Abstract
Proteins and peptides are key components of the labile dissolved organic matter pool in marine environments. Knowing which types of bacteria metabolize peptides can inform the factors that govern peptide decomposition and further carbon and nitrogen remineralization in marine environments. A 13C-labeled tetrapeptide, alanine-valine-phenylalanine-alanine (AVFA), was added to both surface (normoxic) and bottom (hypoxic) seawater from a coastal station in the northern Gulf of Mexico for a 2-day incubation experiment, and bacteria that incorporated the peptide were identified using DNA stable isotope probing (SIP). The decomposition rate of AVFA in the bottom hypoxic seawater (0.018–0.035 μM h-1) was twice as fast as that in the surface normoxic seawater (0.011–0.017 μM h-1). SIP experiments indicated that incorporation of 13C was highest among the Flavobacteria, Sphingobacteria, Alphaproteobacteria, Acidimicrobiia, Verrucomicrobiae, Cyanobacteria, and Actinobacteria in surface waters. In contrast, highest 13C-enrichment was mainly observed in several Alphaproteobacteria (Thalassococcus, Rhodobacteraceae, Ruegeria) and Gammaproteobacteria genera (Colwellia, Balneatrix, Thalassomonas) in the bottom water. These data suggest that a more diverse group of both oligotrophic and copiotrophic bacteria may be involved in metabolizing labile organic matter such as peptides in normoxic coastal waters, and several copiotrophic genera belonging to Alphaproteobacteria and Gammaproteobacteria and known to be widely distributed may contribute to faster peptide decomposition in the hypoxic waters.
Highlights
Proteins and peptides are key components of labile dissolved organic matter (DOM) that supports bacterial growth (Azam, 1998)
These results suggest that some bacterial groups may be more effective at metabolizing peptide-derived organic matter in hypoxic seawater, but direct evidence linking specific bacterial lineages to peptide decomposition has not been reported
AV, FA, VF, and VFA produced during hydrolysis of AVFA (Liu et al, 2013) remained at low levels
Summary
Proteins and peptides are key components of labile dissolved organic matter (DOM) that supports bacterial growth (Azam, 1998). Our previous studies have demonstrated that small peptides decompose more quickly in bottom hypoxic than surface normoxic (normal oxygen-saturated) waters in the northern Gulf of Mexico (nGOM), and that the growth of certain bacterial genera such as Vibrio, Marinobacterium, Neptuniibacter, Pseudoalteromonas, Thalassomonas, Amphritea, Roseobacter and Ruegeria, appears to respond to peptide addition (Liu et al, 2013; Liu and Liu, 2016) These results suggest that some bacterial groups may be more effective at metabolizing peptide-derived organic matter in hypoxic seawater, but direct evidence linking specific bacterial lineages to peptide decomposition has not been reported. As succession of microbial communities often occurs along with development of oxygen minimum zones (Crump et al, 2007; Zaikova et al, 2010; Parsons et al, 2015), studying the response of microbial communities to labile organic matter at different DO levels can provide clues about linkages between microbial niche specialization and their resource utilization (Nelson and Wear, 2014)
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