Bacterial activity and diffusive nutrient supply in the oligotrophic Central Atlantic Ocean

Abstract

Growing evidence of inorganic nutrient limitation on oceanic bacteria suggests a global dependence of bacterial activity and production on rates of nutrient supply. The present study examined whether surface bacterial abundance is significantly related to water column stability, and whether bacterial activity and growth rate are related to the rate of diffusive supply of inorganic nutrients to the mixed layer in the Central Atlantic during 2 meridional cruises. The 2 cruises were run under very different oceanic conditions, with relatively low values of bacterial activity in spring 1995 and relatively higher values in fall 1995. We obtained depth-resolved data in the second cruise and found that the integrated value of bacterial production was also related to the rate of nutrient supply, while integrated particulate primary production and chlorophyll concentration were not. There was also no relationship between particulate primary production and bacterial production. The relationship between nutrient supply and integrated bacterial production was tested with data from a mesocosm experiment showing a good fit to the pattern obtained in the Atlantic. Average bacterial production was ~21% of primary production in the Central Atlantic, with values ranging between 5 and 100%, and higher values in the tropical areas. The demonstration of a direct relationship between nutrient supply and bacterial activity helps to explain a relatively large bacterial biomass as compared to phytoplankton biomass, a low bacterial growth efficiency, and a high bacterial carbon demand relative to contemporaneous primary production often measured in the open ocean, as well as the accumulation of dissolved organic carbon (DOC) observed in nutrient-limited oligotrophic seas.

Quantification of Carbon and Phosphorus Co-Limitation in Bacterioplankton: New Insights on an Old Topic

“…Notably, the response to the P treatment in most cases had a greater magnitude (effect size) than in C treatment. These interpretations of resource limitation are also supported by the values shown by the BCD:EOC ratio, a proxy for bacterial limitation of autochthonous C. This proxy measures the ability of algal C (EOC) to meet bacterial demands for C, based on the reported bacterial preference for this C source [25], [26], [32]. Thus, the BCD:EOC ratio indicates that bacteria were limited by autochthonous C in the eutrophic ecosystem (BCD:EOC>1), but not in the oligotrophic lake (BCD:EOC<1) at starting conditions and over the late period.…”

“…On the other hand, some studies have shown that bacteria are C limited, preferring autochthonous C provided by algae [24]–[26], even though they could use other C resources such allochthonous DOC [24], [27], old DOC (i.e. not recently produced [25], [28]) or semi-labile DOC. Several studies show, however, a prevalence of mineral-nutrient and organic carbon co-limitation in aquatic microbial communities, deduced from stronger responses to combined resources than to single-nutrient additions [13], [26].…”

Forcing of dissolved organic carbon release by phytoplankton by anticyclonic mesoscale eddies in the subtropical NE Atlantic Ocean

“…It has been suggested that the counter-paired cyclonic and anticyclonic eddies generated by the Canary Current and trade wind perturbation by the Canary Islands influence biological processes in the NE Atlantic region (Arístegui et al, 1997;Arístegui and Montero, 2005;González-Dávila et al, 2006;Sangrà et al, 2009). Our study aimed to comprehend the influence of anticyclonic/cyclonic eddies on biological processes including the cell physiological responses to the environmental changes associated with the anticyclonic/cyclonic eddies that may influence the path of the carbon photosynthesized by phytoplankton in the system.…”

“…Limitation by inorganic nutrients such as phosphorus could influence the viability of HB (Lasternas et al, 2010). Indeed, nutrient supply has been shown to explain variability in bacterial activity in the Atlantic Ocean (Gasol et al, 2009), and the negative relationship between the % DC of Synechococcus sp. and the % LC of HB found here may indicate competition for phosphorus in the anticyclonic eddy systems (Zubkov et al, 2007;Lasternas et al, 2010).…”

“…Additional studies have been initiated at this zone to better understand the implication of counter-paired cyclonic and anticyclonic eddies for biological processes and to address their influence in carbon fluxes. Cyclonic eddies promoted nutrient pumping and vertical uplifting of the deep chlorophyll maximum, increasing plankton production, while anticyclonic eddies (AEs) tended to accelerate downstream transport below the photic zone (Arístegui et al, 1997;Arístegui and Montero, 2005;González-Dávila et al, 2006). Since anticyclonic structures can be long-lived coherent structures lasting several months (Sangrà et al, 2005, the magnitude and the efficiency of biological processes such as production and health status of communities can be affected for extended periods and may consequently have significant implications in carbon fluxes.…”