Concurrent Diurnal Variations of High-molecular-weight Dissolved Arsenic and Organic Phosphorus in the Upper Euphotic Zone

The biogeochemical cycling of arsenic in the global ocean has received concerned interests due to its potential competitive inhibition to phosphate. Arsenate can be uptake by phytoplankton or bacterioplankton, followed by transformation into arsenite or methylated arsenic and released into seawater by certain detoxification pathways. The arsenic/phosphate ratio in seawater has been proposed as the factor determining the arsenic speciation in the global ocean, however, the regeneration processes are much more complicated. In this study, ultra-clean sampling and cross-flow ultrafiltration (CFUF) were used to differentiate the molecular weight distribution of arsenic and phosphorous in the oligotrophic Western Philippine Sea. Our results indicated that 21±8% of total dissolved arsenic were in the high-molecular-weight (HMW, 1 kDa – 0.4μm) phase, and HMW phosphorous accounted for a significant amount of 61±16% in the DOP fraction. In the upper euphotic zone, HMW-As exhibited short-term temporal variations, corresponding to the change of the high-molecular-weight dissolved organic phosphorus (HMW-DOP). The low-molecular-weight (LMW, MW < 1 kDa) arsenic, by contrast, was relatively unchanged. The concurrent variations between HMW-As and HMW-DOP may resulted from photoheterotrophy, marine ectoenzymes such as alkaline phosphatase might play an important role in recycling processes. All these evidences highlight the strong biological effect on production and assimilation of HMW materials and its vital importance in marine P and As cycle.