Programmed cell death in the coccoid green microalga Ankistrodesmus densus Korshikov (Sphaeropleales, Selenastraceae)

Abstract

Abstract Reports of programmed cell death (PCD) across the taxonomic spectrum of photosynthetic unicellular organisms raise questions concerning its ecological and evolutionary roles. However, prior to ecological studies or evolutionary interpretations, it is essential to document phenotypic changes associated with PCD at the single-cell level, since death-related responses vary between taxa and within a single taxon depending on environmental stimuli. Here, we report responses to rapidly changing light, temperature and fluctuations in macronutrients in the model selenastracean green microalga Ankistrodesmus densus (Chlorophyta, Chlorophyceae, Sphaeropleales). We used stringent, but environmentally appropriate, conditions of prolonged darkness, nitrogen starvation (4 days), heat (1 h at 44°C) and cold shock (3 h at 2 ± 2°C). PCD phenotypes were examined by ultrastructural changes, phosphatidylserine (PS) externalization and DNA degradation. Flow cytometric Annexin V FITC analyses revealed that darkness and nitrogen-deprived cultures had significantly higher proportions of cells with PS externalization compared with controls (p < 0.05). Heat and cold treatments did not affect PS externalization (p = 0.44 and p = 0.99, respectively). Transmission electron microscopy (TEM) of light-deprived cells demonstrated, among other ultrastructural changes, marked cytoplasmic vacuolization suggesting a subtype of PCD known as vacuolar cell death. Nitrogen-starved cells had less vacuolization but presented more typical ultrastructural markers of PCD such as chromatin condensation and marginalization. In contrast, the more severe heat and cold shock treatments resulted in necrotic-like features. These findings suggest that prolonged darkness and nitrogen starvation induce PCD in a small (8.4 3.5 and 7.42 2.6%, respectively) but significant (p < 0.05) fraction of the A. densus population. Documenting these different death-related phenotypes depending on different environmental inducers is essential for interpreting ecological studies. Furthermore, our data support the hypothesis that autophagic/vacuolar cell death (VCD), which is central to organism homeostasis in plants (Streptophyta), occurs in Chlorophyta. VCD probably arose long before the evolution of multicellularity in plants. HIGHLIGHTS Darkness and nitrogen deprivation induce different programmed cell death markers in Ankistrodesmus densus; Plant vacuolar-like cell death occurs in Chlorophyta; There is crossover between the vacuolar and apoptosis-like death morphotypes.