Ecophysiology and competitive ability of Anabaena circinalis, a bloom-forming cyanobacterium

Blooms of the toxic cyanobacterium A. circinalis are a major problem in Australia. Further knowledge of this species and its competitors is of great importance to understand how the incidence of such blooms may be reduced. Experiments undertaken at a range of scales, from culture tubes to lm high microcosms examined the role of (1) light and mixing, (2) nitrogen type and availability and (3) pH and carbon availability in the dynamics of A. circinalis and its competitors - the diatom Aulacoseira sp. and the cyanobacterium Microcystis aeruginosa. In single species (A. circinalis, Aulacoseira sp.) microcosm experiments with vertically attenuated light environments, the growth of A. circinalis was significantly higher at 48 h mixing intervals than at 10 min intervals at the highest vertical light attenuation tested (k(0.3 = 9.7, Zm :Zeu= 6.25). The difference in growth was due to a combination of light availability and the physiological responses to it (i.e. increased buoyancy). This enabled A. circinalis greater access to light at the longer mixing interval. The contrary was true of Aulacoseira sp. which had significantly slower growth in all light environments at 48 h mixing intervals relative to 10 min intervals. Aulacoseira sp. had lowered access to light due to its high sinking rate. In competition experiments, A. circinalis dominated over Aulacoseira sp. at 48 h and 96 h mixing intervals, however both species co-existed at 10 min intervals. In combined species microcosm experiments (A. circinalis, Aulacoseira sp., Microcystis aeruginosa), there was no difference in species composition when N was supplied as nitrate or ammonium. Where no DIN was supplied, A. circinalis quickly gained dominance. An initial reduction in growth in the non-N2-fixing species (Aulacoseira sp., M. aeruginosa), was followed by a substantial increase. The results suggest the increase in growth was indirectly supported by N leaked into the medium by A. circinalis. In sealed culture experiments, growth rates of A. circinalis did not differ across a broad pH range (7.5 to 9.1). In comparison Aulacoseira sp. had significantly lower growth at high pH (9.0-9.1) relative to low pH (7.5-7.7). Evidence from ˜í¬•13C measurements strongly suggests that both species utilise bicarbonate at high pH, however the difference in growth rate suggests that Aulacoseira sp. is less efficient in utilising carbon via this pathway. All results are discussed in the context of the Australian environment. It is concluded that mixing and light, nitrogen and carbon availability are all potentially significant factors in the formation and/or maintenance of A. circinalis blooms.