This thesis investigates how different pigmented and non-pigmented Pseudoalteromonas strains are able to cope with, and respond to, intense light stress. Since pigments are known to retain a specific fraction of the electromagnetic visible light spectrum, the withdrawn energy is potentially available for the pigment producing organism. Here, both pigmented and non-pigmented Pseudoalteromonas strains were incubated on marine broth agar plates for 48 hours in bright light as well as complete darkness. Subsequently, the bacterial biomass was harvested by scraping-off the cell paste and extracting the biomass with organic solvents. In order to qualify and quantify this potentially light/energy harvesting effect, we first examined the crude extract derived bulk fatty acids (B-FAs) for differences between the seven pigmented and seven non-pigmented Pseudoalteromonas strains grown in the presence and absence of light. Distinct differences in the B-FAs composition were most obvious between the highly pigmented Pseudoalteromonas strains (i.e., #1783, #1784, #2A1 and #1772) derived from shallow waters, and the non-pigmented deep-sea strains (i.e., #2179, #2165, #2201). Particular the need for trans-monounsaturated FAs (trans-MuFAs) in highly pigmented Pseudoalteromonas species, reaching almost 50% of the total B-FAs composition, raised our attention. Furthermore, the light-dependent increase of furan FAs (FuFAs) among all investigated Pseudoalteromonas species was likely correlated to the release of the reactive oxygen species (ROS) hydrogen peroxide (H2O2). That is, all tested crude extracts of all tested Pseudoalteromonas species, compared to the control samples, showed an accelerated rate of H2O2 production. However, we also observed that even negative control samples, when stressed with light for a few hours, showed a significant increase of H2O2 levels, which may explain the need for the up-regulation of FuFAs within all Pseudoalteromonas species when incubated under light stress for up to 48 hours. However, among all tested species, the crude extracts and pigments of the red appearing P. rubra strain showed a remarkable quick response in H2O2 production of less than one hour after the illumination with white light was initiated. Interestingly, the purple pigment violacein isolated from P. luteoviolacea did not show a light-dependent H2O2 production. Finally, we tested the crude extracts, fractions and pure compounds of the same pigmented and non-pigmented Pseudoalteromonas strains against different environmental pathogens for their antibacterial activity. Particularly the highly pigmented P. rubra and P. luteoviolacea extracts showed remarkable antimicrobial activity. Their antibacterial activity was surprisingly strong under dark conditions and comparably lower under full light conditions. We attribute this observation to the damaging effect of light to the light-absorbing pigments. However, the antimicrobial activity was not always caused by the presence of pigments. For example, the pigment violacein isolated from P. luteoviolacea did not show any activity whereas the different prodigiosin derivates isolated from P. rubra clearly showed a potent activity.
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