The relevance of environment vs. composition on dissolved organic matter degradation in freshwaters

Núria Catalán,Carles M Borrego,Rafael Marcé,Jeffrey A Hawkes,Ada Pastor,Daniel Von Schiller,Joan Pere Casas‐Ruiz,Carmen Gutiérrez

doi:10.1002/lno.11606

Abstract

AbstractDissolved organic matter (DOM) composition exerts a direct control on its degradation and subsequent persistence in aquatic ecosystems. Yet, under certain conditions, the degradation patterns of DOM cannot be solely explained by its composition, highlighting the relevance of environmental conditions for DOM degradation. Here, we experimentally assessed the relative influence of composition vs. environment on DOM degradation by performing degradation bioassays using three contrasting DOM sources inoculated with a standardized bacterial inoculum under five distinct environments. The DOM degradation kinetics modeled using reactivity continuum models showed that composition was more important than environment in determining the bulk DOM decay patterns. Changes in DOM composition resulted from the interaction between DOM source and environment. The role of environment was stronger on shaping the bacterial community composition, but the intrinsic nature of the DOM source exerted stronger control on the DOM degradation function.

Highlights

We borrow a simile from cancer research (Mukherjee 2017) by asking what is more important in controlling dissolved organic matter (DOM) decay rates: the soil or the seed? To test this, we conducted a controlled degradation experiment on three contrasting DOM sources under five environments that were selected to cover a wide spectrum of environmental conditions frequently found in freshwaters
We observed a marked decay of the DOMLeaf across Environment levels, while the kinetics were more similar for the DOMAlgae and DOMHumic
Our study indicates that DOM source is more important than environment in determining bulk dissolved organic carbon (DOC) decay, both have a significant effect for the conditions

Summary

Introduction

The growing availability and understanding of high-resolution mass-spectrometry (Koch et al 2008; Patriarca et al 2018) has allowed the identification and alignment of the formulae of thousands of compounds across DOM samples (Kujawinski et al 2002) The degradation of these compounds is expected to be driven by specific relationships between the individual compounds and the metabolic potential of the resident microbiota (Moran et al 2016). Most analytical techniques for DOM characterization have restrictive “analytical windows” (e.g., fluorescence requires a fluorophore, electrospray ionization requires polarity, solid phase extraction requires hydrophobicity), so results are highly biased toward certain groups of compounds These limitations need to be discussed when interpreting DOM compositional changes during incubations. We expected a consistent effect of a given environment on DOM composition and postulate that it is the environment and not the DOM source that drives the variability in DOM degradation kinetics (Fig. 1b), linked to a stronger effect of the environment than of the DOM source on the microbial community

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