Abstract
Microbial heterotrophic activity is a major process regulating the flux of dissolved organic matter (DOM) in the ocean, while the characteristics of this DOM strongly influence its microbial utilization and fate in the ocean. In order to broaden the vertical resolution of leucine-to-carbon conversion factors (CFs), needed for converting substrate incorporation into biomass production by heterotrophic bacteria, 20 dilution experiments were performed in the North Atlantic Ocean. We found a depth-stratification in empirical CFs values from epipelagic to bathypelagic waters (4.00 ± 1.09 to 0.10 ± 0.00 kg C mol Leu−1). Our results demonstrated that the customarily used theoretical CF of 1.55 kg C mol Leu−1 in oceanic samples can lead to an underestimation of prokaryotic heterotrophic production in epi- and mesopelagic waters, while it can overestimate it in the bathypelagic ocean. Pearson correlations showed that CFs were related not only to hydrographic variables such as temperature, but also to specific phylogenetic groups and DOM quality and quantity indices. Furthermore, a multiple linear regression model predicting CFs from relatively simple hydrographic and optical spectroscopic measurements was attempted. Taken together, our results suggest that differences in CFs throughout the water column are significantly connected to DOM, and also reflect differences linked to specific prokaryotic groups.
Highlights
Heterotrophic bacteria are key in the cycling of dissolved organic matter (DOM) because they are major consumers and transformers of the DOM pool in the o cean[1], which in turn supports the metabolic activities and growth of bacterioplankton[2]
A higher spatial resolution of empirical CFs (eCFs) values is required for an accurate estimation of prokaryotic heterotrophic production (PHP) throughout the water column
None of them has investigated the relationship among conversion factors (CFs), bacterial diversity and composition of the DOM pool
Summary
Heterotrophic bacteria are key in the cycling of dissolved organic matter (DOM) because they are major consumers and transformers of the DOM pool in the o cean[1], which in turn supports the metabolic activities and growth of bacterioplankton[2]. The variability of protein and carbon content in bacterial cells growing in different environments (e.g., coastal vs oceanic systems, and/or surface vs deep waters) is not accounted for, potentially leading to misinterpretation of the resultant PHP estimates. There is lack of studies concurrently measuring eCFs, DOM and the taxonomic composition of the microbial community allowing to examine the link among these variables. FDOM measurements at specific excitation/ emission wavelength pairs provide information on humic-like marine substances (peak M, refractory DOM resistant to microbial degradation) and protein-like molecules (peak T, freshly-produced labile DOM)[15,18]. Vertical variability of the eCFs was concomitantly studied with bacterial diversity and the optical signature of DOM at two stations near Cape Finisterre (North Atlantic Ocean off Galicia; NW Iberian Peninsula) and Santander (Bay of Biscay). In the Santander section, in the eastern limit of the upwelling region, the upwelling events are usually shorter and reach lower intensities than off the Galician c oast[23]
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