Role Of Extracellular Phosphatase Produced By Phytoplankton In The Processes Of Phosphorus Cycling In Eutrophic Lakes

Algal bloom is one of the most typical symptom of eutrophication. Phosphorus is the most commonly limiting nutrient in freshwaters. It is well known that phytoplankton can only take up inorganic orthophosphate which is always far less than the amount needed for its metabolism. The following ecological questions arising therefrom remain unclear: 1. Does extracellular phosphatase catalyzing the liberation of orthophosphate from various organic phosphorus compounds play an important role in phosphorus cycling in eutrophic lakes? 2. What kinds of plankton produce extracellular phosphatase in lakes? 3. Can all kinds of phytoplankton produce extracellular phosphatase? 4. Does phytoplankton contribute to dissolved phosphatase? 5. Is the internal phosphorus loading connected with the growth of phytoplankton in water column? 6. How does dredging affect the phosphorus bioavailability and the growth of phytoplankton in water column? Aiming at these questions, the origin, distribution, characteristics of alkaline phosphatase activity (APA) and its role in phosphorus cycling were studied in 16 lakes and 2 fish ponds with different trophic levels from November 2001 to May 2004. The extracellular phosphatase produced by phytoplankton was emphasized. A new technique ELF (Enzyme Labelled Fluoresce) method, which allows an easy visualization of the sites of phosphatase activity under epifluorescence microscopy, and then detects the contributor of the enzyme, was applied together with the traditional method using pNPP as substrate. APA was detected in all lakes and ponds studied. Its sensitive responses to the specific inhibitors of phosphatase (CuSO₄, ZnSO₄, EDTA-2Na and NaWO₄) and surfactants (CTAB and Triton X-100) provided further evidences for the occurrence of theenzyme. The varying responding modes implied its different origin. Based on the data from 7 lakes in Hanyang District of Wuhan City, APA associated with coarser particles ( > 3.0μm) negatively related to soluble reactive phosphorus (SRP) concentrations in general, implying a connection between phosphatase and phosphorus bioavailablity. Thus, the action of phosphatase, as a mechanism of compensating phosphorus deficiency, was active even in the eutrophic lakes. As proven by ELF method, heterotrophic nanoflagellates and phytoplankton were the two main contributors of the extracellular phosphatase associated with the coarser particles. Thus, besides phytoplankton, zooplankton can produce cell bound phosphatase in addition to dissolved one. In Lake Donghu, the ELF labelled cells were detected in 33 algal taxa of many chlorophytes, dinoflagellates, and some diatoms altogether, but never among cyanobacteria. The pattern of ELF labelling was characteristic for some species and formed linear structures, dots or the whole cell surface was evenly labelled. Schroederia sp. was completely labeled in Lake Moshui, while completely non-labeled in Lake Longyang. Meanwhile, The SRP concentration in Lake Moshui was markedly lower than that in Lake Longyang. At the end of the dinoflagellate bloom in Lake Donghu, parts of labelled cells ( ²5% ) in Peridiniopsis sp., was detected, and the percentage of the labelled cells decreased to ⁵% with the increase in SRP. Thus, the production of extracellular phosphatase by phytoplankton was regulated, at least partially by phosphorus bioavailability. For phytoplankton, this enzymatic mechanism for compensation of phosphorus deficiency functions at individual cell or species (rather than assemblage) levels. In all water bodies studied, dissolved APA generally accounted for lager parts of total APA. Fish culture has some influences on its occurrence. In fish ponds with Anabaena sp. dominated, dissolved APA was stimulated by Cu2+. The same effects were observed in the Anabaena sp. culture medium. Thus, There may be a relation between Anabaena sp. and dissolved phosphatase. From January to April 2004,SRP concentrations in both water column and interstitial water increased in Lake Taihu, when it was the highest in interstitial water, the phytoplankton density peaked in water column, with green algae dominated. At the site with lower SRP but higher dissolved organic phosphorus concentrations,the phytoplankton assemblage was dominated by green algae capable of producing extracellular phosphatase as evidenced by the ELF labeling. Thus, the phytoplankton development in water column was related to sediment phosphorus, directly available and phosphatase hydrolyzable. After dredging, interstitial water in Lake Taihu showed remarkably lower contents of total phosphorus, dissolved total phosphorus and SRP, while SRP concentrations in water column remained relatively higher. Phytoplankton was generally unchanged in density and composition. Thus, dredging significantly cut the internal phosphorus loading, but its ecological effectiveness was not observed in a short period. In summary, with the diverse producers (zooplankton, phytoplankton and bacteria plankton) and different forms (associated and dissolved), the extracellular phosphatase is of great ecological significance, which plays an important role in phosphorus cycling in eutrophic lakes. Its production by phytoplankton is species specific reflecting that the different species has distinct phosphorus demand and/or utilizing strategies. The internal phosphorus loading might be causatively linked to the phytoplankton development, but the relationship is so complex that it greatly affects the effectiveness of remediation, such as dredging.