Mogami Yoshihiro, Yamane Akiko, Gino Atsuko, Baba Shoji A
Department of Biology, Ochanomizu University, Otsuka 2-1-1, Tokyo 112-8610, Japan.
J Exp Biol. 2004 Sep;207(Pt 19):3349-59. doi: 10.1242/jeb.01167.
Bioconvection is a result of the negative gravitactic behavior of microorganisms. When the top-heavy density gradient generated by gravitaxis grows sufficiently large, an overturning convection occurs leading to a formation of characteristic patterns, which involve highly concentrated aggregation of cells into extended two-dimensional structures. Although gravity is a crucial factor, few experiments have been done with reference to gravity as an experimental variable. In order to gain an insight into the hydrodynamic as well as biological dependence of the convective motion on gravity, we investigated changes in bioconvective patterns of Tetrahymena under altered gravity acceleration generated by a long-arm centrifuge. Bioconvective patterns recorded of three different cell strains (T. pyriformis, T. thermophila and its behavioral mutant, TNR) were analyzed quantitatively using space-time plot and Fourier analysis. For example, under subcritical conditions, when T. pyriformis (1.0 x 10(6) cells ml(-1)) was placed in a 2 mm-deep chamber, no spatial pattern was observed at 1 g. When the suspension was centrifuged, however, patterns began to appear as acceleration increased over a critical value (1.5 g), and then remained steady. The formation was reversible, i.e. the patterns disappeared again as acceleration decreased. Under supracritical conditions, i.e. when a suspension of the same density was placed in a 4 mm-deep chamber, a steady state pattern was formed at 1 g. The pattern spacing in the steady state was observed to decrease stepwise in response to step increases in acceleration. Fourier analysis demonstrated that for TNR the mean wave number changed almost simultaneously with step changes in acceleration, whereas the responses were less sharp in the wild-type strains. This may suggest that the locomotor phenotype of the cell, such as its avoiding response ability, has a crucial role in bioconvective pattern formation. These findings are discussed in relation to former theoretical studies.
生物对流是微生物负趋重力行为的结果。当由趋重力作用产生的上重下轻的密度梯度增长到足够大时，就会发生翻转对流，从而形成特征性图案，其中包括细胞高度集中聚集形成扩展的二维结构。尽管重力是一个关键因素，但很少有实验将重力作为实验变量进行研究。为了深入了解对流运动对重力的流体动力学以及生物学依赖性，我们研究了在长臂离心机产生的重力加速度改变的情况下，四膜虫生物对流图案的变化。使用时空图和傅里叶分析对记录的三种不同细胞株（梨形四膜虫、嗜热四膜虫及其行为突变体TNR）的生物对流图案进行了定量分析。例如，在亚临界条件下，当将梨形四膜虫（1.0×10⁶个细胞/毫升）置于2毫米深的小室中时，在1g重力下未观察到空间图案。然而，当悬浮液离心时，随着加速度增加超过临界值（1.5g），图案开始出现，然后保持稳定。这种形成是可逆的，即随着加速度降低，图案再次消失。在超临界条件下，即当将相同密度的悬浮液置于4毫米深的小室中时，在1g重力下形成了稳态图案。观察到稳态下的图案间距随着加速度的逐步增加而逐步减小。傅里叶分析表明，对于TNR，平均波数几乎与加速度的阶跃变化同时改变，而野生型菌株的响应则不太明显。这可能表明细胞的运动表型，如其回避反应能力，在生物对流图案形成中起着关键作用。结合以前的理论研究对这些发现进行了讨论。
