BACTERIAL-ALGAL PHYCOSPHERE INTERACTIONS: IS THERE AN EFFECT ON GROWTH AND LIPID PRODUCTION?

The phycosphere is defined as the region in which organic matter excreted by a phytoplankton cell are in high concentration relative to the surrounding environment. The phycosphere often contains specific bacterial communities that use the relatively high concentrations of nutrients for growth. Recent studies have demonstrated the potential for bacterial-algal interactions affecting microalgal growth and biomass accumulation. The purpose of this study was to evaluate how bacterial-algal phycosphere interactions alter the growth rates and lipid content of microalgae and identify secondary metabolites that are correlated with these changes.

To assess the effect of bacteria on microalgae, we investigated 11 co-cultures of a single strain of the diatom Phaeodactylum tricornutum with different species of bacteria isolated from its phycosphere. These co-cultures were compared to an axenic (bacteria-free) culture. Growth rates were determined for all cultures. Cultures that exhibited the highest and lowest growth rates relative to the axenic were tested for lipid/fatty acid analyses and compared. Bacteria were then re-isolated from these co-cultures and maintained axenically. The bacteria were added back to the axenic Phaeodactylum culture at the same concentration found in the original co-cultures. The new co-culture were grown in 2.5 L volumes. The growth rates, lipid content, and fatty acid profiles were compared between the new co-culture, original co-culture, and the axenic culture to isolate the effects of the bacteria. A time of flight- mass spectrometer system was used to identify differences in secondary metabolites between co-cultures that had a significant effect of bacteria: algae and the axenic culture.

For the exponential period of growth there is an average decrease of 32% in growth rate between the top and bottom performing co-cultures. Maximum doublings per day decreases by 46% from the top and bottom performing co-cultures. The co-culture that exhibited the highest growth rate was associated with bacteria from the Muricauda genus. The co-culture with the lowest growth rate was associated with bacteria from the genera Arenibacter. Interestingly both of these genera are from the same family of bacteria, the Flavobacteriaceae.

Most algal species have specific phycosphere associated bacteria, which can have beneficial or detrimental effects on their microalgal hosts. Interactions that increase growth rates and lipid production could improve the production efficiency of biofuels and increase production of essential fatty acids for human health supplements or agriculture/aquaculture feeds. Bioactive compounds that exhibit inhibitory properties could be used to mitigate the effects of harmful algal blooms in coastal waters and fresh water resources.

Supported by: Part of this work performed under the auspices of the U.S. Department of Energy at Lawrence Livermore National Laboratory under Contract DE-AC52- 07NA27344 and the TAMUCC Center for Coastal Studies by DOE-BER's Genomic Sciences Program under the LLNL Biofuels SFA FWP SCW1039-02 and the Center for Coastal Studies support.