In freshwater habitats, aerobic animals and microorganisms can react to oxygen deprivation by a series of behavioural and physiological changes, either as a direct consequence of hindered performance or as adaptive responses towards hypoxic conditions. Since oxygen availability can vary throughout the water column, different strategies exist to avoid hypoxia, including that of active ‘flight’ from low‐oxygen sites. Alternatively, some organisms may invest in slower movement, saving energy until conditions return to more favourable levels, which may be described as a ‘sit‐and‐wait’ strategy. Here, we aimed to determine which, if any, of these strategies could be used by the freshwater ciliate Tetrahymena thermophila when faced with decreasing levels of oxygen availability in the culture medium. We manipulated oxygen flux into clonal cultures of six strains (i.e. genotypes) and followed their growth kinetics for several weeks using automated image analysis, allowing to precisely quantify changes in density, morphology and movement patterns. Oxygen effects on demography and morphology were comparable across strains: reducing oxygen flux decreased the growth rate and maximal density of experimental cultures, while greatly expanding the duration of their stationary phase. Cells sampled during their exponential growth phase were larger and had a more elongated shape under hypoxic conditions, likely mirroring a shift in resource investment towards individual development rather than frequent divisions. In addition to these general patterns, we found evidence for intraspecific variability in movement responses to oxygen limitation. Some strains showed a reduction in swimming speed, potentially associated with a ‘sit‐and‐wait’ strategy; however, the frequent alteration of movement paths towards more linear trajectories also suggests the existence of an inducible ‘flight response’ in this species. Considering the inherent costs of turns associated with non‐linear movement, such a strategy may allow ciliates to escape suboptimal environments at a low energetic cost.

中文翻译：

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淡水纤毛虫的种群统计和运动模式：氧气可用性的影响

在淡水栖息地，需氧动物和微生物可以通过一系列行为和生理变化对缺氧做出反应，这些变化要么是表现受阻的直接后果，要么是对缺氧条件的适应性反应。由于整个水体的氧气利用率可能会有所不同，因此存在不同的策略来避免缺氧，包括从低氧地点主动“飞行”。或者，一些生物体可能会降低运动速度，节省能量，直到条件恢复到更有利的水平，这可以被描述为“坐等”策略。在这里，我们的目的是确定淡水纤毛虫可以使用这些策略中的哪些（如果有的话）嗜热四膜虫当培养基中氧气含量降低时。我们操纵氧通量进入六种菌株（即基因型）的克隆培养物中，并使用自动图像分析跟踪它们的生长动力学数周，从而能够精确量化密度、形态和运动模式的变化。氧气对人口统计学和形态学的影响在不同菌株中是相当的：减少氧通量会降低实验培养物的生长速率和最大密度，同时大大延长其静止期的持续时间。在指数生长期取样的细胞在缺氧条件下更大并且具有更细长的形状，这可能反映了资源投入转向个体发育而不是频繁分裂。除了这些一般模式之外，我们还发现了对氧限制的运动反应的种内变异的证据。一些菌株表现出游泳速度降低，这可能与“静坐等待”策略有关；然而，运动路径向更线性轨迹的频繁改变也表明该物种存在可诱导的“飞行反应”。考虑到与非线性运动相关的转弯的固有成本，这种策略可能允许纤毛虫以较低的能量成本逃离次优环境。