Linking Microbial Functioning and Trophic Pathways to Ecological Status in a Coastal Mediterranean Ecosystem

Franco Decembrini,Gabriella Caruso,Alessandro Bergamasco,Carmela Caroppo

doi:10.3390/w13091325

Abstract

Coastal marine ecosystems host complex microbial communities whose composition and metabolism are influenced by continental inputs and mesoscale properties of seawater masses. The identifying traits of the phytoplankton and bacteria such as biomass, size, shape and their metabolism related to organic matter production and degradation, recognized as indicators of the functioning of an ecosystem, were observed in the Gulf of Manfredonia (South Adriatic Sea, Italy) in late spring. This Gulf area is characterized by terrestrial inputs and mesoscale circulation influence such as coastal waters flowing southward from the North Adriatic and offshore waters interested by the Ionian Sea. Water samples were grouped in clusters (Coastal, Intermediate, Offshore and Deep Systems) according to the water column properties. Phytoplankton community biomass and composition, autotrophic and total prokaryotic abundances and microbial metabolism such as enzyme activity rates and prokaryotic heterotrophic production were analyzed to elucidate the trophic pathways with the objective to infer on the ecosystem status. As expected, size-fractionated phytoplankton biomass and production showed greater concentration in coastal waters with prevalence of the largest fractions (micro- and nano-) supported by the diatoms. Conversely, lower biomass and production were measured in all off-shore waters, mainly sustained by smallest fractions (nano-sized phytoflagellates and picophytoplankton). Total and autotrophic prokaryotic abundance decreased from coastal to offshore stations, inversely with respect to cell volume. Prokaryotic heterotrophic production was just below 50% compared to that of phytoplankton in all waters, evidencing an active biomass synthesis. High alkaline phosphatase and leucine aminopeptidase in coastal and offshore waters suggested the quick regeneration of Phosphorus and protein decomposition, respectively. Different levels of phytoplankton-bacteria association might provide a tool to define the ecological status of the studied system in the observed period; an approach to ecosystem assessment exportable to other coastal systems is proposed.

Highlights

IntroductionA holistic approach to the study of all the components of an ecosystem, including the smallest size ones, is needed to obtain a comprehensive understanding of its functioning
For the first time in the South Adriatic Sea, in this study, we looked with a holistic approach at the overall functioning of the microbial compartment, in order to highlight, if any, functioning differences between coastal and pelagic systems and possible relations with mesoscale processes
To assess the microbial functional role in the Gulf of Manfredonia in the late spring period, as a model of other temperate marine ecosystems, the simplified scheme applied in Figure 7 includes standing stocks and processes that link autotrophic and heterotrophic compartments in a coastal system (CS) subject to terrigenous inputs and connected to a oligotrophic pelagic one (OS) influenced by meso-scale circulation

Summary

Introduction

A holistic approach to the study of all the components of an ecosystem, including the smallest size ones, is needed to obtain a comprehensive understanding of its functioning. Phytoplankton, through the biomass and activity of its different size components (micro-, nano-and pico- fractions), triggers the main pathways of biogenic Carbon assimilated through the food web and affects the whole environmental trophic conditions [2]. Phytoplankton diversity, due to a number of very diverse organisms (prokaryotes and eukaryotes, autotrophic and mixotrophic), has a fundamental role in the functioning of ecosystems. These microorganisms, organized in complex associations, are deeply entangled in food-webs and biogeochemical cycles [3]. Even if few studies are available [6,7] bacterial activity could be a function of bacterial size [8]

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Results

Conclusion