Empirical leucine-to-carbon conversion factors in north-eastern Atlantic waters (50–2000 m) shaped by bacterial community composition and optical signature of DOM

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.

Surface productivity gradients govern changes in abundance and physiological status of deep ocean prokaryotes across the tropical and subtropical Atlantic

“…2, 3 and S5). In fact, there is increasing evidence that leucine-to-carbon conversion factors are highly variable in the ocean, decreasing with depth and increasing with water productivity (del Giorgio and Cole 1998; Orta-Ponce et al 2021; Giering and Evans 2022). Considering these trends, the negative relationship between leucine incorporation in bathypelagic samples and surface productivity would not directly translate to prokaryotic heterotrophic production, as lower conversion factors could be expected in the southern stations relative to the Guinea Dome – Cape Blanc area.…”

Surface productivity gradients govern changes in the viability of deep ocean prokaryotes across the tropical and subtropical Atlantic

“…2, 3, S6). There is increasing evidence that leucine‐to‐carbon conversion factors are highly variable in the ocean, decreasing with depth and increasing with water productivity (del Giorgio and Cole 1998; Orta‐Ponce et al 2021; Giering and Evans 2022). Considering these trends, the negative relationship between leucine incorporation in bathypelagic samples and surface productivity would not directly translate to prokaryotic heterotrophic production, as lower conversion factors could be expected in the southern stations relative to the Guinea Dome‐Cape Blanc area.…”