Effect of Crude Oil on Light Attenuation and the Resulting Impacts on Marine Microbial Phototrophs and Heterotrophs

Oil spills spread over the surface of the seawater which cause the attenuation of light passing through it. Then, we showed that crude oil preferentially attenuates the shorter wavelengths of photosynthetically active radiation. Therefore, we proposed that this change in the spectrum and reduction in light intensity would affect microbial phototrophic community composition, which in turn would alter the heterotrophic community and potentially the rate of hydrocarbon degradation. Experiments were conducted using natural seawater with a crude-oil layer (but not in contact with), a no-oil control, oil spectrum and standard light-spectrum. Treatments included continuous light and light: dark regimes. The impact of oil in phototrophs (Synechococcus and Thalassiosira weissflogii) was measured and their alkane production to sustain hydrocarbonoclastic bacteria (HCB). Chlorophyll a and DNA concentrations were measured as a proxy for biomass. DGGE analysis of bacterial 16S rRNA and psbA genes was also performed. Miseq Illumina sequencing analysis was performed to measure the abundance of bacteria and phototrophs. A qPCR was also performed to quantify phototrophs and HCB and UPLC analysis was done to quantify phototroph`s pigments. With continuous light, cyanobacteria were the most abundant microorganism whereas with light: dark regime, diatoms were the most abundant. The bacterial and phototrophic community had a greater variability with light: dark regime. Synechococcus spp. and T. weissflogii produced different pigments in response to changes in light intensity and spectrum. Synechococcus was able to sustain the growth of Marinobacter hydrocabronocasticus. Thus, we have shown that, in addition to oil having a direct impact on the microbial community in seawater, it has an indirect effect by altering the spectrum and intensity of light.