To Divide or Not to Divide? How Deuterium Affects Growth and Division of Chlamydomonas reinhardtii

Abstract

Extensive in vivo replacement of hydrogen by deuterium, a stable isotope of hydrogen, induces a distinct stress response, reduces cell growth and impairs cell division in various organisms. Microalgae, including Chlamydomonas reinhardtii, a well-established model organism in cell cycle studies, are no exception. Chlamydomonas reinhardtii, a green unicellular alga of the Chlorophyceae class, divides by multiple fission, grows autotrophically and can be synchronized by alternating light/dark regimes; this makes it a model of first choice to discriminate the effect of deuterium on growth and/or division. Here, we investigate the effects of high doses of deuterium on cell cycle progression in C. reinhardtii. Synchronous cultures of C. reinhardtii were cultivated in growth medium containing 70 or 90% D2O. We characterize specific deuterium-induced shifts in attainment of commitment points during growth and/or division of C. reinhardtii, contradicting the role of the “sizer” in regulating the cell cycle. Consequently, impaired cell cycle progression in deuterated cultures causes (over)accumulation of starch and lipids, suggesting a promising potential for microalgae to produce deuterated organic compounds.

Cultivation of the microalgae Chlamydomonas reinhardtii and Desmodesmus quadricauda in highly deuterated media: Balancing the light intensity

“…Therefore, cell division was severely delayed in both D. quadricauda and C. reinhardtii in 90% D 2 O. These results are consistent with previously published results in C. reinhardtii ( Kselíková et al, 2021 ), Parachlorella kessleri ( Zachleder et al, 2021a ; Zachleder et al, 2021b ) or Chlorella ellipsoidea ( Unno et al, 1992 ). Interestingly, Tukey’s HSD test revealed that while both mass and cell number doubling times were affected by deuterium treatment ( Figures 4 , 5 ), the difference between 0% D 2 O and 70% D 2 O was statistically significant only with respect to cell number doubling time.…”

“…In addition, cells in deuterated medium, especially in 90% D 2 O, contained significant amount of starch granules, as shown by Lugol’s staining ( Figures 6C, F, I , 7C, F, I ) as well as by confocal Raman microscopy ( Figures 11B1–3 ). The same trend was observed in other microalgae after treatment with deuterium ( Zachleder et al, 2021a ; Kselíková et al, 2021 ), supraoptimal temperature ( Zachleder et al, 2019 ; Zachleder et al, 2021b ) or nutrient starvation ( Jerez et al, 2015 ). All of the above treatments have one thing in common—they impair/delay cell division, so starch accumulation is in fact a consequence of cell cycle arrest/slowdown.…”

“…In addition, the number of daughter cells formed within 1 cell cycle tended to be lower in 70% D 2 O for both D. quadricauda and C. reinhardtii (compare 0% D 2 O and 70% D 2 O in Figures 6 , 7 ). A reduced number of daughter cells is a typical sign of suboptimal growth conditions or stress in algae dividing by multiple fission and has already been described for C. reinhardtii and Parachlorella kessleri grown in medium containing D 2 O ( Zachleder et al, 2021a ; Kselíková et al, 2021 ). Cultures grown in 90% D 2 O showed barely any increase in cell number regardless of light intensity ( Figures 3C , 5 ; Table 1 ).…”

“…or Chlamydomonas sp. (Unno et al, 1987;Kollars and Geraets, 2008;Zachleder et al, 2021a;Kselíková et al, 2021). Therefore, understanding the stressful effect of deuterium in microalgae is a prerequisite for successful cultivation of microalgae on highly deuterated substrates.…”