Linking the Dynamic Response of the Carbon Dioxide-Concentrating Mechanism to Carbon Assimilation Behavior in Fremyella diplosiphon

Cyanobacteria use a carbon dioxide (CO(2))-concentrating mechanism (CCM) that enhances their carbon fixation efficiency and is regulated by many environmental factors that impact photosynthesis, including carbon availability, light levels, and nutrient access. Efforts to connect the regulation of the CCM by these factors to functional effects on carbon assimilation rates have been complicated by the aqueous nature of cyanobacteria. Here, we describe the use of cyanobacteria in a semiwet state on glass fiber filtration discs—cyanobacterial discs—to establish dynamic carbon assimilation behavior using gas exchange analysis. In combination with quantitative PCR (qPCR) and transmission electron microscopy (TEM) analyses, we linked the regulation of CCM components to corresponding carbon assimilation behavior in the freshwater, filamentous cyanobacterium Fremyella diplosiphon. Inorganic carbon (C(i)) levels, light quantity, and light quality have all been shown to influence carbon assimilation behavior in F. diplosiphon. Our results suggest a biphasic model of cyanobacterial carbon fixation. While behavior at low levels of CO(2) is driven mainly by the C(i) uptake ability of the cyanobacterium, at higher CO(2) levels, carbon assimilation behavior is multifaceted and depends on C(i) availability, carboxysome morphology, linear electron flow, and cell shape. Carbon response curves (CRCs) generated via gas exchange analysis enable rapid examination of CO(2) assimilation behavior in cyanobacteria and can be used for cells grown under distinct conditions to provide insight into how CO(2) assimilation correlates with the regulation of critical cellular functions, such as the environmental control of the CCM and downstream photosynthetic capacity.