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

Abstract

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.

Keywords: Tetrahymena thermophila; cell behaviour; hypoxia; plasticity; sit‐and‐wait.

FIGURE 1

(A) The rate of oxygen renewal is depicted as the absolute difference in DOC between a starting value measured in sterile water immediately after its deoxygenation and a second value measured 3 h later; tubes were kept in conditions identical to the four oxygen treatments, except that no cells of Tetrahymena were present during the trial. Coloured dots represent mean values across five replicates and error bars depict 95% CI. Groups sharing a common lowercase letter are not statistically different from each other, as determined from Tukey's HSD test (α level = .05). (B) Dissolved oxygen concentration in the medium (mg L⁻¹) and cell density (kcells mL⁻¹) were measured throughout the experiment, starting from t = 8 h. Coloured dots and error bars depict mean values and 95% CI across four replicates of the strains D4, D6 and D15 (individual data are depicted in Appendix S2). Only the growth phases and plateau phases are depicted.

FIGURE 2

(a) Proportion of variance (η ²) explained by the factors Oxygen (fixed), Strain (random) and their interaction (random) on parameters of demography, movement and morphology in Tetrahymena thermophila, using a two‐way mixed and crossed ANOVA. Response variables are grouped by demography, movement and morphology respectively. (b) The same analysis was performed by excluding strain D2 from the data set, because its large difference with other strains was responsible for a large part of the total variance, leading to a dilution of the proportion of total variance explained by factors other than Strain; response variables shown here (trajectory linearity and cell size) were most strongly affected by the removal of D2.

FIGURE 3

(A) Growth curves of the 96 populations of T. thermophila monitored throughout the experiment. Population density over time is depicted for each strain in four different oxygenation conditions. Note that replicates being nested within strain, one replicate of a strain should not be directly compared to the same replicate ID in other strains (i.e. comparing panels within a column is not meaningful). (B–D) Three demographic traits were quantified from these growth curves. Coloured dots represent mean values across four replicates and error bars depict 95% CI. Groups sharing a common lowercase letter are not statistically different from each other, as determined from Tukey's HSD test (α = .05) computed separately for each strain.

FIGURE 4

Parameters of movement and morphology quantified during the exponential growth phase. (A) Movement speed (μm s⁻¹). (B) Trajectory linearity (higher means ‘straighter’). (C) Net displacement over 5 s (μm). (D) Cell size (μm²). (E) Cell shape (i.e. major/minor axes ratio of a fitted ellipse, smaller meaning ‘rounder’). Coloured dots represent mean values across four replicates and error bars depict 95% CI. Groups sharing a common lowercase letter are not statistically different from each other, as determined from Tukey's HSD test (α = .05) computed separately for each strain.