Factors affecting the fluctuation of akinete and heterocyst numbers of Aphanizomenon flos-aquae (Cyanobacteria) population
Yamamoto, Yoshimasa; Nakahara, Hiroyuki
The development of a water bloom of Aphanizomenon flos-aquae var. klebahnii Elenk. was monitored in a small artificial pond in 2005. A. flos-aquae occurred in the surface water first on May 8, when the water temperature increased to 16.2 °C, and its biovolume declined sharply within two weeks on August 14, when surface pH dropped from 7.8 to 7.3. Akinete formation was confirmed on the A. flos-aquae filaments at the initial and the final phases of the bloom, although the numbers were very low. The heterocyst frequency was far higher than that of akinete throughout the study period, and changed greatly according to the stages of population growth; the heterocyst density decreased during the exponential and stationary phases, whereas it started to increase after the bloom collapsed. In addition, it became evident that the mean filament lengths of A. flos-aquae were small at the initial and the final phases of the bloom compared with those during the phase of exponential increase from late May to June. These results suggest that changes in akinete and heterocyst densities and filament length can be proper indicators in considering the activity status of the natural population of A. flos-aquae. The effects of physico-chemical factors on the induction of akinete and heterocyst formation in A. flos-aquae were examined in the laboratory. The experiments revealed that adverse conditions for growth, such as low water temperature, low irradiance and low phosphorus concentration triggered high numbers of akinete formation. On the other hand, heterocysts were developed at nearly a constant rate on a filament, whereas the density decreased at high NH4Cl and low Na2HPO4 concentrations.