Molecular characteristics of microbially mediated transformations of Synechococcus‐derived dissolved organic matter as revealed by incubation experiments

Abstract

Summary In this study, we investigated the microbially mediated transformation of labile Synechococcus‐derived DOM to RDOM using a 60‐day experimental incubation system. Three phases of TOC degradation activity (I, II and III) were observed following the addition of Synechococcus‐derived DOM. The phases were characterized by organic carbon consumption rates of 8.77, 1.26 and 0.16 μmol L−1 day−1, respectively. Excitation emission matrix analysis revealed the presence of three FDOM components including tyrosine‐like, fulvic acid‐like, and humic‐like molecules. The three components also exhibited differing biological availabilities that could be considered as labile DOM (LDOM), semi‐labile DOM (SLDOM) and RDOM, respectively. DOM molecular composition was also evaluated using FT‐ICR MS. Based on differing biological turnover rates and normalized intensity values, a total of 1704 formulas were identified as candidate LDOM, SLDOM and RDOM molecules. Microbial transformation of LDOM to RDOM tended to proceed from high to low molecular weight, as well as from molecules with high to low double bond equivalent (DBE) values. Relatively higher aromaticity was observed in the formulas of RDOM molecules relative to those of LDOM molecules. FDOM components provide valuable proxy information to investigate variation in the bioavailability of DOM. These results suggest that coordinating fluorescence spectroscopy and FT‐ICR MS of DOM, as conducted here, is an effective strategy to identify and characterize LDOM, SLDOM and RDOM molecules in incubation experiments emulating natural systems. The results described here provide greater insight into the metabolism of phytoplankton photosynthate by heterotrophic bacteria in marine environments.

Microbial Processing of Sediment‐Derived Dissolved Organic Matter: Implications for Its Subsequent Biogeochemical Cycling in Overlying Seawater

“…Three fluorescent components were analyzed (supporting information Figure S4). Comparison to previous studies indicated that C1 exhibited fluorescence peak typical of protein‐like (tyrosine‐like) component, and the C2 and C3 are often referred to as humic‐like components (Catalá et al, ; Guo et al, ; Jørgensen et al, ; Zheng et al, ). Trends of these components were monitored during the 110‐day incubation and shown in supporting information Figure S4.…”

Highly enriched N‐containing organic molecules of Synechococcus lysates and their rapid transformation by heterotrophic bacteria

“…XM-24 was isolated from coastal seawaters nearby Xiamen Island (~24 N,~118 E, China) using SN medium (Waterbury 1986;Zheng et al 2018), and its associated heterotrophic bacteria were self-selected natural assemblages. The XM-24 coculture comprised a pure Synechococcus strain and a heterotrophic bacterial community that primarily consisted of several dominant bacterial species (Zheng et al, 2018(Zheng et al, , 2019. The coculture was incubated at 25 C with 10 μmol photonsÁm −2 Ás −1 irradiation in SN liquid medium for the experiments described here.…”

“…DOM that is released by phytoplankton and further transformed by heterotrophic bacteria affects the molecular composition of environmental DOM pools (Azam et al 1983; Zhao et al 2019; Zheng et al 2019). Further, the chemical composition of DOM produced by different marine phytoplankton taxa (e.g., Prochlorococcus , Synechococcus , Thalassiosira , and Phaeodactylum ) partially reflects their taxonomic origins, wherein closely related phytoplankton species share similar profiles of released DOM (Becker et al 2011).…”

“…Further, distinct molecular signatures of Synechococcus ‐derived DOM are produced via exudation, mechanical cell lysis and viral lysis, with cyanophage‐mediated Synechococcus lysis contributing higher molecular weight N‐containing DOM compounds to DOM pools (Ma et al 2018). Coastal bacterioplankton can quickly utilize and transform lysis products of picocyanobacteria, suggesting a potential mechanism for the correspondence between bacterial populations and Synechococcus ‐derived DOM molecules (Zhao et al 2019; Zheng et al 2019).…”

Stable Isotope Probing Identifies Bacterioplankton Lineages Capable of Utilizing Dissolved Organic Matter Across a Range of Bioavailability

“…This discrepancy between the analyses of unfractionated community structure and DNA-SIP identified DOM incorporators highlights the advantage of using the sensitive SIP technique to identify response of oligotrophs to varying DOM sources and quality. Syn lysate is a mixture of abundant labile and less abundant recalcitrant DOM (Zheng et al, 2019a), therefore 13 C incorporation by both copiotrophs and oligotrophs was possible. For example, members of the Acidobacteria clade, a responding lineage to the Syn lysate treatment has been shown to take up cellulose, a byproduct of cyanobacteria production (Nobles et al, 2001;Eichorst and Kuske, 2012;Zhao et al, 2015).…”