 m were selected for our study. For the determination of CDOM absorption, 200 mL of seawater was immediately filtered on Micropore filters of 0.2 µm pore size using Nalgene filtration units previously rinsed twice with HCL and stored in SCHOTT ® glass bottles, previously combusted (450 • C, 6 h) and rinsed twice with HCL.…”