The effects of temperature, light intensity and nutrient depletion on akinete formation in seven strains of plank tonic Anabaena spp.: A. mucosa TAC426; A. crassa TAC436; A. spiroides TAC443 and TAC444; A. flosaquae TAC446; and A. ucrainica TAC448 and TAC449 were examined. A Marked effect of temperature on akinete formation was observed at 40 mu mol photons.m(-2).sec(-1) and nutrient-sufficient conditions. At 20 degrees C, akinetes did not develop in A. mucosa TAC426, A. crassa TAC436, A. spiroides TAC443, A. flos-aquae TAC446, or A. ucrainica TAC443 but were formed at frequencies of a little over 11% (ratio of filaments with akinetes to total filaments) in A. spiroides TAC444 and A. ucrainica TAC448. None of the strains formed akinetes or heterocysts at 30 degrees C and 35 degrees C. At lower temperature (10 degrees C and 15 degrees C), akinetes developed in all the strains at maximum frequencies of 13.4-77.4 % during the late exponential phase or late exponential to stationary phases of growth. With only one exception, low light or nutrient deletion did not lead to the induction of akinete differentiation at 20 degrees C. Only akinete formation in A. flosaquae TAC446 was induced by nitrogen deletion with a frequency of 12.1%, similar to that induced by low temperature, but the initiation of akinete formation in the strain was delayed compared to treatment with low temperature. These results show that temperature was the most important environmental factor triggering akinete formation in these species. In A. crassa TAC436 and A. spiroides TAC443 and TAC444, akinetes developed during the late exponential growth phase even though heterocysts were formed at a 100% frequency (ratio of filaments with heterocysts to total filaments) throughout the entire growth phase. In A. mucosa TAC426, A. flos-aquae TAC446, and A. ucrainica TAC448 and TAC449 there was a positive correlation between heterocyst and akinete formation, suggesting that the presence of a heterocyst may play a rob in akinete formation.
