The rice-field isolate of
Gloeocapsa sp. grew aerobically at the expense of N
2 as a nitrogen source and showed DCMU-sensitive aerobic nitrogenase activity under photoautotrophic conditions. The two pre-emergence rice- field 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 (Mac
r and Bas
r) without affecting aerobic diazotrophy. The nonrevertible Mac
s Bas
s strain resulting from Ethidium bromide treatment of Mac
r Bas
r Gloeocapsa strain stopped growth, N
2-
fixation and photosynthetic O
2 evolution when inoculated in growth medium containing 10 or 20μg ml
-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 (Mac
r Bas
r) strain dependent on photosynthesis for aerobic diazotrophy and that the genes for the two herbicide-resistant phenotypes (Mac
r Bas
r) are possibly plasmid-borne. The present findings will have a significant bearing on the future of cyanobacterial biotechnology in agriculture.
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