Microbial control in P particle formation

Phosphorous is an essential nutrient for living organisms, and limits marine phytoplankton growth in surface waters, but little is known about its effect in bottom water. Cycling of phosphorus in the ocean is considered to be geologically driven by particle sedimentation as the main export process. However, little is known about biogeochemical processes and geomicrobial interactions in the sediment-water column interface. Phosphorus occurs mainly in one oxidation state and exists as one stable isotope (31P). Therefore, direct evidence for active microbial cycling is addressed by labelling experiments with radioactive tracers. The radioactive isotope 33P can be quantified by bulk measurements of radioactivity decay. Additionally, the decay product of 33P, (33S) can be detected by mass spectrometry. This offers the possibility to trace P uptake in the bulk and single-cell level, to obtain new insights of microbial interactions with P. This study presents preliminary results of novel approaches to study microbial formation of particulate P, aiming to assess the potential of novel methodologies and understand their limitations Radiotracer incubations were carried out on pure cultures, enrichment cultures as well as mixtures of cells and particles. Partitioning of P uptake in different P pools was performed using a modified sequential extraction (SEDEX) protocol. Bulk uptake showed clear differences in biogenic P when compared to a killed control. Present results indicate that biogenic P uptake is not limited to cellular incorporation and that the microbial influence on particulate P formation cannot be assessed by bulk methodologies alone. Therefore, investigations on the single cell level require high resolution imaging techniques such as nanoSIMS analysis. These novel approaches in marine microbiology can further increase the knowledge about single-cell P-uptake processes. The combination of 33P incubations and subsequent analysis of 33S on the single-cell level has not been widely applied so far, but have a high potential to determine the enrichment in P of different cells as well as inorganic particles. Assessing these methods is importance for their application in the environment, with exciting and promising discoveries for future studies