Explanation

Anabaena, nostoc, Aulosira and Tolypothrix are free living nitrogen-fixing cyanobacteria, whereas Rhizobium sp, Frankia, etc, are symbiotic nitrogen-fixing bacteria and Azotobacter(aerobic), Clostridium(anaerobic), Beijerinckia (aerobic), Rhodosprillum, Chromatium, Rhodopseudomonas are free living nitrogen-fixing bacteria. Aspergillus niger causes aspergillosis and produce citric acid, oxalic acid, etc.