Potential role of bacterial communities in the bloom formation of harmful dinoflagellate Prorocentrum after an oil spill

Abstract

Introduction The association between phytoplankton blooms and oil spills is still controversial despite numerous studies. Surprisingly, to date, there have been no studies on the effect of bacterial communities exposed to crude oil on phytoplankton growth, even though crude oil leads to variation in bacterial communities, and this variation can affect phytoplankton growth and species composition.

Results and Discussion In this study, to investigate the impact of altered bacterial communities exposed to crude oil on the growth of dinoflagellate, we exposed free-living bacteria isolated from a Prorocentrum texanum culture to crude oil (100 ppm) for a month, and then investigated the growth change in P. texanum after addition of these oil-treated bacteria. As a result, the growth rate and yield of P. texanum in bacterial treatment was clearly enhanced, compared to control. To gain more direct evidence for the role of oil-degrading bacteria in bloom formation, we isolated oil-degrading bacteria from sediment samples collected from oil-contaminated sites after the Texas City "Y" oil spill, and investigated variation in dinoflagellate growth after co-culture with single bacterial isolates. A total of seven oil-degrading bacterial isolates were established, and two bacterial isolates clearly enhanced the growth rate and yield of six dinoflagellate cultures; axenic Amphidinium carterae and Peridinium sociale, and xenic Karenia brevis, P. gracile, P. minimum, and P. texanum. To determine whether or not these bacteria can enhance dinoflagellate growth by releasing nutrients, nutrient limited medium was prepared by removing each one of the components (nitrogen, phosphorous, trace metals or vitamins), and C1-T3 and E1-Gal-T2 were inoculated into each nutrient limited media, containing A. carterae and Pe. sociale. These two bacterial isolates greatly enhanced the growth rate and yield of the two dinoflagellates, regardless of any nutrient-limited media. Together with these findings, oil-degrading bacteria may enhance the growth of dinoflagellates and this growth enhancing activity may not be derived from nutrients released from the bacteria.