Interactions between benthic copepods, bacteria and diatoms promote nitrogen retention in intertidal marine sediments.

Willem Stock,Koen Sabbe,Marleen De Troch,Kim Heylen,Anne Willems,Candida Savage

doi:10.1371/journal.pone.0111001

Abstract

The present study aims at evaluating the impact of diatoms and copepods on microbial processes mediating nitrate removal in fine-grained intertidal sediments. More specifically, we studied the interactions between copepods, diatoms and bacteria in relation to their effects on nitrate reduction and denitrification. Microcosms containing defaunated marine sediments were subjected to different treatments: an excess of nitrate, copepods, diatoms (Navicula sp.), a combination of copepods and diatoms, and spent medium from copepods. The microcosms were incubated for seven and a half days, after which nutrient concentrations and denitrification potential were measured. Ammonium concentrations were highest in the treatments with copepods or their spent medium, whilst denitrification potential was lowest in these treatments, suggesting that copepods enhance dissimilatory nitrate reduction to ammonium over denitrification. We hypothesize that this is an indirect effect, by providing extra carbon for the bacterial community through the copepods' excretion products, thus changing the C/N ratio in favour of dissimilatory nitrate reduction. Diatoms alone had no effect on the nitrogen fluxes, but they did enhance the effect of copepods, possibly by influencing the quantity and quality of the copepods' excretion products. Our results show that small-scale biological interactions between bacteria, copepods and diatoms can have an important impact on denitrification and hence sediment nitrogen fluxes.

Highlights

Over the past century anthropogenic activities have dramatically increased the amount of reactive nitrogen on Earth [1]
The initial ammonium concentration was highest in the copepod + diatom treatment, but the difference was only significant compared to the increased NO3- treatment
Effect of copepods The presence of copepods or their spent medium resulted in elevated phosphate and ammonium concentrations in the microcosms

Summary

Introduction

Over the past century anthropogenic activities have dramatically increased the amount of reactive nitrogen on Earth [1]. It has been estimated that nitrogen inputs have increased as much as tenfold in coastal ecosystems [2,3]. As a result, these often nitrogenlimited areas [4] have experienced severe eutrophication, resulting in anoxia and changes in community structure [5]. Denitrification and anaerobic ammonium oxidation (anammox) are capable of countering eutrophication by removing reactive nitrogen from the ecosystem as nitrous oxide (N2O) or nitrogen gas (N2) [6]. Anammox, denitrification and DNRA are all catalysed in the anoxic sediment, but by different microbial assemblages [7]

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