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A polyclonal antibody against the potent hepatotoxic cyclic peptide microcystins and nodularins was used in conjunction with immuno-gold labelling to localize the toxins in three strains of cyanobacteria. Ultrastructurally, there were no major differences between unicellular Microcystis aeruginosa strain PCC 7820 (toxin-producing strain) and M. aeruginosa strain UTEX 2063 (non-toxin-producing strain), except that M. aeruginosa PCC 7820 cells had a sheath. The thickness of the sheath was about 12 nm and was distinguishable from the cell wall at the ultrastructural level only when the specimen was stained en bloc with uranyl acetate. Microcystins and nodularin were found in M. aeruginosa PCC 7820 and Nodularia spumigena strain L-575 respectively, but not in nontoxic M. aeroginosa UTEX 2063. In M. aeruginosa PCC 7820 cells, microcystin was found primarily in the thylakoid area and nucleoid, with smaller amounts in the cell wall and sheath. Only nonspecific labelling was found in other cellular inclusions, such as polyhedral bodies, cyanophycin granules and membrane-limited inclusions. In strain N. spumigena L-575, nodularin was found in both vegetative cells and heterocysts with a distribution similar to that in M. aeruginosa PCC 7820.

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The symbiosis between a bacterium and the West African rain forest yam Dioscorea sansibarensis is described for the first time at the ultrastructural level. The bacteria are harboured in glands which run the entire length of the pronounced ‘drip‐tip’ leaf acumenae of the host plant. Each acumen, which may be up to 12 cm long in very large mature leaves, contains from two to six bacterial glands. The glands are kidney‐shaped in cross section and contain numerous multicellular simple trichomes which arise from the epidermis of the gland floor and project into the lumen of the gland. The bacteria are Gram‐negative and variously rod, ovoid, and coccoid in shape. The bacterial cells contain mesosomes, polyhydroxybutyrate granules and large electron‐dense bodies. Bacteria‐free plants grow more slowly and produce one yellowish‐green leaf per node in contrast to the vigorous growth habit of infected plants, with two deep green leaves per node. Infected plants exposed to a variety of atmospheres containing acetylene, both in the light and in the dark, failed to reduce acetylene to ethylene, indicating that nitrogen fixation is not a function of this symbiosis.