The responses of indigenous oil-degrading bacteria to oil exposure in Phragmites australis-dominated marsh soil: a mesocosm study

Abstract

This study examined impacts of Macondo MC252 oil from the Deepwater Horizon spill on oil-degrading bacteria in fresh and oligohaline marshes dominated by the common reed Phragmites australis (Cav.) Trin. ex Steud. and correlated microbial changes to soil variables. We hypothesized that indigenous oil-degrading bacteria and soil characteristics in the marshes effectively respond to Macondo oil. We tested this hypothesis with a greenhouse mesocosm study. Weathered and emulsified Macondo oil was applied to P. australis sods at different rates (0, 4, 8, 12, and 16 L m−2). Populations of oil degrading bacteria containing alkane monooxygenase and PAH-ring hydroxylating dioxygenase alpha subunit (PAH-RHDα) genes, the expression of these genes, soil respiration rate, residual total petroleum hydrocarbon, redox potential (Eh), as well as dissolved organic carbon (DOC) and its aromaticity index of SUVA254 were measured in soils 2 months after oil addition. Oil exposure generally increased oil-degrading bacterial populations, in particular Gram-positive (GP) PAH-degraders, soil respiration rate, DOC concentration, and aromaticity of DOC, but decreased Eh values. GP PAH-RHDα genes accounted for approximately 98% of total detected genes, and expression of these genes increased by a factor of 3 to 5 at various oil dosages. Both abundance and expression of GP PAH-RHDα genes significantly correlated to SUVA254 (P < 0.05). The study revealed that indigenous oil-degrading bacteria effectively responded to weathered Macondo oil in the P. australis marsh soil wherein GP bacteria with PAH-RHDα genes played a major role in biodegradation of PAH-enriched petroleum hydrocarbons.