SPECIES-SPECIFIC RESPONSE IN ALKALINE PHOSPHATASE ACTIVITY OF FRESHWATER PHYTOPLANKTON IN A NUTRIENT ENRICHMENT EXPERIMENT

A new method was utilized to study species-specific responses of phytoplankton to phosphorus limitation in a nutrient enrichment experiment. A substrate, ELF, produces a fluorescent precipitate at the sites of alkaline phosphatase (AP), which makes it possible to visually detect phosphorus (P) limitation in individual cells of multiple species. Lake water was incubated in the laboratory to induce nitrogen (N) or P limitation. Initially, little or no ELF labeling was observed for any of the phytoplankton species, indicating a general lack of P limitation. This observation was supported by low bulk AP activity in the initial field samples. During the experiment, several chlorophyte taxa (Coelastrum, Eudorina, a solitary spiny coccoid) were driven to P limitation, as evidenced by a high percentage of cells displaying ELF labeling when inorganic N was added. Taxa such as Actinastrum and Dictyosphaerium, on the contrary, were never P limited. Little or no ELF was observed in cyanobacterial species, suggesting that P limitation was not achieved in these organisms. Using traditional bulk AP activity, significantly higher levels of AP activity were observed in treatments with inorganic N additions, compared to those with phosphate additions. ELF labeling generally followed the trend of bulk AP, except in species that did not dominate the biomass. Finally, we noted that all species observed were ELF labeled at least on one occasion, except for fragile flagellates which did not withstand the labeling procedure.