Isotope insights into the phosphorus cycle of marinesediments

Abstract

The aim of the present dissertation was to investigate the microbial contribution to biogeochemical phosphorus (P) cycling in marine sediments. This essential nutrient controls marine primary production on geologic and recent time scales. Regeneration of phosphate (Pi) from organic matter and sequestration in phosphorites are major processes in the ocean's P cycle, in which the benthic microbial community has decisive influence on the source-sink balance. Two isotopic methods elucidated these biogeochemical transformations of Pi in detail and allowed new insights that extended the scope of investigations of Pi concentrations and pools alone.Oxygen isotopes of dissolved Pi (delta-18OP) have proven a suitable biosignature for microbial Pi regeneration and cycling. We refined and validated a micro extraction protocol for the separation and mass spectrometry of ultra low Pi samples (< 1 micromoles) of marine pore waters. We obtained a novel dataset on delta-18OP from two sediment cores off Morocco. With increasing depth, isotopic equilibration of Pi with water indicated thorough microbial Pi turnover, while disequilibria at the top pointed to Pi release from organic matter.In a comprehensive study on benthic Pi cycling in the Benguela upwelling system, we combined pore water inventories and modeling with the investigation of delta-18OP, in order to detect the microbial control on Pi regeneration and the influence of different sedimentary settings of mineralization and Pi availability. Pattern of delta-18OP in the sediments were more complex than expected from geochemical evidence. Isotope disequilibria were preserved at low Pi deep-sea sites, and thorough equilibria at sites with high mineralization and Pi concentration. Bottom water data pointed to a discontinuity between water column and pore water Pi. An isotope mass balance model, considering enzyme and substrate systems, allowed us to estimate the Pi recycling efficiency of the benthic community. We believe that different microbial Pi uptake strategies, controlled by ambient Pi availability, strongly influence the preservation of pore water delta-18OP.We finally sought to elucidate the role of microorganisms in authigenic phosphorite formation, and applied a radioisotope tracer (33P) to track sedimentary Pi transfers in incubation experiments. Our results constitute the first direct evidence that the presence of live bacteria is pivotal to phosphorite precipitation. Sequestration of Pi outbalanced regeneration under anoxic conditions, and Pi uptake by Thiomargarita namibiensis strongly suggested that these large sulfur bacteria provide a transient Pi reservoir. The unique physiology of Pi uptake and release in these bacteria may antagonize the positive feedback between enhanced Pi regeneration, stimulated primary production, and anoxia in bottom waters of upwelling regions.