GROWTH AND CELL CYCLE OF ULVA COMPRESSA (ULVOPHYCEAE) UNDER LED ILLUMINATION 1 Kazuyoshi Kuwano, 2 Naoko Abe Graduate School of Science and Technology, Nagasaki University, Bunkyo-machi, Nagasaki 852-8521, Japan Yukari Nishi, Hiromi Seno Faculty of Fisheries, Nagasaki University, Bunkyo-machi, Nagasaki 852-8521, Japan Gregory N. Nishihara Institute for East China Sea Research, Nagasaki University, Taira-machi, Nagasaki 851-2213, Japan Masafumi Iima Faculty of Environmental Studies, Nagasaki University, Bunkyo-machi, Nagasaki 852-8521, Japan and Vil em Zachleder Laboratory of Cell Cycles of Algae, Institute of Microbiology, Academy of Sciences of the Czech Republic (ASCR), Opatovick y ml yn, 37981 T rebo n, Czech Republic The cell-cycle progression of Ulva compressa is diurnally gated at the G 1 phase in accordance with light–dark cycles. The present study was designed to examine the spectral sensitivity of the G 1 gating system. When blue, red, and green light-emitting diodes (LEDs) were used for illumination either alone or in combination, the cells divided under all illumination conditions, suggesting that all colors of light were able to open the G 1 gate. Although blue light was most effective to open the G 1 gate, red light alone or green light alone was also able to open the G 1 gate even at irradiance levels lower than the light compensation point of each color. Occurrence of a period of no cell division in the course of a day suggested that the G 1 gating system normally functioned as under ordinary illumination by cool-white fluorescent lamps. The rise of the proportion of blue light to green light resulted in increased growth rate. On the other hand, the growth rates did not vary regardless of the proportion of blue light to red light. These results indicate that the difference in growth rate due to light color resulted from the difference in photosynthetic efficiency of the colors of light. However, the growth rates significantly decreased under conditions without blue light. This result suggests that blue light mediates cell elongation and because the spectral sensitivity of the cell elongation regulating system was different from that of the G 1 gating system, distinct photoreceptors are likely to mediate the two systems. Key index words: cell cycle; cell division; cell elongation; diurnal rhythm; G 1 gate; Ulva Abbreviations : DO, dissolved oxygen; LED, light- emitting diode Diurnal cell division has been investigated by many biologists (for example, see Sweeney and Hastings 1958, Edmunds 1966, Spudich and Sager 1980). The diurnal periodicity in cell division results from diur- nal regulation of the cell cycle and the mechanism of the cell-cycle regulation has been the focus of interest to researchers. The regulation by an endogenous cir- cadian clock is one of the most notable models, and there are many reports assuming that the cell cycle is regulated by an endogenous clock (Edmunds and Adams 1981, Homma and Hastings 1989, Makarov et al. 1995, L€ uning et al. 1997). However, there are other models that can account for periodical regula- tion of the cell cycle. The cell cycle of Chlamydomonas reinhardtii has recently been demonstrated to be regu- lated by the growth rate at a given illumination condi- tion and temperature without contribution of any hypothetical endogenous clock previously suggested (V  ıtov a et al. 2011a,b). Cell-cycle regulation is one of the most fundamental functions of life. The regula- tion system forms the basis of various biological processes, so a clear understanding of cell-cycle regu- lation is essential to the elucidation of these pro- cesses. Ulva, multicellular green algae, are prominent components of the marine algal flora worldwide, and are important contributors to the primary pro- duction in coastal ecosystems. The tremendous growth rate of these species can cause nuisance 1 Received 18 December 2013. Accepted 7 April 2014. 2 Author for correspondence: e-mail kkuwano@nagasaki-u.ac.jp. Editorial Responsibility: M. Cock (Associate Editor) J. Phycol. 50, 744–752 (2014) © 2014 Phycological Society of America DOI: 10.1111/jpy.12207 744