Comparative Study On The Ecophysiological Responses Of Different Groups Of Phytoplankton To Urea

Both field survey and lab-based incubation experiments were used to study theecophysiological responses of different groups of phytoplankton to Urea. The fieldvoyages surveyed the distribution of urea concentrations in estuaries and coastalwaters, as well as plankton urease activity, to study urea availability in the waters andits effect on phytoplankton’s population growth and community succession. In thelaboratory, six typical species from different groups of phytoplankton, including:dinoflagellate （Karenia mikimoto）, diatom （Skeletonema costatum）, cryptophyte（Proteomoms sp.）, haptophyte （Phaeocystis globosa）, chlorophyte （Pycnococcusprovasol） and cyanophyte （Synechococus sp.） were selected to study effects ofdifferent nitrogen sources （NO₃^-, NH₄⁺and urea） on the growth and physiology ofalgae. Furthermore, the15N isotope labeling technique were used to study nitrate,ammonium and urea uptake kinetics of K. mikimotoi, S. costatum and Prorocentrumdonghaiense under nitrogen starved condition, the results were as follows:Results from the field studies in the Yangtze River Estuary, the Daya Bay, theZhujiang Estuary and Qinhuangdao coastal waters showed that, high ureaconcentrations were mostly distributed in the estuaries, farming areas and the denselypopulated coastal areas. Land runoff of urea was responsible for the increase of ureaconcentrations in the estuarine and coastal waters. The urea levels in the nearshorewaters of China are relative low compared with those in other countries. In theQinhuangdao coastal waters, ratios of urea to DIN were1:5.13and1:4in May andJuly, the urea concentrations in the Daya Bay even exceeded the DIN concentrationsin the summer and autumn. Hence, we concluded that urea was an important nitrogensource, which could not be ignored in the sea.Urease activity can be used to indicate the capability of plankton in utilizing urea.Significant difference in plankton urease activities were found between the seasons inDaya Bay. Urease activity was low in winter and spring, many stations below thedetection limit; while urease activities were high in autumn and summewith the valuesof1.17±0.65and0.56±0.91μmol-N/L·h. Urease activity in autumn was mainly affected by the dinoflagellate abundance. when A. anophagefferens bloom occurred inthe Qinhuangdao sea, plankton urease activities reached20.57±8.33μmol-N/L·h,which were significantly higher than in any other study sea areas. The mechanisms onPlankton urease activities were integrated. Urease activity was influenced by manyfactors, such as temperature, salinity, nutrient concentrations and their ratios,phytoplankton community structure and abundances. The effectiveness of factorsshifted with the variation of natural condition.In the lab, results showed that high concentration of NH₄⁺（200μmol/L） hadtoxic effect on the growth of K. mikimotoi and Proteomoms sp., but no effect on S.costatum, P. provasoli, P. globosa and Synechococeus sp.; Under low concentration（30μmol/L）, the growth of the target species except P. provasoli, Proteomoms sp.didnot showed significant difference in different nitrogen source （NO₃^-, NH₄⁺and urea）,which indicated that the capability of K. mikimotoi、S. costatum, P. provasoli inutilizing urea for growth were comparable to in utilizing NO₃^-and NH₄⁺, but theability of P. globosa and Proteomoms sp. in utilizing urea were weak. Compared theparticulate carbon, nitrogen and phosphorus content in different cell types for theresponse of the three nitrogen sources,Data from cellular carbon, nitrogen andphosphorus and theirs ratios also supported the results. The particulate carbon,nitrogen and phosphorus content of P. provasoli and K. mikimotoi in urea culture wereeven higher than that in NO₃^-or NH₄⁺culture. Therefore, the results showed thatdifferent groups of phytoplankton had difference response in growth and physiologyto urea.15N uptake kinetics experiments results showed that, when NO₃^-, NH₄⁺and ureaexisted at the same time, algae would absorb different nitrogen sourcessimultaneously, three sets of absorption system playing roles at the same time.However, the absorption rates of in different nitrogen sources were different. K.mikimotoi and S. costatum had the highest uptake rate in NH₄⁺, followed by urea andNO₃^-. The nitrogen preference of K. mikimotoi was in the order of urea> NO₃^-> NH₄⁺;S. costatum prefered s for NO₃^-> NH₄⁺> urea in sequence; while P. donghaienseprefered for urea> NH₄⁺> NO₃^-in sequence.The results indicated that the dinoflagellates K. mikimotoi and P. donghaiense prefered for the reduced form ofnitrogen source, such as urea and NH₄⁺, while the diatom S. costatum prefered foroxidized nitrogen source, such as NO₃^-.In this paper, the results showed that urea was an important component of theavailable nitrogen source in the coastal waters of China. For example, in the Daya Bay,the urea contents could be higher than DIN especially in the summer and autumn. Inthe Yangtze River Estuary and its adjacent sea areas, the urea level sometimes wasgreater than DIN. Differences were found among the target species from differentgroups of phytoplankton in urea uptake and assimilation. We concluded thatterrigenous input of urea would play an important role in phytoplankton populationgrowth and even its community succession in the inshore sea areas.