Insights on the microbial carbon pump in the global ocean with spectroscopic technique

Abstract

The transformation of biologically labile organic matter into refractory compounds by prokaryotic activity has been termed the ‘microbial carbon pump’ (MCP) and may constitute an effective mechanism to store reduced carbon in the dark ocean. Understanding its generation and its role in carbon sequestration is crucial to assess its relevance in the context of the global carbon cycle. The main aim of this PhD thesis is to test the significance of the chromophoric (CDOM) and fluorescent (FDOM) fractions of dissolved organic matter (DOM) as tracers for the microbial production of recalcitrant DOM in the global ocean. All its content is framed in the Malaspina 2010 circumnavigation, which allowed us to produce the first global inventory of the optical properties of DOM in both the surface ocean (<200 m), gathered by Longhurst’s biogeographic provinces, and the dark ocean (>200 m) by the main water masses. In the dark ocean, ideal age and ageing (apparent oxygen utilization) of the main water masses were tracked along the global thermohaline circulation, allowing the estimation of net production/consumption rates of CDOM and FDOM and their respective turnover times. We found that CDOM was generated in situ by microbial metabolism (at a global rate of 3.3 ± 0.5 x 10–5 m–1 yr–1), with a turnover time of ca. 625 years and was accumulated in the dark ocean due to its recalcitrant nature, with an increase in the degree of aromaticity and molecular weight along the thermohaline circulation. We identified two distinct chromophores. One was centred at 302 nm (UV chromophore) and the other one at 415 nm (Visible chromophore). The UV chromophore was attributed partially to nitrate and likely to the antioxidant gadusol and presented a turnover time of ca. 345 years. The Visible chromophore was related to the respiratory enzyme cytochrome c and presented a turnover time of ca. 356 years. The analysis of the fluorescent properties of DOM in both the surface (< 200 m) and dark ocean (> 200 m) allowed us to identify four ubiquitous fluorophores. Two fluorophores were humic-like (C1, C2) components and the other two were amino acid-like (tryptophan-like C3, tyrosine-like C4) components. The robustness in the level of explanation for humic-like and amino acid-like components by biogeochemical variables was much higher for the humic-like components than for the amino acidlike components both in the surface and the dark ocean (~80% vs ~30%). The fluorescent humic-like material was explained by water ageing and showed positive power functions both in the surface and in the dark ocean. In the dark ocean, C1 showed a higher production rate than C2, with a net production rate of 2.3 ± 0.2 x 10–5 and 1.2 ± 0.1 x 10–5 RU yr–1 and turnover times of 530 and 740 years, respectively. However, in the surface ocean both rates were similar. In the dark ocean C1 and C2 showed higher conversion efficiencies per unit of utilized oxygen than in the surface ocean, likely due to photobleaching. In the dark ocean, tyrosine-like C4 presented an inverse power relationship with the apparent oxygen utilization, decreasing at a rate of –1.1 ± 10–5 RU yr–1. On the contrary, the tryptophan-like component C3 did not show a pattern with ageing. In the surface ocean, the amino acid-like components were apparently more affected by physical processes, although the positive relationship of C4 with Chl a also implies a microbial influence on this component. The in situ production of the DOM fractions as by-products of microbial metabolism identified as water masses turned older and the long turnover times of the humic-like components indicated the relevant role of the MCP in the carbon sequestration in the dark ocean. Thus, the initial hypothesis of this PhD thesis that was: “are the chromophoric and fluorescent fractions of DOM key components of the recalcitrant DOM pool?” has been verified. Similarly, the fact that chromophores and fluorophores can be used as tracers of the water mass mixing and biogeochemical processes operating at centennial time scales will bring new insights into the ocean carbon storage.