Adaptation and population dynamics of Azotobacter vinelandii during aerobic biological treatment of olive-mill wastewater

Olive-mill wastewater (OMW) has a high organic and polyphenol content and i s resistant to biodegradation. Its disposal leads to a major environmental pollution problem in the Mediterranean basin. The detoxification of OMW fol lowing inoculation with Azotobacter vinelandii (strain A) was performed for two successive 5-day-period cycles in an aerobic, biowheel-type reactor, u nder non-sterile conditions. The phytotoxicity of the processed product was reduced by over 90% at the end of both cycles. To exclusively monitor the A. vinelnandii population in the reactor a most probable number-PCR approac h was employed and applied daily to serial dilutions of total DNA extracted from reactor samples. PCR sensitivity was independent of the presence of O MW or non-target DNA. The A. vinelandii population dynamics were successful ly monitored, showing an initial adaptation period, followed by a sharp pop ulation maximum on the fourth day of both cycles (1.6 x 10(8) and 9.6 x 10( 7) cells ml(-1) respectively), after a major phytotoxicity decline. N-2 fix ation rates were estimated using the acetylene reduction assay and reached a peak during the first 1-2 days of each cycle (36 and 29 nmol C2H2 ml(-1) h(-1) respectively). The data are consistent with an initial physiological adaptation phase, where the presence of phenolic compounds limits A. vinela ndii growth but stimulates N2 fixation, followed by a rapid growth phase as phytotoxicity declines. (C) 1999 Published by Elsevier Science B.V. All ri ghts reserved.