Microbial community structure and variability in the tropical Pacific

Abstract The spatially extensive tropical Pacific includes regions that are limited by macronutrients or iron, and is thus broadly representative of open-ocean systems in which microbial communities predominate. Despite strong physical forcing due to the El Nino-Southern Oscillation cycle and the local effects of tropical instability waves, microbial abundances from a variety of JGOFS and related studies show similar, modest levels of variability in the high-nutrient, low-chlorophyll (HNLC) equatorial upwelling region, the oligotrophic, western Pacific Warm Pool, and the North Pacific central gyre. Mean 0–50 m abundances of some of the groups distinguished by flow cytometry are significantly enhanced in the HNLC region, including heterotrophic bacteria (HBACT; 720,000 versus 440,000 cells ml −1 ), Synechococcus spp. (SYN; 9800 versus 2000 cells ml −1 ) and pico-eukaryotic algae (PEUK; 6300 versus 800 cells ml −1 ). However, Prochlorococcus spp. (PRO) are slightly more abundant in the low-nitrate regions (180,000 versus 150,000 cells ml −1 ). The higher HNLC concentrations of SYN and PEUK are part of a broader expansion of the phytoplankton community over the relatively constant PRO base when the limiting nutrient (iron) pool is increased. Elevated biomass and production of phytoplankton and the greater availability of DOC presumably explain the higher HNLC abundances of HBACT. The mean biomass (±standard deviation) of bacterial populations for cross-equatorial transects (14.1±2.8 μg C l −1 ) is similar to that in the subtropics (11.6±2.7 μg C l −1 ), with cruise variations falling generally within a 2-fold range. Heterotrophs comprise a significantly higher mean percentage of total prokaryote biomass (59±9%) in the HNLC region than in the low-nutrient subtropics (42±6%). The biomass production of photosynthetic bacteria (PRO and SYN) in the central equatorial Pacific is conservatively twice that of HBACT, but total carbon flux through bacteria (44–75% of phytoplankton 14 C-production) is dominated by the high respiration, hence carbon demand, of heterotrophs. Given, the very different growth limiting factors (Fe, N, P, and organic carbon) among the various subregions and microbial groups in the tropical Pacific, it seems unlikely that direct controls on growth rate are sufficiently precise to account for the relatively low microbial variability observed. Among factors affecting loss rates, the regulatory role of viral lysis remains largely unexplored, as in most open-ocean systems. However, there is relatively good evidence, including the grazing response to the IronEx II perturbation and multi-level cascade influences, that protistan grazer are generally able to suppress large excursions in microbial abundance and biomass. The key elements of such a control mechanism are size or surface-chemistry characteristics that link the dynamics of different microbial populations to common (nanoflagellate) predators and the fact that such predators are held well below their maximum growth rate potential at ambient food concentrations. This latter point, in particular, ensures a rapid and approximately linear increase in protistan growth and grazing pressure up to prey concentrations many times ambient levels.