The dark portion of the Mediterranean Sea is a bioreactor of organic matter cycling

Abstract

[1] Total prokaryotic abundance, prokaryotic heterotrophic production and enzymatic activities were investigated in epi-, meso-and bathypelagic waters along a longitudinal transect covering the entire Mediterranean Sea. The prokaryotic production and enzymatic activities in deep waters were among the highest reported worldwide at similar depths, indicating that the peculiar physico-chemical characteristics of the Mediterranean Sea, characterized by warm temperatures (typically 13 C also at abyssal depths), support high rates of organic carbon degradation and incorporation by prokaryotic assemblages. The higher trophic conditions in the epipelagic waters of the Western basin resulted in significantly higher prokaryotic production and enzymatic activities rates than in the Central-Eastern basin. While all of the variables decreased significantly from epi-to meso-and bathypelagic waters, cell-specific hydrolytic activity and cell-specific carbon production significantly increased. In addition, the deep-water layers were characterized by low half-saturation constants (K m ) of all enzymatic activities. These findings suggest that prokaryotic assemblages inhabiting the dark portion of the Mediterranean Sea are able to channel degraded carbon into biomass in a very efficient way, and that prokaryotic assemblages of the deep Mediterranean waters work as a "bioreactor" of organic matter cycling. Since prokaryotic production and enzymatic activities in deep water masses were inversely related with oxygen concentration, we hypothesize a tight link between prokaryotic metabolism and oxygen consumption. As climate change is increasing deep-water temperatures, the predicted positive response of prokaryotic metabolism to temperature increases may accelerate oxygen depletion of deep Mediterranean waters, with cascade consequences on carbon cycling and biogeochemical processes on the entire deep basin.

Extracellular DNA can preserve the genetic signatures of present and past viral infection events in deep hypersaline anoxic basins

“…We found that DNase activities, even when normalized to cell abundance, were higher in the sediments of La Medee than in the oxic sediments outside the anoxic basin. Conversely, aminopeptidase, b-glucosidase and alkaline phosphatase activities (used as a proxy for potential mobilization of proteins, carbohydrates and organophosphate monoesters) [54] were up to one order of magnitude lower in La Medee than in oxic sediments. We cannot exclude the possibility that the measured rates might be influenced by sediment decompression [55,56].…”

An exploratory study of heterotrophic protists of the mesopelagic Mediterranean Sea

“…However, mesopelagic fish biomass appears to be comparatively high based on the characteristics of the deepscattering layer, from which fish biomass has been modelled (Proud et al, 2017). With regard to microbes, the metabolic activity of prokaryotes appears to be higher for mesopelagic Mediterranean populations compared with those found at similar depths in other systems, related possibly to the relatively warm temperature of the deep waters, 13 C (Luna et al, 2012). Also, the abundances of typical bacterivores, heterotrophic nanoflagellates, appear to be low relative to abundances of prokaryotes compared with other systems perhaps because of high ciliate concentrations (Aristegui et al, 2009).…”

Nutrient Cycling in the Mediterranean Sea: The Key to Understanding How the Unique Marine Ecosystem Functions and Responds to Anthropogenic Pressures

“…This can be explained by the faster recycling of organic P and organic N back into the dissolved inorganic pools in the Western Mediterranean Sea. Evidence for this faster recycling of organic matter can be seen in the higher population densities of bacteria observed in the Western Mediterranean Sea, which are four to six times greater than in the Eastern Mediterranean Sea, as well as the higher concentrations of alkaline phosphatase, which are about three times greater than in the Eastern Mediterranean Sea [21]. Alkaline phosphatases are a group of enzymes that are designed to break the chemical bonds that hold P locked into organic compounds, thereby releasing dissolved phosphate to seawater.…”