Colloidal organic phosphorus in the South China Sea


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Phosphorus (P) is an essential and growth-limiting nutrient for all living organisms and controls primary producers in marine environments. Dissolved Inorganic Phosphorus (DIP) in the surface ocean is often depleted, resulting in fast turnover times and stress responses as in genome adaptation and changes in cellular compositions for the microbes. The larger amount of Dissolved Organic Phosphorus (DOP) in surface waters is believed to be the crucial P source in the surface waters, which promotes microbial production in the euphotic zone of the oligotrophic ocean. In our previous report (Lee et al., 2017), diurnal variations of HMW-DOP were first revealed in the upper euphotic zone of the Western Philippine Sea (WPS). Here we found that more than 90% of the dissolved phosphorous is of organics in the surface water of the South China Sea and 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 indicated it is a major inventory to promote microbial growth. Concentration-depth profiles indicated surface enrichment of HMW-DOP, whereas LMW-DOP exhibited almost no change throughout the water column and SCS basin. Results not only showed the assimilation by microorganisms was preferentially selective, with fast turnover rates in the euphotic zone, which were seemly driven by cyanobacteria. The difference highlighted that phosphorous in the ocean has different recycling pathways for low molecular weights and colloidal fractions. Overall, the molecular weight distribution of DOP in this study reveals for the first time on DOP characteristics and reactivity, which further provide insight into microbial P utilization in the ocean.

Figure 1. Schematic concentration-depth profiles of colloidal and low-molecular-weight (< 1 kDa) organic phosphorous in the South China Sea with sampling stations and satellite-acquired chlorophyll concentration in surface waters.



Lee C.-P., Cheng C.-Y., Wen* L.-S. (2017) Vertical distribution and diurnal variations of high-molecular-weight dissolved arsenic in the oligotrophic ocean. Limnology and Oceanography, 62, 2200-2212, DOI: 10.1002/lno.10560.

Lee C.-P., Wen* L.-S. (2022) Colloidal organic phosphorous in the South China Sea. Marine Chemistry, 246, DOI: 10.1016/j.marchem.2022.104179.