| Abstract: | Colonies of the stream-inhabiting cyanobacterium Nostoc parmelioides Kützing often contain a single endosymbiotic dipteran larva Cricotopus nostocicola (Wirth), which induces a morphological change from small, spherical colonies to larger, ear-shaped colonies. At a current velocity of 0 cm · s?1, whole colonies containing the midge showed overall rates of 14CO2 uptake and nitrogenase activity that were higher than those when the midge was absent (sphere-shaped colonies). Spherical colonies incubated at current velocities of 5-10 cm · s?1did not show higher rates of 14CO2 or 15N2 incorporation than those with the larvae (ear-shaped colonies). Ear-shaped colonies extended well into regions of higher current velocity, whereas spherical colonies did not. Photosynthesis of ear-shaped colonies was stimulated by increased current velocity, increased inorganic C and decreased O2 concentrations. Moreover, levels of O2 at the surface of midge-inhabited colonies decreased with increased current velocity. The morphological change induced by the larva is detrimental (lowers photosynthesis and N2 fixation) in quiescent water but not at current velocities above 10 cm · s?1. This is probably a result of higher diffusion of O2 and CO2 associated with the midge-induced morphology. |
