Chytrid fungi shape bacterial communities on model particulate organic matter.

Cordelia Roberts,Kimberley E Bird,Ro Allen,Michael Cunliffe

doi:10.1098/rsbl.2020.0368

Abstract

Microbial colonization and degradation of particulate organic matter (POM) are important processes that influence the structure and function of aquatic ecosystems. Although POM is readily used by aquatic fungi and bacteria, there is a limited understanding of POM-associated interactions between these taxa, particularly for early-diverging fungal lineages. Using a model ecological system with the chitin-degrading freshwater chytrid fungus Rhizoclosmatium globosum and chitin microbeads, we assessed the impacts of chytrid fungi on POM-associated bacteria. We show that the presence of chytrids on POM alters concomitant bacterial community diversity and structure, including differing responses between chytrid life stages. We propose that chytrids can act as ecosystem facilitators through saprotrophic feeding by producing ‘public goods’ from POM degradation that modify bacterial POM communities. This study suggests that chytrid fungi have complex ecological roles in aquatic POM degradation not previously considered, including the regulation of bacterial colonization, community succession and subsequent biogeochemical potential.

Highlights

Particulate organic matter (POM) in aquatic ecosystems acts as ‘hotspots’ for bacteria [1,2,3] and fungi [4,5]
After 2 h, the bacterial communities attached to the chitin microbeads in the established treatment were distinct from the other treatments, dominated by Burkholderiales, Chromatiales and to a lesser extent Neisseriales and Pseudomonadales
Understanding microbial–POM interactions has been largely dominated by bacteria-focused studies

Summary

Introduction

Particulate organic matter (POM) in aquatic ecosystems acts as ‘hotspots’ for bacteria [1,2,3] and fungi [4,5]. Bacteria–POM studies have characterized the microscale interactions between bacteria and particles, including the composition of colonizing communities [8,9,10], interactions between attached bacteria [11] and the dynamics of POM degradation [9,10,11,12]. Laboratory-based incubations with chitin microbeads as model POM have identified bacteria that colonize and degrade POM using extracellular enzymes, producing a pool of more freely available substrates, including dissolved organic matter (DOM), considered ‘public goods’ for other bacteria in the community to utilize [8]. Studies of freshwater leaf-degrading dikaryan fungi show that as bacteria lack key enzymes associated with plant polymer degradation [21], the production of low and intermediate weight DOM by fungi [22] may support enhanced bacterial growth on allochthonous leaf litter [21]

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