Effects of Crude Oil on Growth Rate and Variable Fluorescence of Marine Cyanobacteria

The effects of crude oil were studied in dose-response experiments measuring growth and photosynthetic physiology of three strains of the marine cyanobacteria, Synechococcus (an estuarine, coastal and an open ocean strain), and an open ocean strain of Prochlorococcus. Oil was added (0-250 ppm by weight) as freshly-made water-oil emulsions. Physiological reaction time to oil addition was quantified using variable chlorophyll fluorescence (F_v, F_v/F_m) following oil additions. Decrease in physiological state (measured by F_v) occurred almost immediately (within 2 minutes) following oil addition. Growth rates were measured over several days of oil exposure. For the three Synechococcus strains, all strains exhibited significantly decreased growth rates at 100 ppm and 250 ppm. The open ocean and estuarine strain also exhibited significant decreases in growth rate at 50 ppm, while the coastal strain only showed a significant decrease in growth rate at 100 ppm and above, suggesting some strain-specific differences in oil tolerance. However, a single dose response curve adequately fit all three species, demonstrating a fairly consistent overall negative relationship between oil concentration and Synechococcus growth across strains isolated from different habitats. Significant declines in growth rate measured over several days were consistent with significant decreases in F_v/F_m after only 2-4 hours, suggesting that variable fluorescence could be used as a rapid assay for oil effects on cyanobacteria. Results for Prochlorococcus will be considered in the context of the Synechococcus results.