Microorganisms have evolved sophisticated sensor-actuator circuits to perform taxis in response to various environmental stimuli. How any given circuit can select between different taxis responses in noisy vs. saturated stimuli conditions is unclear. Here, we investigate how Euglena gracilis can select between positive vs. negative phototaxis under low vs. high light intensities, respectively. We propose three general selection mechanisms for phototactic microswimmers, and biophysical modeling demonstrates their effectiveness. Perturbation and high-speed imaging experiments show that of these three mechanisms, the “photoresponse inversion mechanism” is implemented in E. gracilis : a fast, light-intensity-dependent switching between two flagellar beat states responsible for swimming and turning causes positive vs. negative phototaxis at low vs. high light intensity via run-and-tumble vs. helical klinotaxis strategies, respectively. This coordinated beat-switching mechanism then also accounts for a larger set of previously reported E. gracilis behaviors; furthermore, it suggests key design principles for other natural as well as synthetic microswimmers.

中文翻译：

---

两种鞭毛跳动状态之间的光依赖性切换在微游泳者中选择了多功能的趋光性策略


微生物已经进化出复杂的传感器-致动器电路，以响应各种环境刺激来执行出租车。目前尚不清楚任何给定电路如何在嘈杂和饱和刺激条件下在不同的出租车响应之间进行选择。在这里，我们研究了细裸藻如何分别在低光强度和高频光强度下在正趋光性和负趋光性之间进行选择。我们提出了趋光微游泳者的三种一般选择机制，生物物理模型证明了它们的有效性。扰动和高速成像实验表明，在这三种机制中，“光响应反转机制”在 E. gracilis 中实现：负责游泳和转弯的两个鞭毛跳动状态之间的快速、光强度依赖性切换，分别通过运行和翻滚与螺旋趋变策略在低光强度与高光强度下导致正趋光性与负趋光性。这种协调的节拍切换机制也解释了以前报道的一大组 E. gracilis 行为;此外，它还为其他自然和合成微型游泳者提出了关键设计原则。