All-In-One: Microbial Response to Natural and Anthropogenic Forcings in a Coastal Mediterranean Ecosystem, the Syracuse Bay (Ionian Sea, Italy)

Gabriella Caruso,Stefano Miserocchi,Yiyong Zhou,Chunlei Song,Maria Grazia Giacobbe,Filippo Azzaro,Marcella Leonardi,Xiuyun Cao,Franco Decembrini

doi:10.3390/jmse10010019

Abstract

Bacterial and phytoplankton communities are known to be in close relationships, but how natural and anthropogenic stressors can affect their dynamics is not fully understood. To study the response of microbial communities to environmental and human-induced perturbations, phytoplankton and bacterial communities were seasonally monitored in a Mediterranean coastal ecosystem, Syracuse Bay, where multiple conflicts co-exist. Quali-quantitative, seasonal surveys of the phytoplankton communities (diatoms, dinoflagellates and other taxa), the potential microbial enzymatic activity rates (leucine aminopeptidase, beta-glucosidase and alkaline phosphatase) and heterotrophic culturable bacterial abundance, together with the thermohaline structure and trophic status in terms of nutrient concentrations, phytoplankton biomass (as Chlorophyll-a), and total suspended and particulate organic matter, were carried out. The aim was to integrate microbial community dynamics in the context of the environmental characterization and disentangle microbial patterns related to natural changes from those driven by the anthropic impact on this ecosystem. In spite of the complex relationships between the habitat characteristics, microbial community abundance and metabolic potential, in Syracuse Bay, the availability of organic substrates differently originated by the local conditions appeared to drive the distribution and activity of microbial assemblage. A seasonal pattern of microbial abundances was observed, with the highest concentrations of phytoplankton in spring and low values in winter, whereas heterotrophic bacteria were more abundant during the autumn period. The autumn peaks of the rates of enzymatic activities suggested that not only phytoplankton-derived but also allochthonous organic polymers strongly stimulated microbial metabolism. Increased microbial response in terms of abundance and metabolic activities was detected especially at the sites directly affected by organic matter inputs related to agriculture or aquaculture activities. Nitrogen salts such as nitrate, rather than orthophosphate, were primary drivers of phytoplankton growth. This study also provides insights on the different seasonal scenarios of water quality in Syracuse Bay, which could be helpful for management plans of this Mediterranean coastal environment.

Highlights

Coastal aquatic environments are extremely vulnerable areas due to the presence of multiple interacting forcings, both natural and anthropogenic, that occur in these environments and that may affect the dynamics of marine biota [1,2]
With respect to microbial metabolism, several positive associations were observed between proteolytic activity, T and dissolved 37.056 oxygen (DO), while negative associations were found with N salts (NH4 and NO3 )
The organic matter and nutrient inputs that characterized the human-impacted stations (HIS) were reflected in significant increases of Chl a, total phytoplankton and heterotrophic bacterial abundances and metabolic activities (LAP, GLU, alkaline phosphatase (AP))

Summary

Introduction

Coastal aquatic environments are extremely vulnerable areas due to the presence of multiple interacting forcings, both natural and anthropogenic, that occur in these environments and that may affect the dynamics of marine biota [1,2]. Maritime transport, tourism, as well as aquaculture and fishery may coexist both spatially and temporally in these areas and cause severe impacts such as pollution, biodiversity loss and habitat destruction This justifies the need for coastal monitoring as a key tool to acquire updated information on the environmental status, in relation with the new European environmental challenges mainly focused on the achievement of the Good. Many complex interactions occur at different cellular levels among representatives belonging to different microbial species; this results in difficulties in estimating metabolic interactions among microorganisms, in consideration that a large fraction of microbial diversity remains largely undiscovered in terms of metabolic needs and biosynthetic capabilities [8] Different relationships such as mutualism and competition connect phytoplankton to bacterial species. The physical attachment of bacteria to phytoplankton cells was shown to play a role in biogeochemical cycling in the ocean, favoring the sinking of organic matter and the overall increase in the flux of organic carbon from the surface to deep waters [11,12]

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