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 N2 fixation rates and plankton abundances demonstrate the central role of N2 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).
Highlights
Subtropical oligotrophic gyres, accounting for 45 % of global ocean area, represent one of the earth’s largest biomes (Sarmiento et al, 2004)
Strong relationships between N2 fixation rates and plankton abundances demonstrate the central role of N2 fixation in regulating ecosystem processes in the western tropical South Pacific (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)
Plotting bacterial abundances against the bacterial production data reported by Van Wambeke et al (2018), and interpreting regression slopes using the criteria described by Ducklow et al (1992) in Zu, we found no evidence of bottom–up control of bacteria populations at MA, LDB or GY
Summary
Subtropical oligotrophic gyres, accounting for 45 % of global ocean area, represent one of the earth’s largest biomes (Sarmiento et al, 2004). The groups of phytoplankton that numerically dominate the open ocean – Prochlorococcus, Synechococcus, and picophytoeukaryotes (PPE) – are of a small size, gener-. Over the last 10 years, molecular methodologies have allowed for the characterization of these groups at the taxon or species level, revealing enormous diversity across all trophic levels of marine microbial communities and identifying numerous ecotypes occupying distinct ecological niches (Carlson et al, 2007; Venter et al, 2004). Environmental sequencing has revealed a surprising diversity of small-sized PPE and new eukaryotic lineages continue to be discovered and characterized (Kashtan et al, 2014; Kim et al, 2016; Lepère et al, 2009; Rii et al, 2016a), while revealing the importance of viruses in regulating phytoplankton communities (Brum et al, 2015; Huang et al, 2015)
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