Differentiating the role of different-sized microorganisms in peptide decomposition during incubations using size-fractioned coastal seawater

•Tetrapeptide AVFA was incubated in size-fractioned coastal seawater.•Gammaproteobacteria, mainly Alteromonas, played the dominant role in taking up small peptides.•Decomposition rates of AVFA were significantly higher in large size fractions (> 5 μm).•Large-size microorganisms like protists may have contributed to the decomposition of AVFA.

Peptide decomposition by different-sized microorganisms was compared by incubating tetrapeptide alanine–valine–phenylalanine–alanine (AVFA), a fragment of RuBisCO, in coastal seawater after size-fraction by filtration. The size-fractioned seawater included < 0.8-μm filtered (free-living bacteria), < 5-μm filtered (free-living bacteria + heterotrophic nanoflagellates), < 20-μm filtered (free-living and particle-attached bacteria + heterotrophic nanoflagellates + other small protists), and unfiltered whole water collected from Texas coast in the western Gulf of Mexico. Decomposition rates of AVFA in the < 20-μm and unfiltered seawater were significantly higher than those in the < 0.8-μm and < 5-μm seawater in the December 2011 incubation. The higher decomposition rate in the large size fractions can be attributed to activities of particle-attached bacteria and/or large-size microorganisms, such as osmotrophic protists. However, the role of particle-attached bacteria in explaining this decomposition difference might be limited, as bacterial abundance and community structure did not differ much among the 4 treatments. Consistently, the June 2013 incubation indicated that AVFA decomposed most rapidly in the unfiltered seawater with > 20-μm microorganisms. This study provides insights into the relative role of different-sized microorganisms in regulating the recycling of labile organic matter in coastal waters.