Aluminum (Al) is one of the key parameters of GEOTRACES, which can be used as the tracer of terrestrial input from river and/or atmospheric deposition. Recent results in the central Yellow Sea have shown that dissolved Al was scavenged significantly during the phytoplankton bloom and had similar vertical profiles with silicate, which indicate the biological regulation of dissolved Al. However, detailed scavenging mechanism of dissolved Al by phytoplankton remains unclear. In the present study, the scavenging mechanism of dissolved Al by diatoms (Nitzschia closterium, Skeletonema coasatum, Thalassiosira weissflogii) and dinoflagellate (Prorocentrum donghaiense) were investigated under different Al additions incubation conditions. The algae chosen here were typical phytoplankton species in the marginal seas of China. The cell were grown in filtered and sterilized seawater enriched with f/2 medium under 2000 lx (12:12 LD circle) at 20 ℃. The algae were cultivated triplicate in 2 L polycarbonate (PC) bottle under different Al enrichment conditions, including control group with no Al enrichment, 300 nmol/L and 600 nmol/L Al enrichment groups. Al was added as form of Al-EDTA complex in order to prevent the hydrolysis of free Al³⁺ ion. Dissolved Al concentrations as well as other relevant parameters (e.g., cell density, chlorophyll-a and silicate etc.) were analyzed during the incubation experiments.
The results showed that the growth rates of cultivated diatoms and dinoflagellate under 300 nmol/L and 600 nmol/L enrichment groups decreased about 3%~23% compared to the control group, indicating that the enrichment of dissolved Al didn't inhibit the growth of algae but delay the exponential growth period slightly. Dissolved Al concentrations decreased dramatically within two days for all incubation groups of diatoms, and then the decline slowed down until exponential growth period when Al was scavenged to nearly 20 nmol/L. The concentrations of dissolved Al increased slightly after the degradation of phytoplankton due to the release of dead cells. In contrast with diatom, the decreasing of dissolved Al concentration was not obvious during the incubation of dinoflagellate. Intra-and surface adsorbed Al pools in the cultivated algae was differentiated using oxalate reagent (Oxalate-EDTA-Citrate) to remove surface adsorbed Al from phytoplankton cells. The results showed that dissolved Al was scavenged onto the surface of cells during initial incubation period, and then part of surface adsorbed Al was converted into intra-cellular Al pool with the growth of phytoplankton. The percentage of intra-cellular Al in phytoplankton was approximately 20%-80% during the exponential growth period, and increased slightly during stable growth period. This indicates that the biogeochemical cycle of Al in the ocean is affected by diatoms. Using dissolved Al as tracer of terrestrial material input should appropriate correct the impact from biological activity, especially at the marginal seas influenced significantly by the phytoplankton bloom.