Kinetics Of Phosphorus In Marine Ciliates (Euplotes Sp.) Feeding On Microalgae

Ciliates, as an important member in microbial food, link the microbial food web with the classic food web. Over the past decades, the biogeochemical cycles of phosphorus (P) in aquatic ecosystems have gained wide attention. Therefore, the speciation, distribution, and cycling of P in aquatic environments have been extensively studied. Ciliates can assimilate dietary P and eliminate it to produce regenerated P, which is significant for the P cycle of the microbial food web and even for the whole ecosystem. However, the study on the transfer of P in ciliates is obviously not enough, especially regarding the kinetic parameters.Ciliate (Euplotes sp.) is a common species widely found in Hong Kong coastal waters. In the present study, three different microalgae species (Isochrysis sp.、 Chlorella autotrophica and Thalassiosira pseudonana) were radiolabelled with 33P and then fed to ciliates at different food concentrations. The kinetic parameters including the dietary assimilation efficiency and efflux rate constants were then quantified to examine the P transfer and recycle as a result of ciliate feeding. The results are summarized as the followings:1. Dietary assimilation of P:The P assimilation efficiency (AE) by ciliates fed with three different microalgae (Isochrysis sp.、 C. autotrophica and T. pseudonana) were 14-46%,12-37%, and 20-41%, respectively. In the food concentration experiments, each microalga had four cells densities (0.04、0.16、0.80 and 4.00 mg C·L-1). When the food was sufficient, the AEs were relatively lower, whereas the AEs increased significantly when the food was deficient.2. Efflux of P:During the 36-h depuration, P was rapidly eliminated from the ciliates initially (i.e. fast compartment, ke1) and was then eliminated at a much slower rate (i.e. slow compartment, ke2)-The P efflux rate constant ke1 in the ciliates feeding on Isochrysis sp., C. autotrophica and T. pseudonana were 0.87-1.86 d-1,0.54-2.29 d-1, and 1.65-1.89 d-1, and the ke2 was 0.09-0.38 d-1,0.09-0.15 d-1, and 0.23-0.60 d-1, respectively. Overall, the efflux rates were very high, and the fast compartment efflux rates were much higher than the slow compartment ones. These data suggested the high turnover rate of P in ciliates.3. The uptake rate constant:The uptake rates of Isochrysis sp.、C. autotrophica and T. pseudonana from dissolved inorganic P (DIP) were 51.7,23.2 and 13.0 L·g-1·h-1. For the regenerated P (RP), the corresponding uptake rates were 16.1,19.2 and 13.0 L·g-1·h-1, respectively. The RP was composed of DIP and dissolved organic P (DOP). The results demonstrated that all the three microalgae species had relatively high utilization efficiency of DP, and the uptake rates of DIP was significantly higher than that of DOP. However, the uptake rate of Euplotes sp. from the DIP was significantly lower, which was only 0.38 L·g-1·h-1. However, Ciliates uptake of regenerated P: During the depuration period, RP may be recycled by the ciliates by either direct uptake of dissolved (regenerated) P or ingestion of algal particle P (PP) due to rapid absorption of DP. Direct uptake of RP by the ciliates was rather slow, representing 3.7% of the total RP pool without algae during the 6-h of exposure, and the calculated uptake rate constant was 0.58 L·g-1·h-1. On the contrary, the uptake of PP by ciliates was significantly higher than that of RP, representing 10-34% of the total PP pool as a result of ingestion of algae. This may be explained by two mechanisms. The first was due to the complicated membrane structure on the surface of ciliates, which prevented the diffusion and absorption of dissolved P. The second was due to the appropriated size of food particle which favored the capture and ingestion by the ciliates. Therefore, uptake of PP from algal diet was the main pathway for marine ciliates to utilize the regenerated P.4. By integrating each dynamic parameter, we established a simple model to quantify the transfer and distribution of P during ciliate feeding on common microalgae species. The model shows that the AE of PP in marine ciliate was greatly higher than that of DP, which indicated that PP was the major source for marine ciliates. Meanwhile, when the food was in deficiency, the ciliate may increase the AE of PP through their self-regulated process to cope with the adverse environment. Furthermore, marine ciliates can accelerate the P cycle in microbial food web and even in the whole marine ecosystem due to their regeneration and efflux of P. Therefore, the ciliates play an important role in the biogeochemical cycle of P in marine ecosystem.