Trophic Interactions among Marine Microbes in Oil-contaminated Seawater on a Mesocosmic Scale

Abstract

We studied the effects of oil spills on the trophic interactions among microorganisms in marine environments on a mesocosmic scale. When Bunker-A oil or kerosene was introduced into the mesocosm, bacterial numbers increased for the first 3 days, then decreased with a concomitant propagation of either heterotrophic nanoflagellates (HNFs) or viruses. We hypothesized that the sensitivity of HNFs to each oil resulted in this trophic relationship. To test this hypothesis, the rates at which HNFs grazed on bacteria were examined by conducting microcosm experiments in the presence of one of these oils. The rates were markedly reduced by kerosene but slightly increased by Bunker-A oil. The removal of HNFs by filtration led to a fluctuation in the numbers of bacteria and viruses that was quite similar to that in the mesocosm with kerosene. We conclude that the inhibitory effect of oil components on unicellular microorganisms is one of the key factors controlling the microbial food-web in oil-contaminated seawater.