Abstract

Multiple fission is a cell cycle variation leading to the production of more than two daughter cells. Here, we used synchronized cultures of the chlorococcal green alga Parachlorella kessleri to study its growth and pattern of cell division under varying light intensities. The time courses of DNA replication, nuclear and cellular division, cell size, total RNA, protein content, dry matter and accumulation of starch were observed at incident light intensities of 110, 250 and 500 µmol photons m−2s−1. Furthermore, we studied the effect of deuterated water on Parachlorella kessleri growth and division, to mimic the effect of stress. We describe a novel multiple fission cell cycle pattern characterized by multiple rounds of DNA replication leading to cell polyploidization. Once completed, multiple nuclear divisions were performed with each of them, immediately followed by protoplast fission, terminated by the formation of daughter cells. The multiple fission cell cycle was represented by several consecutive doublings of growth parameters, each leading to the start of a reproductive sequence. The number of growth doublings increased with increasing light intensity and led to division into more daughter cells. This study establishes the baseline for cell cycle research at the molecular level as well as for potential biotechnological applications, particularly directed synthesis of (deuterated) starch and/or neutral lipids as carbon and energy reserves.

Highlights

  • Growth of algae is affected by a few main factors: (1) temperature, that affects the kinetics of the entire metabolism, (2) the source of energy and (3) the source of carbon.For autotrophically grown algae in nature, the levels of carbon dioxide, their carbon source, are stable

  • We describe a novel pattern of multiple fission cell cycle

  • The pattern utilized by cells of P. kessleri consists of multiple rounds of DNA replication, which, after completion of the last one, are followed by successive nuclear divisions

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Summary

Introduction

Growth of algae is affected by a few main factors: (1) temperature, that affects the kinetics of the entire metabolism, (2) the source of energy and (3) the source of carbon. For autotrophically grown algae in nature, the levels of carbon dioxide, their carbon source, are stable. Light and temperature are the two main factors that fluctuate the most and which affect growth rates of light-grown algae. Light serves as the main energy source; with increasing light intensity, growth rates will increase [3,4] until the light reaches inhibitory levels, whereupon growth is negatively affected. In chlorococcal and volvocean algae, including Parachlorella kessleri, increasing light intensity will induce the formation of larger mother cells and lead to the production of more daughter cells [3,4].

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