Response of Microphytobenthos and Benthic Bacteria Viability to Eutrophication in a Benthic–Pelagic Coupling Mesocosm Experiment

Irini Tsikopoulou,Ioulia Santi,Panagiotis D Dimitriou,Paraskevi Pitta,Nafsika Papageorgiou,Ioannis Karakassis

doi:10.3389/fmars.2020.00270

Irini Tsikopoulou, Ioulia Santi + Show 4 more

Open Access

https://doi.org/10.3389/fmars.2020.00270

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Abstract

Excessive primary productivity due to nutrient inputs is a potential problem in coastal areas when resulting in high organic matter sedimentation rates. Microphytobenthos and heterotrophic bacteria are two components of the benthic ecosystem that contribute to nutrient cycling and decomposition of organic matter. In this context, the effects of nutrient addition and the associated in situ produced organic matter on microphytobenthos community composition and benthic bacterial viability were assessed in a mesocosm experiment for 58 days. The experimental setup included triplicate mesocosms filled with sediment and water under three levels of nutrient addition (‘control’, ‘low’ and ‘high’). Benthic algal community composition was assessed using chemotaxonomy and bacterial viability was estimated using flow cytometry and a double-staining protocol. Multivariate analysis detected a significant effect of treatment and time on microphytobenthic community composition indicating a difference between control and low mesocosms and also between low and high treatments at Day 12 and Day 24 of the experiment. Nonetheless, microphytobenthos implied high resistance and redundancy of benthic algae to disturbance as all three treatments showed no significant difference in community structure between Day 0 and Day 58. Bacterial viability responded quickly to the high nutrient addition and was significantly lower than in the ‘control’ and ‘low’ treatments at Days 6 and 12. Both pelagic and benthic environmental variables were correlated to these changes in benthic community.

Highlights

Marine sediments represent perhaps the most complex habitats on Earth and constitute an important element in marine biogeochemical cycles (Gray and Elliott, 2009)
The results indicated that both microphytobenthos community composition and benthic bacterial viability changed during the experiment under conditions of organic enrichment induced by nutrient addition in the water column and by the consequent increase in organic matter sedimentation from the pelagic zone
Microphytobenthic community composition fluctuated during the experiment but returned to the initial composition, while bacterial viability was only affected by high nutrient inputs

Summary

Introduction

Marine sediments represent perhaps the most complex habitats on Earth and constitute an important element in marine biogeochemical cycles (Gray and Elliott, 2009). As part of the benthic ecosystem, microphytobenthos, Eutrophication Effects on Microphytobenthos and Bacteria the unicellular eukaryotic algae, and Cyanobacteria that grow on the sediments, represent a key component of the carbon budget accounting for more than 50% of the total primary production in coastal areas where the euphotic zone reaches the sediment surface (Cahoon, 1999). These communities can radically modify the sediment function (Baustian et al, 2011, 2013). Biomass and the production of extracellular polymeric substances by these organisms represent an important source of fresh organic matter, which can fuel heterotrophic bacteria (Middelburg et al, 2000)

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