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Beyond Recycling: A Closer Look into the Association Between Algae and Bacteria

Charles Davis1, Dr. Deana Erdner2

Rice University1, University of Texas Marine Science Institute2

Although phytoplankton and bacteria exist in two separate domains of the phylogenetic tree of life, the two have not only evolved together but also lived in close association over millions of years. Bacteria in ocean water ecosystems are found as free-living organisms, as organisms attached to detritus, or as algae associated bacteria. Although it is known that phytoplankton and bacteria live in close association, the nature of the relationship between the two requires more analysis. Previous studies in the Erdner lab have shown that a bacterized (xenic) culture of the toxic dinoflagellate Alexandrium tamarense experience higher growth rates, cell yield, cell volume, and toxin content compared to an axenic sub-culture. To test whether bacteria could “rescue” the axenic strain, bacterial communities from the xenic culture were added to the axenic culture. The resulting “rexenic” strains were monitored at 9 and 15 months after inoculation. Even though the reintroduction of the bacteria to the algae was shown to “rescue” the culture this effect was observed over a long 15-month period. One of the two rexenic cultures physiologically resembled their xenic parent at end of the 15-month period, and the other rexenic culture was healthier than the axenic parent but not as much as the other rexenic culture. This study examines how the composition of these free and attached communities change over time. To determine community composition, we sequenced the V4 region of the 16S RNA gene as a taxonomic marker. The sequence data was processed using the program QIIME. This consisted of quality filtering of the sequences, clustering into Operational Taxonomic Units (OTUs) at 97% sequence similarity, removal of chimeric sequences, and rarefaction of samples to an equal sequence depth. There were differences between free and attached communities in both the xenic and rexenic samples. The attached communities’ composition initially resembled the xenic parent, but diverged over time. Since one of the two rexenic cultures resembled the xenic parent at the end of the 15-month growth period, we conclude that bacterial communities can have different composition yet still have similar effect on the physiology of the algae.

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