Abstract
In most of the aquatic ecosystems, microorganisms are major players in the biogeochemical and nutrients cycles (Carbon Nitrogen, Phosphorus), through their enzymatic activities (leucine aminopeptidase, alkaline phosphatase and beta-glucosidase) on organic polymers such as polypeptides, organophosphate esters and polysaccharides, respectively. The small monomers released by decomposition are metabolised by microbes, supporting their growth. Most of the extracellular enzymes are adaptative and their synthesis and activity is strongly affected by environmental factors, consequently the relative importance of leucine aminopeptidase, alkaline phosphatase and beta-glucosidase reflects differences in the composition of organic matter and assume a different meaning.Since more than two decades, at the CNR the influence of climate changes, seasonal variability, depth and coastal input on the patterns of enzymatic activities in the Mediterranean Sea have been studied. Its particular characteristics of a semi-closed basin, high summer evaporation and the occurrence of important water dynamics, make this ecosystem particularly suitable as a model site for climate changes-related observations.The present paper reviews the current information of environmental changes on extracellular enzymatic activity obtained in the Mediterranean areas with the aim of evaluating the effects of environmental changes on the microbial activities. The obtained results revealed significant variations in the rates of hydrolytic activities in relation to space and time, with the highest levels generally found in the epipelagic layer (0–100m) and in coastal zones during warm periods. In the Central Mediterranean Sea their relationship with temperature changes was demonstrated.Spatial variations in the relative enzyme activities also suggested a modulation in the metabolic profiles of the prokaryotic communities, with biogeochemical implications in nutrient regeneration.Long term studies on microbial activity and abundances in relation with rising temperatures can have a predictive value to describe the evolutionary scenario of microbial processes and the response of microbial metabolism to climate changes in the Mediterranean Sea.
Highlights
Marine microorganisms are the most abundant organisms in the oceans; in spite of their small size, they account for a significant fraction of living planktonic biomass and play a major role in biogeochemical processes in aquatic ecosystems [1]
Heterotrophic bacteria contribute to nutrient cycles through the production of new bacterial biomass which acts as a temporary Carbon storage, influencing the amount of particulate organic matter (POM) available to higher trophic levels or exported to deeper waters
A relatively warmer water in the deep zones compared to deep oceans, the prevalence of small heterotrophs and small phototrophs with a microbially dominated food web [13,14], the presence of Levantine Intermediate Waters [at about 300–400 m depth] and dense water formation phenomena that occur in specific sites such as the Gulf of Lions, the Adriatic Sea, and the Aegean Sea [15,16]
Summary
Marine microorganisms are the most abundant organisms in the oceans; in spite of their small size, they account for a significant fraction of living planktonic biomass and play a major role in biogeochemical processes in aquatic ecosystems [1]. In oligotrophic water bodies the role of microorganisms becomes even more important, because in such environments with low nutrient concentrations, prokaryotic heterotrophs compete with phytoplankton for the uptake of the low amounts of available nutrients, mostly derived by regeneration processes [2]. Understanding how complex organic matter is transformed and remineralized by marine heterotrophs, and how this activity varies geospatially, is fundamental to our knowledge of the microbially-mediated carbon cycle on a global scale. The levels of phosphate are the main limiting factor for biological productivity in the eastern Mediterranean, the presence of Atlantic waters with low salinity and temperature in the West basin and the intense water evaporation in the Levantine basin [11,12],. A relatively warmer (about 13 °C) water in the deep zones compared to deep oceans, the prevalence of small heterotrophs and small phototrophs with a microbially dominated food web [13,14], the presence of Levantine Intermediate Waters [at about 300–400 m depth] and dense water formation phenomena that occur in specific sites such as the Gulf of Lions, the Adriatic Sea, and the Aegean Sea [15,16]
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