Demography and movement patterns of a freshwater ciliate: The influence of oxygen availability

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.