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), Rhodospirillum, Chromatium, Rhodopseudomonas are free living nitrogen fixing bacteria. Aspergillus niger causes Aspergillosis and produce citric acid, oxalic acid etc.