Date of Award

Spring 1983

Project Type


Program or Major


Degree Name

Doctor of Philosophy


Novel aspects of morphology and ultrastructure were examined in two species of cyanobacteria. A complete, three-dimensional, ultrastructural reconstruction of the unicellular cyanobacterium Agmenellum quadruplicatum was carried out by means of high-voltage electron microscopy of thick sections and computer-aided reconstruction of serial thin sections. The photosynthetic thylakoid system consisted of 3-6 membrane sheets that traversed and were parallel to the long axis of the cell. These sheets were arranged as an anastomosing network of concentric shells. They coalesced and approached the cytoplasmic membrane at three peripheral loci along the entire length of the cell. The central cytoplasm of the cell was completely surrounded by thylakoids; this appeared to be a true form of compartmentalization in a prokaryotic organism. The thylakoid membranes clearly interconnected with the cytoplasmic membrane at several locations within the cell. Some of the various intracellular inclusion bodies were always peripherally located, while others were always centrally located. The detailed three-dimensional arrangement of subcellular features was remarkably consistent from one cell to another.

The morphology and ultrastructure of the branching, filamentous (stigonematalean) cyanobacterium Mastigocladus laminosus were examined with conventional light and electron microscopy. The vegetative morphology and ultrastructure of M. laminosus were similar to those of Fischerella ambigua, the only stigonematalean cyanobacterium examined in detail to date. The ultrastructural characteristics of mature heterocysts in M. laminosus were distinctly different from those of other cyanobacteria; the former lacked certain extra wall layers, lacked cyanophycin-like plugs, contained large numbers of closely packed intracytoplasmic membranes, and contained a previously unreported type of inclusion body. The heterocyst differentiation process also differed from that seen in other cyanobacteria; the earliest events involved internal changes rather than external and bundles of stacked, lamellar membranes were formed. These results showed that heterocyst differentiation and ultrastructure in different genera of cyanobacteria vary more widely than has been thought to be the case. M. laminosus was also shown to undergo aging and morphogenetic processes analogous to some of those known to occur in eukaryotic organisms.