The Production of Non-Methane Hydrocarbons by Marine Plankton

The oceans are a small source of non-methane hydrocarbons (NMHC), a suite of volatile organics whose chemical destruction mechanism by reaction with hydroxyl radical can significantly affect the oxidation capacity of the atmosphere. Little is known about the water column cycling processes that constrain this source; previous work has established a photochemical source for many alkenes, and a phytoplanktonic source for isoprene. The focus of this thesis was to gain further insight on marine microbiological cycling of NMHC. This included investigations on two main themes. The first was the effect of different physiological conditions on phytoplanktonic isoprene production. A variety of phytoplankton were examined for the ability to produce isoprene. All were found to have constant isoprene production rates per cell during exponential growth, with decreasing rates as the populations senesced. A positive allometric relationship between isoprene production rate and cell volume was found; highest production rates were found for the largest cell tested, Emiliania huxleyi, and lowest rates for Prochlorococcus, the smallest. Isoprene production in Prochlorococcus was found to be a function of light intensity and temperature, with patterns similar to the relationships between growth rate of this species and these environmental parameters. The second theme investigated was the effect that heterotrophic marine plankton might have on NMHC cycling. We detected no clear production or consumption of any NMHC, except isoprene, from any of the phytoplankton or other organisms tested. The heterotrophic bacteria examined had no detectable effect on isoprene production per Prochlorococcus cell in a dual-species culture, but a temporary production of isoprene was detected from bacterial cultures grown in organically-enriched media. Nanoflagellate grazing by Cafeteria roenbergensis on Prochlorococcus had no detectable effect on NMHC cycling except to control the total phytoplankton counts, and thus total isoprene production. Besides controlling phytoplankton counts, phage lysis of Prochlorococcus had no detectable effect on NMHC cycling except to decrease isoprene production per Prochlorococcus cell during the latent period of infection. Any other effect these particular organisms may have on NMHC cycling likely involves other processes, such as photochemistry.