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

13 pages, 4 figures, 3 tables.-- This article is licensed under a Creative Commons Attribution 4.0 International License

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

Funding for sampling and analyses was supported by the projects “Fuentes de Materia Orgánica y Diversidad Funcional del Microplancton en las aguas profundas del Atlántico Norte” (MODUPLAN, Ref. CTM 2011-24008, 2012-2015, Plan Nacional I+D+I; https://moduplansarmiento2014.wordpress.com/) to M.M.V., and “Deep Standard Oceanographic Sections monitoring program RADIALES-PROFUNDOS”, funded by the Instituto Español de Oceanografía (IEO). Additional funding was provided by the Axencia Galega de Innovación (GAIN, Xunta de Galicia) through IEO-GAIN Programme Contracts (Contratos Programa). M.N.-C. was funded by the projects HOTMIX and FERMIO (MINECO, CTM2011-30010-C02-MAR and CTM2014-57334-JIN, respectively), both co-financed with FEDER funds. T.R.-R. was supported by MICINN program “Personal Técnico de Apoyo a la Investigación” (PTA, Ref. PTA2015-10948-I) and the project iFADO (Innovation in the Framework of the Atlantic Deep Ocean (EAPA_165/2016), financed by the INTERREG Atlantic Area. E.G.-F. was supported by the BIO-PROF and MODUPLAN projects. C.P.O.-P. was supported by The Nippon Foundation and the Partnership for Observation of the Global Ocean through the NF-POGO Shipboard Training Fellowship

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