Day/Night Separation of Oxygenic Energy Metabolism and Nuclear DNA Replication in the Unicellular Red Alga Cyanidioschyzon merolae

Abstract

The transition from G1 to S phase and subsequent nuclear DNA replication in the cells of many species of eukaryotic algae occur predominantly during the evening and night in the absence of photosynthesis; however, little is known about how day/night changes in energy metabolism and cell cycle progression are coordinated and about the advantage conferred by the restriction of S phase to the night. Using a synchronous culture of the unicellular red alga Cyanidioschyzon merolae, we found that the levels of photosynthetic and respiratory activities peak during the morning and then decrease toward the evening and night, whereas the pathways for anaerobic consumption of pyruvate, produced by glycolysis, are upregulated during the evening and night as reported recently in the green alga Chlamydomonas reinhardtii. Inhibition of photosynthesis by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) largely reduced respiratory activity and the amplitude of the day/night rhythm of respiration, suggesting that the respiratory rhythm depends largely on photosynthetic activity. Even when the timing of G1/S-phase transition was uncoupled from the day/night rhythm by depletion of retinoblastoma-related (RBR) protein, the same patterns of photosynthesis and respiration were observed, suggesting that cell cycle progression and energy metabolism are regulated independently. Progression of the S phase under conditions of photosynthesis elevated the frequency of nuclear DNA double-strand breaks (DSB). These results suggest that the temporal separation of oxygenic energy metabolism, which causes oxidative stress, from nuclear DNA replication reduces the risk of DSB during cell proliferation in C. merolae. IMPORTANCE Eukaryotes acquired chloroplasts through an endosymbiotic event in which a cyanobacterium or a unicellular eukaryotic alga was integrated into a previously nonphotosynthetic eukaryotic cell. Photosynthesis by chloroplasts enabled algae to expand their habitats and led to further evolution of land plants. However, photosynthesis causes greater oxidative stress than mitochondrion-based respiration. In seed plants, cell division is restricted to nonphotosynthetic meristematic tissues and populations of photosynthetic cells expand without cell division. Thus, seemingly, photosynthesis is spatially sequestrated from cell proliferation. In contrast, eukaryotic algae possess photosynthetic chloroplasts throughout their life cycle. Here we show that oxygenic energy conversion (daytime) and nuclear DNA replication (night time) are temporally sequestrated in C. merolae. This sequestration enables “safe” proliferation of cells and allows coexistence of chloroplasts and the eukaryotic host cell, as shown in yeast, where mitochondrial respiration and nuclear DNA replication are temporally sequestrated to reduce the mutation rate.

The Unicellular Red AlgaCyanidioschyzon merolae—The Simplest Model of a Photosynthetic Eukaryote

“…In C. merolae cultured under LD, the levels of photosynthetic and respiratory activities peaked in the morning and then decreased toward the evening and at night, whereas the pathways for the anaerobic consumption of pyruvate, produced by glycolysis, were upregulated during the evening and at night ( Miyagishima et al 2019 ), as also reported for the green alga C. reinhardtii ( Strenkert et al 2019 ). Also, the day/night rhythm of respiration depends on the photosynthetic activity.…”

Diurnal transcript profiling of the diatom Seminavis robusta reveals adaptations to a benthic lifestyle

“…A preference for genome replication taking place around dusk is observed in most lineages of microalgae and is thought to avoid DNA damage by ultraviolet radiation or photosynthetic reactive oxygen species during this sensitive phase of the cell cycle (Miyagishima et al ., 2019; Nikaido and Johnson, 2000). This is reflected in the expression of S phase genes in the afternoon and at dusk in transcriptomes of N .…”

Efficient open cultivation of cyanidialean red algae in acidified seawater

“…Other cultivations in laboratory (Fig. 5 ) were performed at 100 μmol m −2 s −1 which is the optimal intensity for C. merolae 30 . In addition, in the outdoor cultivation, the light intensity at noon was 300–2000 μmol m −2 s −1 depending on the weather.…”