Dissolved organic matter released by two marine heterotrophic bacterial strains and its bioavailability for natural prokaryotic communities

Abstract

Marine heterotrophic prokaryotes (HP) play a key role in organic matter processing in the ocean; however, the view of HP as dissolved organic matter (DOM) sources remains underexplored. In this study, we quantified and optically characterized the DOM produced by two single marine bacterial strains. We then tested the availability of these DOM sources to in situ Mediterranean Sea HP communities. Two bacterial strains were used: Photobacterium angustum (a copiotrophic gammaproteobacterium) and Sphingopyxis alaskensis (an oligotrophic alphaproteobacterium). When cultivated on glucose as the sole carbon source, the two strains released from 7% to 23% of initial glucose as bacterial derived DOM (B-DOM), the quality of which (as enrichment in humic or proteinlike substances) differed between strains. B-DOM induced significant growth and carbon consumption of natural HP communities, suggesting that it was partly labile. However, B-DOM consistently promoted lower prokaryotic growth efficiencies than in situ DOM. In addition, B-DOM changed HP exoenzymatic activities, enhancing aminopeptidase activity when degrading P. angustum DOM, and alkaline phosphatase activity when using S. alaskensis DOM, and promoted differences in HP diversity and composition. DOM produced by HP affects in situ prokaryotic metabolism and diversity, thus changing the pathways for DOM cycling (e.g. respiration over biomass production) in the ocean.

Microbial transformation of distinct exogenous substrates into analogous composition of recalcitrant dissolved organic matter

“…Photochemical reactions (or photo‐bleaching) solely have been demonstrated to transform DOM from refractory to a relatively labile state (Medeiros et al ., 2015, Stubbins et al ., 2010), and this abiotic reaction is restricted in the photic zone. In the deep and dark ocean interior, the molecular conformity of DOM could be majorly linked with microbial activities (Ortega‐Retuerta et al ., 2021), but the experimental evidence is limited.…”

Increased microbial and substrate complexity result in higher molecular diversity of the dissolved organic matter pool

“…An earlier study has demonstrated that Alteromonas have a limited ability to degrade DOM compared to natural microbial communities over longer terms (Pedler et al 2014). We also found a difference in DOC utilization between Alteromonas and natural microbial community, which could be due to that natural populations undergo succession which increase the niche breadths and DOM degradation (Bunse and Pinhassi 2017; Ortega‐Retuerta et al 2021). We further assessed the extent to which DOM or microbial composition is potentially responsible for this divergence.…”

Epiphytic Bacteria Are Essential for the Production and Transformation of Algae-Derived Carboxyl-Rich Alicyclic Molecule (CRAM)-like DOM

“…Previous studies have shown that axenic cultures of multiple species of algae contribute to detectable humic-like DOM in the ocean ( 21 ), while others indicated that aggregation of recalcitrant DOM results from microbial processing (mainly bacteria) of planktonic-precursors ( 22 ). Incubation experiments have also demonstrated that heterotrophic bacteria in the marine environment can convert glucose, glutamic acid, oligosaccharides, and algae extracts into highly diverse recalcitrant components ( 16 , 23 , 24 ). These exometabolites directly released by bacteria upon utilization of labile substrates ought to have intrinsic properties rendering them resistant to decomposition and therefore allowing them to remain recalcitrant for a long time ( 16 , 21 , 25 – 27 ).…”