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Bioconvection Patterns In Cultures Of A Calcifying Alga.   S.J. Chen, and P.J. Duke.  Dept. of Orthodontics, Dental Branch, University of Texas Health Science Center, Houston, TX.

    Bioconvection is fluid motion resulting from the collective behavior of directionally swimming microorganisms. Uneven density distribu-tions in surface accumulations of upswimming microorganisms produce overriding instabilities, with subsequent downward plummeting of fluid and cells. These downflows are visible with the naked eye, in the case of algal cells, as green striations or plumes. Between plumes are regions of upwelling fluid with fewer cells. From above, downflowing regions appear as green dots, surrounded by lighter green areas of upwelling fluid. Pattern formation is associated with particular culture conditions (e.g. density, height of fluid).   The objective of the current study was to determine conditions affecting bioconvection patterns in cultures of a calcifying green alga. Stock cell cultures of Pleurochrysis carterae,  Plymouth strain 136, were grown in F/2 medium (18^oC) in t-flasks, until bioconvection occurred as indicated by plume formation. To determine effects of medium depth,  5, 7 or 10 ml of stock solution were placed in 35mm culture dishes. For density studies, the solution was concentrated by centrifuging the cells and resuspending them in half as much medium.  Prior to image collection, the bioconvection pattern in each dish was destroyed by gentle back and forth movement of the dish, and the time to reform the pattern was determined. Phases of pattern formation were photographed, and number of dots/cm² determined. Pattern reformation begins with the establishment of “rolls” which appear from above as dark wavy lines. Rolls separate into dots, each of which is a column of downflowing fluid and cells. The depth of the culture medium (5,7, or 10 mm deep) influenced the number of columns formed/cm²: 27 vs.20 vs.2). Time to reform pattern and time to pattern stability were shortest in cultures with the highest density.  Studies of the swimming behavior of microorganisms in space or on the ground requires that bioconvection be taken into account, because cells that swim up must come down at some point.

(Support:  NIH T32DE015355.)