Diatom Dominance Enhances Resistance of Phytoplanktonic POM to Mesopelagic Microbial Decomposition

Miguel Cabrera-Brufau,Laura Arin,Cèlia Marrasé,Pedro Cermeño,Maria Montserrat Sala

doi:10.3389/fmars.2021.683354

Abstract

Particulate organic matter (POM) lability is one of the key factors determining the residence time of organic carbon (OC) in the marine system. Phytoplankton community composition can influence the rate at which heterotrophic microorganisms decompose phytoplankton detrital particles and thus, it controls the fraction of OC that reaches the ocean depths, where it can be sequestered for climate-relevant spans of time. Here, we compared the degradation dynamics of POM from phytoplankton assemblages of contrasting diatom dominance in the presence of mesopelagic prokaryotic communities during a 19-day degradation experiment. We found that diatom-derived POM exhibited an exponential decay rate approximately three times lower than that derived from a community dominated by flagellated phytoplankton (mainly coccolithophores and nanoflagellates). Additionally, dissolved organic matter (DOM) released during the degradation of diatom particles accumulated over the experiment, whereas only residual increases in DOM were detected during the degradation of non-diatom materials. These results suggest that diatom-dominance enhances the efficiencies of the biological carbon pump and microbial carbon pump through the relatively reduced labilities of diatom particles and of the dissolved materials that arise from their microbial processing.

Highlights

Marine phytoplankton are responsible for approximately half of global net primary production (NPP), i.e., 53.6 ± 7.4 Pg C per year (Dunne et al, 2007) compared to a mean of 55 Pg C per year of terrestrial ecosystems (Cramer et al, 1999)
The NPSi incubation produced a diatom-dominated community in which this group accounted for 98% of the biomass, including 47% in the form of Chaetoceros resting spores
We have shown that the dynamics of particulate organic matter (POM) bacterial remineralization is strongly dependent on its phytoplanktonic origin

Summary

Introduction

Marine phytoplankton are responsible for approximately half of global net primary production (NPP), i.e., 53.6 ± 7.4 Pg C per year (Dunne et al, 2007) compared to a mean of 55 Pg C per year of terrestrial ecosystems (Cramer et al, 1999). Prokaryotic decomposition activities are considered the main sink of marine POM in the mesopelagic ocean (200–1000 m depth), being responsible for up to 85–92% of particle remineralization by various estimates (Anderson and Tang, 2010; Giering et al, 2014). This implies the solubilization of sinking particles into DOM prior to prokaryotic uptake through zooplankton processing (Strom et al, 1997) and bacterial cleavage by extracellular enzymes (Smith et al, 1992). The attenuation of POM flux with depth derives from the combined effects of particle remineralization into CO2 and solubilization into DOM (Benner and Amon, 2015)

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Discussion

Conclusion