Filamentous Cyanobacteria as a Prototype of Multicellular Organisms

Abstract

Filamentous cyanobacteria belong to the oldest organisms on our planet. Many cyanobacteria exist in the form of trichomes, i.e., cell chains comprising hundreds of cells connected by intercellular interactions. Under deficiency of environmental nitrogen, the cells in trichomes of some cyanobacteria undergo specialization to perform separate functions of oxygenic photosynthesis and nitrogen fixation. Thus, the trichome transforms into a complex organism (complex system), in which vegetative cells and the heterocysts exchange with photosynthetic and nitrogen fixation products. The transmission of metabolites may proceed via the periplasmic space or through the special contact structures called microplasmodesmata, septosomes, septal contacts, or nanopores. In filamentous cyanobacteria, the storage and transmission of energy at the cellular level is accompanied by electrical processes occurring in cell membranes. Theoretical and model analysis of extracellular currents induced by the local illumination in trichomes of Phormidium uncinatum showed that the trichomes are cell associations organized into unified cables capable of transferring energy along the trichome. From the viewpoint of modern molecular genetics, filamentous cyanobacteria showing the distribution of functions between neighboring cells are the prototype of a multicellular organism and a convenient model for elucidating the regulatory mechanisms of multicellularity, which, apparently, appeared more than once during the evolution in different phylogenetic groups, including bacteria, fungi, algae, and plants.