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

Abstract

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.