P Fluxes and Prokaryotic Cycling at Benthic Boundary Layer in the Deep Southeastern Mediterranean Sea

The role of prokaryotic microbial productivity in P cycling in the benthic boundary layer (BBL) of deep-sea sediments (350–1,900 m bottom depths) was studied in the P-impoverished southeastern Mediterranean Sea (SEMS) using sediment cores and incubation reactors. Our results show that orthophosphate (PO₄) benthic fluxes were negative (consumption) at a rate of −11.0 ± 3.3 µmolem⁻²d⁻¹, while the dissolved organic phosphate (DOP) flux was positive (production) at rate of 4.5 ± 1.6 µmolem⁻²d⁻¹. Therefore, the SEMS sedimentary BBL acts as an active net sink for P, similar to other impoverished subtropical oceanic gyres, yet at much shallower water depths and much closer to the terrestrial margin. Considering the water column integrated P mass, and the sedimentary net removal fluxes of P, we calculated a turnover time of ∼90 years for the bottom water in the SEMS, which is similar to the deep water residence time in this basin. Apart abiotic removal of PO₄ under oxic conditions (e.g., co-precipitation with iron, forming iron-phosphate minerals), prokaryotic microbial utilization played an important role in generating DOP through aerobic respiration in the BBL, resulting in a net release of DOP. Our study demonstrates that microbial communities in the BBL play an important role in regulating P concentrations, fluxes and forms in the energy-starved environment of the SEMS seabed. These results may contribute to our understanding of water column P dynamics in the low nutrient low chlorophyll basins.