UC San Diego

The majority of the deep ocean is an extreme environment characterized by low temperature and high pressure, which drives bacteria to evolve unique adaptations in the deep sea. Analyses of cold-adapted microbes, psychrophiles, have suggested that their adaptations, which increase fluidity in membrane composition are similar to the adaptations of high-pressure-adapted microbes, piezophiles. To determine the adaptation mechanics of Antarctic seawater microbes, I performed adaptive laboratory evolution experiments using microbes from Antarctic surface seawater samples, by gradually increasing the incubation pressure of the microbes. ALE experiments were carried out with both the original microbial communities and bacterial strain isolates from the samples, by gradually increasing the incubation pressure of the microbes. This paper presents the evaluations of changes in community structures through the ALEs and characterizations of the strains isolated at high pressure.

In all the ALEs, I observed decreased growth rates at high pressure for both microbial communities and isolates, as well as decreased diversities within microbial communities at high pressure, suggesting that high pressure hinders microbial growth. However, further growth analysis of Psychrobacter isolate ALE at 50 MPa after 6 rounds of transfers, surviving bacteria were capable of growing better at a higher pressure compared to the ancestral strains. Genome resequencing of the Psychrobacter strains suggested that the bacteria could have possibly adapted to growing at a higher pressure through mutations of proteins coding for transmembrane transport, membrane proteins and peptidoglycan production. In conclusion, this thesis project presents primary analysis that shows psychrophilic bacteria were capable of adapting to higher pressure environments, which might suggest that bacteria adapted to both low temperature and high pressure environment, such as the deep sea, could have first evolved to become psychrophilic which in turn enabled them to adapt to high pressure.