Microbial interactions in marine water amended by eroded benthic biofilm: A case study from an intertidal mudflat

TL;DR

This study investigates how eroded benthic biofilm influences microbial and viral dynamics in intertidal mudflats, revealing seasonal variations in microbial loop stimulation driven by biofilm input.

Contribution

It provides novel insights into the seasonal effects of benthic biofilm on pelagic microbial and viral interactions in intertidal ecosystems.

Findings

Biofilm input stimulates microbial and viral activity differently in March and July.

Benthic microorganisms strengthen biofilm effects on microbial communities.

Seasonal shifts alter the microbial and viral loop responses to biofilm input.

Abstract

In shallow macrotidal ecosystems with large intertidal mudflats, the sediment-water coupling plays a crucial role in structuring the pelagic microbial food web functioning, since inorganic and organic matter and microbial components (viruses and microbes) of the microphytobenthic biofilm can be suspended toward the water column. Two experimental bioassays were conducted in March and July 2008 to investigate the importance of biofilm input for the pelagic Microbial and Viral Loops. Pelagic inocula (<0.6$\mu$ and <10$\mu$ filtrates) were diluted either with \textless{}30kDa-ultrafiltered seawater or with this ultrafiltrate enriched with the respective size-fractionated benthic biofilm or with \textless{}30kDa-benthic compounds (BC). The kinetics of heterotrophic nanoflagellates (HNF), bacteria and viruses were assessed together with bacterial and viral genomic fingerprints, bacterial enzymatic activities and viral life strategies. The experimental design allowed us to evaluate the effect of BC modulated by those of benthic size-fractionated microorganisms (virus+bacteria, +HNF). BC presented (1) in March, a positive effect on viruses and bacteria weakened by pelagic HNF. Benthic microorganisms consolidated this negative effect and sustained the viral production together with a relatively diverse and uneven bacterial assemblage structure; (2) in July, no direct impact on viruses but a positive effect on bacteria modulated by HNF, which indirectly enhanced viral multiplication. Both effects were intensified by benthic microorganisms and bacterial assemblage structure became more even. HNF indirectly profited from BC more in March than in July. The Microbial Loop would be stimulated by biofilm during periods of high resources (March) and the Viral Loop during periods of depleted resources (July).