Prokaryotes Regulate Particulate Organic Carbon Export in Suspended and Sinking Particle Fractions

Abstract

BackgroundOceans are crucial regulators of the global carbon cycle. Understanding the oceanic biological carbon pump (BCP) and its contribution to carbon export has been the subject of extensive research. These studies have provided quantitative evidence regarding the centrality of phytoplankton throughout the water column. In the Southern Ocean, the biological carbon pump is driven primarily by phytoplankton productivity and is an effective organic matter sink. There is evidence showing that sinking particulate organic matter (POM) sustains microbial communities with different ecological strategies (i.e., r-/K-strategists). These results suggest that the role of microbial communities on the effectiveness of the biological carbon pump should not be underestimated. However, we lack mechanistic insights regarding the importance of these microorganisms, their diversity and influence on the efficiency of the BCP.Results Here, we provide the first insights regarding prokaryotic metabolic capacity linked to suspended and sinking particles to improve our understanding of microbial contributions towards POM export in the Southern Ocean. A Marine Snow Catcher (MSC) was deployed at several stations southwest of Tasmania in the Subantarctic zone to obtain suspended and sinking particulate material for determining carbon and nitrogen flux. Metagenomic analysis and metagenomic-assembled genomes showed that both the suspended and sinking particle-pools were dominated by bacteria with metabolic capacity for degrading POM (e.g. Gammaproteobacteria MAGs). Archaeal genomes (Poseidoniia and Nitrososphaeria) appear to drive nitrogen metabolism via nitrite and ammonia oxidation in these communities. Results suggest that for bacteria r-strategists were more ubiquitous in the suspended pool while K-strategists were more prevalent in the sinking particle-pool, with the opposite being true for archaea. In addition, metabolic reconstructions suggest that prokaryotes harbour substantial genetic capacity for degrading complex POM and chemoautotrophic synthesis of recalcitrant dissolved organic carbon (RDOC) from CO2. ConclusionThe metagenome assembled genomes from sinking and suspended size fractions and carbon flux determinations revealed striking trends regarding prokaryotic contributions in the water column. The results show that the predicted lifestyles for bacteria and archaea may differ substantially in sinking and suspended fractions, suggesting niche specificity. Together, our data suggest that prokaryotes in suspended and sinking particles may enhance POM export via the production of RDOC in the Southern Ocean.

Functions of extracellular polymeric substances in partitioning suspended and sinking particles in the upper oceans of two open ocean systems

“…Another needed improvement in our approach is to include microbial processes that are intimately involved in EPS and aggregate dynamics (Nguyen et al 2022). Suspended and sinking particles serve as microhabitats for diverse microbial communities, including bacteria, fungi, and viruses (Boeuf et al 2019; Yamada et al 2020; Dithugoe et al 2023). Bacteria not only degrade EPS, but also produce them (Passow 2002; Mari et al 2017).…”