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PHOTOTAXIS AND AEROTAXIS IN A CALCIFYING ALGA. W. Callejas1, L. Doan2, M. Marsh3, and P.J. Duke2.  1University of Houston- Downtown, 2Dept. of Orthodontics, Dental Branch, 3Dept. of Biochemistry and Molecular Biology, Medical School, University of Texas Health Science Center at Houston, Houston, TX.

   In previous studies, we determined that cells of Pleurochrysis cartarae do not start to move in a measurably oriented manner until a certain cell density is reached, concomitant with the start of bioconvection (con-vection due to movement of microorganisms.)  The process begins with the accumulation of cells at the surface of the medium, which in turn requires that the cells swim up. The objective of the current study was to determine if other taxes, specifically photo- and aerotaxis, were the cause of surface accumulation of cells. Because our experiment using this alga was deselected from spaceflight on ISS due to lack of videomicroscopy, macroscopic methods were used in these experiments. Cells from a stock cell culture of P.carterae, Plymouth strain 136,  were grown overnight in F/2 medium (18oC), then divided into 4 t-flasks.  After 24 hours of culture, a green halo of cells surrounded the air bubble in each flask, demonstrating aerotaxis.   For phototaxis, box covers, with only a small opening at the bottom of one side, were placed over the t-flasks as an obstruction of the light source. Cells in these cultures did not accumulate in any region of the culture, and cultures did not grow due to the lack of  light. The t-flasks were then placed horizontally in the incubator with the boxes covering all but the top 25% of the flask. The air bubble was located in the dark region of the flask. When the covers were removed, cells were found in the uncovered region of the t-flasks, demonstrating positive phototaxis. No cells accumulated around the air bubble, showing that phototaxis was stronger than aerotaxis.  Upswimming both in the lab and in nature is likely due to phototaxis and /or aerotaxis, not negative geo-or gyro-taxis.  This knowledge can be used to design spaceflight experiments, and the development of a macromethod obviates the need for videomicroscopy to see overall cell accumulations. Support: UTHSC Summer Research Program for High School Students 2004 to W. Callejas.