Colloidal organic phosphorus in the South China Sea

Abstract

Phosphorus (P) is a growth-limiting nutrient that controls primary producers in marine environments. Recent studies suggested that dissolved organic P (DOP) may promote microbial production in the euphotic zone of the oligotrophic ocean. The characteristics of dissolved organic P (DOP) are crucial because it affects microbial utilization of P in the organic pool. Here we provide evidence of colloidal organic P (high-molecular-weight [HMW]-DOP, 1 kDa – 0.45 μm) in the South China Sea (SCS) and highlight the importance of size fractionation in the marine P cycle. Both soluble reactive phosphorus and DOP exhibited geographic variations, indicating that P cycling is strongly affected by hydrography and water mixing. DOP accounted for more than 90% of the P in the surface water of the SCS. A positive correlation between DOP and chlorophyll-a concentrations suggested that primary producers played a crucial role in DOP production. HMW-DOP comprised 75% of the surface DOP. The enrichment of P in the colloidal fraction compared with its low-molecular-weight (LMW, <1 kDa) counterpart suggested that HMW-DOP is a significant inventory to promote microbial growth. HMW-DOP depth profiles indicated surface enrichment, whereas LMW-DOP exhibited low variation throughout the water column. The difference in the features of these depth profiles indicated that P has different recycling pathways for the two size fractions. Overall, the size distribution of DOP reveals information on DOP characteristics and reactivity, which further provide insight into microbial P utilization in the ocean.