Partitioning of CO 2 Fixation in the Colonial Cyanobacterium Microcystis aeruginosa : Mechanism Promoting Formation of Surface Scums

Abstract

Constraints on inorganic carbon (C i ) availability stimulated buoyancy in natural, photosynthetically active populations of the colonial blue-green alga (cyanobacterium) Microcystis aeruginosa. In nonmixed eutrophic river water and cultures, O 2 evolution determinations indicated C i limitation of photosynthesis, which was overcome either by CO 2 additions to the aqueous phase or by exposure of buoyant colonies to atmospheric CO 2 . Microautoradiographs of M. aeruginosa colonies revealed partitioning of 14 CO 2 fixation and photosynthate accumulation between peripheral and internal cells, particularly in large colonies. When illuminated colonies were suspended in the aqueous phase, peripheral cells accounted for at least 90% of the 14 CO 2 assimilation, whereas internal cells remained unlabeled. However, when 14 CO 2 was allowed to diffuse into colonies 15 min before illumination, a more uniform distribution of labeling was observed. Resultant differences in labeling patterns were most likely due to peripheral cells more exclusively utilizing CO 2 when ambient C i concentrations were low. Among colonies located at the air-water interface, internal cells showed an increased share of photosynthate production when atmospheric 14 CO 2 was supplied. This indicated that C i transport was restricted in large colonies below the water surface, forcing internal cells to maintain a high degree of buoyancy, thus promoting the formation of surface scums. At the surface, C i restrictions were alleviated. Accordingly, scum formation appears to have an ecological function, allowing cyanobacteria access to atmospheric CO 2 when the C i concentration is growth limiting in the water column.