Photosynthesis of two morphologies of Nostoc parmeloides (Cyanobacteria) as related to current velocities and diffusion patterns

Colonies of the stream-inhabiting cyanobacterium Nos-toc parmelioides Kitzing often contain a single endo-symbiotic dipteran larva Cricotopus nostocicola (Wirth),which induces a morphological change from small, spher-ical colonies to larger, ear-shaped colonies. At a currentvelocity of 0 cm. whole colonies containing the midgeshowed overall rates of "CO2 uptake and nitrogenase ac-tivity that were higher than those when the midge wasabsent (sphere-shaped colonies). Spherical colonies incu-
bated at current velocities of 5-10 cm. did not showhigher rates of "CO2 or "N2 incorporation than those withthe larvae (ear-shaped colonies). Ear-shaped colonies ex-tended well into regions of higher current velocity, whereasspherical colonies did not. Photosynthesis of ear-shapedcolonies was stimulated by increased current velocity, in-creased inorganic C and decreased 02 concentrations.Moreover, levels of 02 at the surface of midge-inhabitedcolonies decreased with increased current velocity. Themorphological change induced by the larva is detrimental(lowers photosynthesis and N2 fixation) in quiescent water
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but not at current velocities above 10 cm. s-'. This is prob-ably a result of higher diffusion of 02 and CO2 associatedwith the midge-induced morphology.
Key index words: current; inorganic carbon; mutualism;nitrogen fixation; Nostoc; photosynthesis