Clean 2008, 36 (7), 593 – 600 V. Krumins et al. 593 Valdis Krumins 1 Eun-Kyeu Son 1 Gediminas Mainelis 1 Donna E. Fennell 1 1 Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey, USA. Research Article Retention of Inactivated Bioaerosols and Ethene in a Rotating Bioreactor Constructed for Bioaerosol Activity Studies Available literature raises the possibility that airborne microorganisms are not just passively transported through the air, but may be metabolically active while “in transit”. To investigate the metabolic activity of bacteria suspended in air enriched with volatile growth substrates, a rotating bioreactor system was developed and tested which allows prolonged suspension of bacteria in air. The bioreactors were 0.32 m 3 stainless steel drums rotated at 1.3 revolutions per minute and equipped with on-board pressure, temperature and humidity transducers with real-time data collec- tion. Bioreactors were tested for retention of a volatile organic compound, ethene, and for suspension of an inactivated laboratory-generated Pseudomonas fluorescens bio- aerosol. At an initial ethene concentration of 560 ppmv the first order abiotic loss of ethene was 0.051 l 0.010 d –1 (average l standard deviation) or a half life of 13.6 days. Inactivated airborne bacteria (size mode 0.5 to 0.7 lm) remained suspended with a half life of 8.5 days. Thus, the bioreactors maintained bacteria suspended in the pres- ence of a volatile growth substrate for a time period theoretically sufficient to observe metabolic activity. Bioreactors will be used to test natural bioaerosols and active labo- ratory-generated bioaerosols during enrichment with volatile organic compounds to assess growth and substrate biodegradation. Keywords: Activity; Bacteria; Bioaerosol; Biodegradation; Bioreactor; Received: January 5, 2008; revised: April 4, 2008; accepted: April 12, 2008 DOI: 10.1002/clen.200800004 1 Introduction Microorganisms of anthropogenic, terrestrial or aquatic origin may be aerosolized by a variety of mechanisms, become airborne and can remain airborne for days and may even be transported between continents [1]. It is estimated that air contains approximately 10 4 to 10 5 bacterial cells per m 3 [2]. Airborne bacteria may exist as single cells or, more often, as aggregates of bacteria, fungal or plant detri- tus, and water, with typical particles sizes from 0.3 to 100 lm [3]. Most aerobiology research has focused on quantifying airborne microbial loads [e. g., 3] and elucidating the transmission or decay of pathogens or biowarfare agents [1, 3 – 10]. Generally, airborne microorganisms have been assumed to be resting or inactive [11] and, thus, the possibility that bacteria suspended in natural ambi- ent air can metabolize substrates, grow, or divide has not been con- sidered. Although ultraviolet radiation, drying conditions, and pres- ence of radicals can make the atmosphere a harsh environment, association with particles offers bacteria some protection [1, 3]. Fur- thermore, aerial environments, in addition to supplying oxygen, transiently contain moisture, nutrients and many organic com- pounds that are known to support microbial growth. Global emis- sions of non-methane volatile organic compounds (VOCs) are esti- mated to be 142 Tg per year [12]. Concentrations of various VOCs have been widely measured in outdoor air and vary substantially on a geographical, temporal and seasonal basis. Baker et al. [13] measured VOCs in 28 US cities. Ethane varied from 560 to 8740 pptv; combined benzene, toluene, ethylbenzene, and xylenes was 1 to 4 ppbv; and isoprene was up to 2590 pptv. Schneider et al. [14] reported benzene at up to 3 ppbv and toluene at up to 7 ppbv in some urban environments. Concentrations of car- boxylic acids may be higher, for example 8500 ppmv acetic acid was reported [15]. Ethene was reported to be from less than 1 ppbv to greater than 60 ppbv [13, 16, 17]. The fine particulate matter (PM) found in air, PM10 and PM2.5 (diameters less than 10 and 2.5 lm, respectively), consist of 20 to 50% hydrophobic and hydrophilic organic compounds, many of which are bacterial substrates [15]. Furthermore, VOCs can sorb onto particles or dissolve into water droplets, thus increasing the concentration of these substances in PM. Geller [18] found that partitioning of background aerial VOCs into liquid bacterial growth media resulted in dissolved organic car- bon (DOC) concentrations comparable to those found in oligotro- phic aquatic environments capable of supporting microbial metab- olism. It is possible, therefore, that atmospheric background levels of VOCs either directly adsorbed to bacterial cells or to hydrome- Correspondence: Prof. D. E. Fennell, Rutgers University, School of Envi- ronmental and Biological Sciences, Department of Environmental Scien- ces, 14 College Farm Road, Room 231, New Brunswick, NJ 08901, USA. E-mail: fennell@envsci.rutgers.edu Abbreviations: DOC, Dissolved organic carbon; PM, Particle Matter; VOC, Volatile organic compounds i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com