Richter Peter, Helbling Walter, Streb Christine, Häder Donat-P
Institut für Biologie, Friedrich-Alexander-Universität, Erlangen, Germany.
Photochem Photobiol. 2007 Jul-Aug;83(4):818-23. doi: 10.1111/j.1751-1097.2007.00134.x.
Recently it was shown that the unicellular flagellate Euglena gracilis changes the sign of gravitaxis from negative to positive upon excessive radiation. This sign change persists in a cell culture for hours even if subsequently transferred to dim light. To test the ecological relevance of this behavior, a vertical column experiment was performed (max. depth 65 cm) to test distribution, photosynthetic efficiency and motility in different horizons of the column (surface, 20, 40 and 65 cm). One column was covered with a UV cut-off filter, which transmits photosynthetically active radiation (PAR) only, the other with a filter which transmits PAR and UV. The columns were irradiated with a solar simulator (PAR 162 W m(-2), UV-A 32.6 W m(-2), UV-B 1.9 W m(-2)). The experiment was conducted for 10 days, normally with a light/dim light cycle of 12 h:12 h, but in some cases the light regime was changed (dim light instead of full radiation). Under irradiation the largest fraction of cells was found at the bottom of the column. The cell density decreased toward the surface. Photosynthetic efficiency, determined with a pulse amplitude modulated fluorometer, was negligible at the surface and increased toward the bottom. While the cell suspension showed a positive gravitaxis at the bottom, the cells in the 40 cm horizon were bimodally oriented (about the same percentage of cells swimming upward and downward, respectively). At 20 cm and at the surface the cells showed negative gravitaxis. Positive gravitaxis was more pronounced in the UV + PAR samples. At the surface and in the 20 and 40 cm horizons photosynthetic efficiency was better in the PAR-only samples than in the PAR + UV samples. At the bottom photosynthetic efficiency was similar in both light treatments. The data suggest that high light reverses gravitaxis of the cells, so that they move downward in the water column. At the bottom the light intensity is lower (attenuation of the water column and self shading of the cells) and the cells recover. After recovery the cells swim upward again until the negative gravitaxis is reversed again.
最近研究表明，单细胞鞭毛虫纤细裸藻在受到过量辐射后，其重力趋性的方向会从负向变为正向。即便随后转移到弱光环境中，这种方向变化在细胞培养物中仍会持续数小时。为了测试这种行为的生态相关性，进行了一项垂直柱实验（最大深度65厘米），以测试柱体不同深度（表面、20厘米、40厘米和65厘米）处的分布、光合效率和运动性。其中一个柱体覆盖了一个截止紫外线的滤光片，该滤光片仅透射光合有效辐射（PAR），另一个柱体覆盖的滤光片则透射PAR和紫外线。柱体由太阳模拟器照射（PAR为162瓦每平方米，UV-A为32.6瓦每平方米，UV-B为1.9瓦每平方米）。实验持续了10天，通常采用12小时光照/12小时弱光的光周期，但在某些情况下改变了光照条件（用弱光代替全辐射）。在照射条件下，柱体底部发现的细胞比例最大。细胞密度朝着表面降低。用脉冲幅度调制荧光计测定的光合效率在表面可忽略不计，并朝着底部增加。虽然细胞悬液在底部表现出正向重力趋性，但在40厘米深度处的细胞呈双峰取向（分别向上和向下游动的细胞比例大致相同）。在20厘米深度和表面处，细胞表现出负向重力趋性。正向重力趋性在紫外线+PAR样本中更为明显。在表面以及20厘米和40厘米深度处，仅PAR样本中的光合效率比PAR+紫外线样本更好。在底部，两种光照处理下的光合效率相似。数据表明，强光会使细胞的重力趋性逆转，从而使它们在水柱中向下移动。在底部，光强度较低（水柱的衰减和细胞的自我遮光），细胞恢复。恢复后，细胞再次向上游动，直到负向重力趋性再次逆转。
