Evidence for genetic control of herbicide resistance in a rice-field isolate of Gloeocapsa sp. capable of aerobic diazotrophy under photoautotrophic conditions.

The rice-field isolate of Gloeocapsa sp. grew aerobically at the expense of N2 as a nitrogen source and showed DCMU-sensitive aerobic nitrogenase activity under photoautotrophic conditions. The two pre-emergence ricefield herbicides Machete and Basalin strongly inhibited growth, photosynthesis, respiration and nitrogenase activity in Nostoc muscorum in a concentration hardly inhibitory to these processes in Gloeocapsa sp. Ethidium bromide treatment of Gloeocapsa sp. resistant to growth inhibition by Machete and Basalin resulted in non-revertible loss of both herbicide resistant phenotypes (Macr and Basr) without affecting aerobic diazotrophy. The nonrevertible Macs Bass strain resulting from Ethidium bromide treatment of Macr Basr Gloeocapsa strain stopped growth, N2fixation and photosynthetic 02 evolution when inoculated in growth medium containing 10 or 20 jig m1-1 of Machete, Basalin or both. These findings suggest that the rice field isolate of Gloeocapsa sp. is a naturally occurring Machete and Basalin resistant (Macr Basr) strain dependent on photosynthesis for aerobic diazotrophy and that the genes for the two herbicide-resistant phenotypes (Macr Bas') are possibly plasmid-borne. The present findings will have a significant bearing on the future of cyanobacterial biotechnology in agriculture.