Cyanophycin mediates the accumulation and storage of fixed carbon in non-heterocystous filamentous cyanobacteria from coniform mats

Thin, filamentous, non-heterocystous, benthic cyanobacteria (Subsection III) in some marine and thermal environments aggregate into macroscopic cones and conical stromatolites. We investigated the uptake and storage of inorganic carbon by cone-forming cyanobacteria from Yellowstone National Park using high-resolution stable isotope mapping of labeled carbon (H13CO3-) and immunoassays. Observations and incubation experiments in actively photosynthesizing enrichment cultures and field samples reveal the presence of abundant cyanophycin granules in the active growth layer of cones. These granules are ultrastructurally heterogeneous and rapidly accumulate newly fixed carbon, storing about 20% of the total particulate labeled carbon after 2 hours of incubation. These experiments demonstrate an unexpectedly large contribution of PEP carboxylase to carbon fixation, and a large flow of carbon and nitrogen toward cyanophycin in thin filamentous, non-heterocystous cyanobacteria. This pattern does not occur in obvious response to a changing N or C status. Instead, it suggests an unusual interplay between the regulation of carbon concentration mechanisms and accumulation of photorespiratory products in cone-forming cyanobacteria.