Investigating relationships between hydrocarbonoclastic bacteria and micro-algae

Abstract

Crude oil spills damage marine ecosystems due to the potentially toxic nature of the petrochemical hydrocarbon constituents and their recalcitrance to degradation. Polycyclic aromatic hydrocarbon components (PAHs) are one group of hydrocarbons in crude oil that are of particular concern due to their genotoxicity and potential to bioaccumulate. Their potential to cause damage in marine environments can be mitigated by the presence and activities of hydrocarbonoclastic bacteria. The phycosphere of marine eukaryotic phytoplankton (micro-algae) has recently been shown as an important biotope where hydrocarbonoclastic bacteria can be found, and the association between these organisms is largely unexplored. This thesis presents new insight into the relationship between these organisms by performing enrichment experiments with crude oil and individual hydrocarbon substrates, as well as wholegenome analysis of an algal-associated hydrocarbon-degrader, and using molecular probes for the in situ visualization (by CARD-FISH) and whole-community analysis (by Flow-FISH) of hydrocarbonoclastic bacteria associated with laboratory cultures and field samples of micro-algae. Results demonstrated variations in the tolerance of different diatom species to PAHs, and that intermediate metabolites formed from the partial biodegradation of PAHs can be more harmful to diatoms compared to that of their parent PAH compounds. Thalassiosira pseudonana presence enhanced phenanthrene dissolution and PAH-degrading bacteria formed cell clusters in EPS aggregates. The genome of an obligate hydrocarbonoclastic bacterial species (Polycyclovorans algicola) found associated with marine micro-algae possessed genes involved in cell communication, horizontal gene transfer and nutrient sharing that may play an important role in the organism’s association with its eukaryotic host cells. Indeed, these interactions are likely to allow these hydrocarbonoclastic bacteria to be supported on the phycosphere of micro-algae in the absence of petrochemical exposure. Microcosm experiments with field samples of micro-algae and bacterial consortia showed that crude oil biodegradation, in particular the PAH fraction, was enhanced compared to that by the free-living bacterial community, and revealed certain groups (e.g. members of the order BD7-3) that had not previously been reported to become enriched in the presence of crude oil. Using Flow-FISH, epibiotic members of the genus Marinobacter were found associated with micro-algae in sea surface field samples, and CARD-FISH was used to show this same group associated with the soft tissues of the coral Lophelia pertusa from polyp samples collected from different coral mounds in the deep North Atlantic.