Microbial enzymes in the Mediterranean Sea: relationship with climate changes

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.

Regulation of Microbial Activity Rates by Organic Matter in the Ross Sea during the Austral Summer 2017

“…The LAP/GLU ratio in our study was different from the ranges observed by Sala et al [ 14 ] in the Arctic ocean (85–100), and from temperate regions (as in the oligotrophic Mediterranean Sea) where LAP prevail over other enzymatic activities [ 13 , 46 ]. Nevertheless, a prevalence of GLU on LAP was observed in Arctic fjords [ 20 ], and this suggests that patterns of peptide hydrolysis may differ between temperate and high latitude sites [ 52 ].…”

“…In addition, AP may be produced both by prokaryotes (bacteria and cyanobacteria) and eukaryotes (phyto- and zoo-plankton). Other organisms possess phosphoester hydrolases (phosphatases), able to release inorganic phosphorus for metabolic needs [ 46 ]. It has been demonstrated that phagotrophic nanofagellates and marine invertebrates might release extracellular enzymes through major physiological processes such as excretion or sloppy feeding.…”

“…In many marine environments, proteins are often processed faster than polysaccharides, as showed by LAP to GLU ratios being >1 [ 24 , 31 , 46 ]. On the contrary, in our study, a balance between the two activities or a prevalence of GLU on LAP were observed, with LAP/GLU ratio < 1.…”

“…On the contrary, in our study, a balance between the two activities or a prevalence of GLU on LAP were observed, with LAP/GLU ratio < 1. Considering these molar ratios as an indicator of microbial metabolism of organic C and N, C was preferentially mobilized compared to N due to high abundance of digestible carbohydrates [ 46 ].…”

Spatial patterns of ectoenzymatic kinetics in relation to biogeochemical properties in the Mediterranean Sea and the concentration of the fluorogenic substrate used

“…In these five studies the lowest substrate concentration used was 50 nM (typically the lower concentration in the set is between 1 and 5 µM), while the highest concentration was 1200 µM (the range of the higher concentrations in the set is 5-1200 µM, with a median of 200 µM). Another compilation of data from the Mediterranean Sea (Zaccone and Caruso, 2019) showed that 6 out of 22 studies used a single concentration (assumed to be saturating) with a median of 125 µM for leucine 7-amido-4-methyl coumarin and 50 µM for methylumbelliferyl-phosphate. Likewise, the remaining studies assessed enzyme kinetics with a highly variable range of substrate concentrations (lowest concentrations of 0.025-200 µM, with a median of 0.1 µM; highest concentrations of 1-4000 µM, with a median of 20 µM).…”