Abstract
To study the response of bacteria to different size-fractions of naturally occurring dissolved organic matter (DOM), a natural prokaryotic community from North Atlantic mesopelagic waters (1000 m depth) was isolated and grown in (i) 0.1-μm filtered seawater (CONTROL), (ii) the low-molecular-weight (<1 kDa) DOM fraction (L-DOM), and (iii) the recombination of high- (>1 kDa) and low-molecular-weight DOM fractions (H + L-DOM), to test the potential effect of ultrafiltration on breaking the DOM size continuum. Prokaryotic abundance and leucine incorporation were consistently higher in the H + L-DOM niche than in the L-DOM and CONTROL treatments, suggesting a different interaction with each DOM fraction and the disruption of the structural DOM continuum by ultrafiltration, respectively. Rhodobacterales (Alphaproteobacteria) and Flavobacteriales (Bacteroidetes) were particularly enriched in L-DOM and closely related to the colored DOM (CDOM) fraction, indicating the tight link between these groups and changes in DOM aromaticity. Conversely, some other taxa that were rare or undetectable in the original bacterial community were enriched in the H + L-DOM treatment (e.g., Alteromonadales belonging to Gammaproteobacteria), highlighting the role of the rare biosphere as a seed bank of diversity against ecosystem disturbance. The relationship between the fluorescence of protein-like CDOM and community composition of populations in the H + L-DOM treatment suggested their preference for labile DOM. Conversely, the communities growing on the L-DOM niche were coupled to humic-like CDOM, which may indicate their ability to degrade more reworked DOM and/or the generation of refractory substrates (as by-products of the respiration processes). Most importantly, L- and/or H + L-DOM treatments stimulated the growth of unique bacterial amplicon sequence variants (ASVs), suggesting the potential of environmental selection (i.e., changes in DOM composition and availability), particularly in the light of climate change scenarios. Taken together, our results suggest that different size-fractions of DOM induced niche-specialization and differentiation of mesopelagic bacterial communities.
Highlights
Dissolved organic matter (DOM) is a complex mixture of compounds that constitutes a major source of carbon and energy in aquatic ecosystems (Hansell et al, 2009)
The mean PO43− initial concentration was similar in all treatments (0.99 ± 0.03, 0.97 ± 0.02 and 0.96 ± 0.03 μM in CONTROL, L- and H + L-dissolved organic matter (DOM), respectively, ANOVA p > 0.05)
In terms of bulk measurements, communities growing in the H + low-molecular-weight (
Summary
Dissolved organic matter (DOM) is a complex mixture of compounds that constitutes a major source of carbon and energy in aquatic ecosystems (Hansell et al, 2009). The sizereactivity continuum model hypothesizes how the bioreactivity of natural organic matter decreases, in general, along a continuum of size, diagenetic alteration and age, giving rise to a net flow of organic carbon from larger to smaller size classes with increasing decomposition (Benner and Amon, 2015). This reactivity is strongly dependent on environmental variables, such as inorganic nutrients or temperature (Arnosti et al, 2011). The reactivity of DOM does depend on the size and chemical characteristics of the organic compounds, but should be dependent on the composition of the consumer community
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