Experimental evidence of the potential bioavailability for marine heterotrophic bacteria of aerosols organic matter

Abstract. The surface ocean receives important amounts of organic carbon from atmospheric deposition. The degree of bioavailability of this source of organic carbon will determine its impact on the marine carbon cycle. In this study, the potential availability of dissolved organic carbon (DOC) leached from both desert dust and anthropogenic aerosols to marine heterotrophic bacteria was investigated. The experimental design was based on 16-days incubation, in the dark, of a marine bacterial inoculum into artificial seawater amended with water-soluble Saharan dust (D-treatment) and anthropogenic (A-treatment) aerosols, so that the initial DOC concentration leachate from aerosols is 36 µM C. Glucose-amended (G) and non-amended (control) treatments were run in parallel. Over the incubation period, an increase in bacterial abundance (BA) and bacterial production (BP) was observed first in the G-treatment, followed then by D and finally A treatments, with bacterial growth rates significantly higher in the G and D treatments than the A treatment. Following this growth, maxima of BP reached were similar in D (879 ± 64 ng C L−1 h−1; n = 3) and G (648 ± 156 ng C L−1 h−1; n = 3) treatments and were significantly higher than in A-treatment (124 ± 39 ng C L−1 h−1; n = 2). The DOC consumed over the incubation period was similar in A (9 ± 4 µM; n = 2) and D (9 ± 2 µM; n = 3) treatments and was significantly lower than that consumed in the G-treatment (22 ± 3 µM). Nevertheless, the bacterial growth efficiency (BGE) in the D treatment (14.2 ± 5.5 %; n = 3) compared well with the G treatment (7.6 ± 2 %; n = 3), suggesting that the metabolic use of the labile DOC fraction in both conditions was energetically equivalent. In contrast, the BGE in the A-treatment was lower (1.7 ± 0.1 %; n = 2), suggesting that the most part of used labile DOC was catabolized. The results obtained in this study highlight the potential of aerosol organic matter to sustain the metabolism of marine heterotrophs and stress the need to include this external source of organic carbon into biogeochemical models, for a better constraining of the carbon budget.