Comparative bioavailability of ammonium, nitrate, nitrite and urea to typically harmful cyanobacterium Microcystis aeruginosa.

Phosphorus is generally considered as the prime limiting nutrient responsible for cyanobacterial blooms. However, recent research is drawing attention to the importance of bioavailable nitrogen (N) in freshwater eutrophication. This study investigated the bioavailability of NO3(-)-N, NO2(-)-N, NH4(+)-N and Urea-N under different concentrations of 1.2, 3.6 and 6.0mgL(-1) to Microcystis aeruginosa. Overall, Urea-N ranked the first in promoting M. aeruginosa growth, followed by NO3(-)-N and NO2(-)-N. However, the algal growth cultured in NH4(+)-N was depressed under test N levels. The bioavailability of N to M. aeruginosa was seriously influenced by both N forms and N concentrations (p<0.01). Total N concentrations in Urea-N treatment decreased the fastest, which were corresponding with the μ values of M. aeruginosa. The high enzymic activities of nitrate reductase, nitrite reductase and glutamine synthetase indicated that the decomposition process for urea is effective, which contributed in N assimilation and utilization in M. aeruginosa cells.