Diversity and microbial community structure at a former military ranges in Vieques (Puerto Rico)

Abstract

The eastern part of the island of Vieques (Puerto Rico) was subjected to live fire bombing by the US Navy and Marine Corps for approximately 60 years, leaving multiple unexploded ordnances in terrestrial and marine environments. The microbial community structure under different redox conditions were examined in soil and marine sediment samples exposed to 2,4-Dinitrotoluene (2,4-DNT) and Hexahidro-1,3,5-Trinitro-1,3,5- Triazina (RDX). Samples from the former training ranges were obtained and separately subjected to four treatments (no addition of explosives, 2,4-DNT or RDX, addition of 2,4-DNT or RDX and a carbon source, and a killed control with 2,4-DNT or RDX and a carbon source) under two redox conditions for four (4) weeks (oxic and sulfate-reducing). The influence of explosives on the bacterial community structure was analyzed by correspondence analysis (CA) of terminal restriction length polymorphism (T-RFLP) profiles and through the cloning and sequencing of 16S rRNA genes. Soil microbial communities exhibited the metabolic capability to mineralize 2,4-DNT under aerobic conditions as evidenced by the decreased of 2,4-DNT coupled with the stochiometric production of nitrite. Specific changes in microbial community during the active degradation process of 2,4-DNT (day 8 to 16) suggest an enrichment of possible degraders. Additionally, comparison of 2,4-DNT amended and un-amended clone libraries from day 8, showed an increment in the Comamonadaceae family (29 to 44%) in amended systems. The recovered Comamonadacea sequences were closely related to Variovorax genus. Finally, in-silico T-RF profiles of Comamonadaceae family are also dominant in T-RFLP profiles from 2,4-DNT amended microcosms. In contrast, soil concentration of RDX in aerobic microcosm did not decrease during the experiment but significantly changed the microbial community T-RFLP profile, even more dramatic than 2,4-DNT treatments. In marine sediments, the persistence of RDX did not influence the microbial structure. Under sulfate reducing conditions, 2,4-DNT and RDX rapid decreases on soil and marine sediment microcosms even in abiotic controls, suggesting that under this conditions explosives are degraded or become less bio-available thus decreasing its impact on the ecosystem. It was observed as well that the addition of RDX to aerobic soil microcosms decreased the relative abundance of OTUs of the fragments related to the Commamonadaceae family. Therefore mix contaminations with RDX and 2,4-DNT in aerobic soil could affect the 2,4-DNT degrading community and thus interfering with its natural attenuation. Microbial community analysis provides key ecological criteria’s to better understand the fate of pollutants and to identify indigenous microbial potential to assist environmental restoration.