Carbon remineralization in the Mediterranean Sea

The resulting enhancement of the natural Greenhouse Effect has been tempered by oceanic uptake by sequestration of CO2 in the deep oceanic waters, which accounts for nearly a third of anthropogenic carbon added to the atmosphere. In fact, topical studies have supported the significant role of the oceanic biological pump and continental shelf pump with respect to the atmospheric CO2 build-up. In this context, the Eastern Mediterranean Transient (EMT) event resulted as a particular case of continental shelf pump that has significantly affected the deep respiration firstly in the Ionian Sea and afterwards in all the Mediterranean Sea. With the aim of evaluating the effect of EMT spreading throughout the Mediterranean and its impact on the microbial compartment, in October-November 2004 a trans-mediterranean cruise was performed, spanning longitudinally from the Gibraltar Strait to Creta Island. The study of prokaryotic biomass and respiratory rates (ETS) from surface to bottom depth was carried out and particular emphasis was given to the deeper layers. Prokaryotic abundances and respiration classically decreased with increasing depth. The longitudinal patterns of the prokaryotic cell abundance along the Mediterranean showed statistically different distribution between Western and Eastern basins at all layers with high values in the Western basin and a decreasing Eastward trend, in accordance with the well-known Mediterranean trophic gradient. On the contrary the cell sizes enhanced Eastward and with depth. Different patterns in respiration were observed in the Western and Eastern basins due to both the circulatory pathway of water masses and the trophic gradients existing on the whole Mediterranean basin. In this context, both the prokaryotic abundances and respiratory rates seem to be suitable markers to describe the variability in the dark deep waters of the Mediterranean Sea.