Selection of oil-tolerant cyanobacteria on oil polluted sediments
Abed, Raeid M.M.; Golubic, Stjepko
An experimental approach was used to explore the identity of oil-tolerant cyanobacteria combining direct microscopy and Denaturing Gradient Gel Electrophoresis (DGGE). Microbial mat inhabiting cyanobacteria were grown over sediment in an open mesocosm setting (aquarium). Cultures were started from a mixed inoculum of mats from three different geographic locations, exposed to conditions imitating oil spill, with an oil-free control. The analyses by microscopy and denaturing gradient gel electrophoresis were performed at the start of the experiment and after 2, 4 and 8 months of growth. Oil contamination resulted in development of a compact cohesive mat, different from the fluffy, loosely attached mat developed over the oil-free sediment. Both, direct microscopy and DGGE, documented a significant shift in composition of cyanobacterial community over time, favouring morphotypes with narrow trichomes; however, the change in oil-free and oil-contaminated cultures took different directions. The relatively large Oscillatoria-like cyanobacteria of the initial phase of the mat development yielded Leptolyngbya-like morphotypes, with bands that clustered with Halomicronema excentricum and a sequence identified as Leptolyngbya sp. respectively, which persisted over 8 months of incubation. The oil-exposed mats were dominated by organisms from the same Halomicronema excentricum cluster, which shared the dominance with two different phylotypes, one related to Phormidium minutum D5 sequence, and another one close to Phormidium ambiguum M71. After 8 months, two cyanobacteria phylogenetically close to Halomicronmena excentricum and Phormidium minutum overtook all others. DGGE showed clearly the replacement of all original community members as well as changes in the composition of cyanobacterial community over time in both oil-exposed and oil-free settings. The results indicate that thin filamentous cyanobacteria contain a group of opportunistic microorganisms that tend to prevail after perturbations in ecosystems. This group may include oil-tolerant cyanobacteria that dominate the mats immediately after oil spill events.