Supervisors: TÜ MRI professor Ain Heinaru ja TÜ MRI vanemteadur Eve Vedler.
Opponent: Professor Kornelia Smalla, Julius Kühn'i Instituut, Föderaalne Kultuurtaimede Uurimiskeskus, Epidemioloogia ja Patogeenide Diagnostika Instituut, Braunschweig, Saksamaa.
The Baltic Sea is considered one of the most polluted reservoirs in the world, which is a result of accelerated shipping traffic and intensive human activity in its catchment area. Oil and oil spills, along with eutrophication, constitute the main threats to the Baltic Sea ecosystem because of the large amount of oil used, transported and stored in the region. In case of oil pollution, toxic aromatic compounds like benzene, toluene and naphthalene get into the water thereby endangering the vulnerable ecosystem of the Baltic Sea. Bioremediation, removal of contaminants by using microorganisms, is thought to be an effective and promising technology to cope with the problem of environmental pollution. Bacteria have adapted to degradation of aromatic compounds through evolvement of different catabolic (degradative) pathways. Genes encoding the enzymes that form these catabolic pathways are located either on the chromosome or on separate DNA molecules termed plasmids within the bacterial cell. Plasmids contribute to horizontal transfer of catabolic genes from one bacterial cell to the other through establishment of physical contact between two bacterial cells, which facilitates the spread of degradative capabilities within a bacterial community. Despite the assured importance of catabolic plasmids for bioremediation, and years of research on the bacterial community structure of the Baltic Sea, the study of the Baltic Sea plasmid community has so far been neglected.
The present work was dedicated to investigation of catabolic plasmids that reside in cultivable bacteria of the Baltic Sea and in its catchment area. Particularly, the work was focused on identification and taxonomy of plasmids, characterization of their architecture and genetic content.
Ten (10) out of sixty one (61) plasmid-bearing bacterial strains isolated from Baltic Sea water were found to carry plasmids belonging to the famous IncP-9 family of catabolic plasmids. Plasmids of this group have also been detected in all Baltic Sea water samples using a cultivation-independent approach that is based on DNA analysis of the total bacterial community. In addition, IncP-9 plasmids were isolated from the Baltic Sea catchment area, from polluted soil and river water samples taken near the oil shale mining and processing industry in Estonia. During separate laboratory experiments and in conjunction with the case study under natural conditions in the environment, the horizontal transfer of these plasmids has been detected. As a result, the recipient strains gained the ability to degrade aromatic compounds and adapted to toxic environmental conditions. The IncP-9 plasmid family can be considered as an important plasmid group for keeping and dissemination of catabolic genes in nature. The present work brings this plasmid group forward as proper candidates for development of bioremediation technologies.
Besides the IncP-9 plasmid family the work concerns new types of catabolic plasmids. Most of the plasmids highlighted in this study, did not fall within well-known plasmid families, which are usually associated with bacterial degradative capabilities, referring to the unique genetic composition of the Baltic Sea catabolic plasmids. Analysis of the plasmid pD2RT, a new type of catabolic plasmid that enables the host bacterial strain Pseudomonas migulae D2RT to degrade toluene, revealed its close relatedness to the plasmids of plant pathogenic bacteria. Thus, plasmids could be considered as universal carriers and propagators of a variety of genes in nature that may encode distinct functions.
The present research provides valuable information on the degradative bacterial strains isolated in order to use them in the development of effective bioremediation technologies for the Baltic Sea and its surrounding area.