Diversity shifts and crude oil transformation in polluted microbial mat microcosms

Diversity shifts linked to crude oil transformation were studied in microbial mats developed under laboratory conditions using a molecular approach and basic biomass determination (protein and predominant pigment content). For this purpose two different microcosms were established, one simulating a four-day black tide and the other trying to reflect the colonisation of contaminated black sediment by the indigenous populations of Ebro Delta microbial mats inoculated over the polluted substrate. The crude oil used as a contaminant was Casablanca, a light oil with low viscosity and low sulphur content similar to Arabian light. In the first microcosm, Microcosm I, neither crude oil degradation nor changes of total and phototrophic biomass were detected. Nevertheless, DGGE profiles showed noticeable changes that suggested a slight increase in the diversity of the bacterial populations, assuming that the number of bands was directly correlated to the diversity of species. Furthermore, when attention is focused on the Chromatiaceae family, the same behaviour was observed in the pigmented zone, while in the black sediment diversity diminished over time. Cyanobacteria exhibited a decrease in the diversity in both depths analysed. In the second microcosm, Microcosm II, small changes to the aliphatic hydrocarbon fraction were observed in the initial millimetres of the contaminated black sediment. An increase in the total and phototrophic biomass was observed over time and when DGGE patterns were analysed, shifts were observed and the final consequence was a decrease in the diversity of total bacterial populations. The same trend was observed for Cyanobacteria and for the Chromatiaceae family. Despite the pressure on indigenous bacterial populations from Ebro delta microbial mats, they can grow after a short black tide and are able to successfully colonise contaminated sediment to ultimately form a well-established microbial mat. © 2004 Taylor & Francis Group, LLC.