Synchronous Growth and Plastid Replication in the Naturally Wall-less Alga Olisthodiscus luteus

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Abstract
Olisthodiscus luteus is a unicellular biflagellate alga which contains many small discoidal chloroplasts. This naturally wall-less organism can be axenically maintained on a defined nonprecipitating artificial seawater medium. Sufficient light, the presence of bicarbonate, minimum mechanical turbulence, and the addition of vitamin B12 to the culture medium are important factors in the maintenance of a good growth response. Cells can be induced to divide synchronously when subject to a 12-hour light/12-hour dark cycle. The chronology of cell division, DNA synthesis, and plastid replication has been studied during this synchronous growth cycle. Cell division begins at hour 4 in the dark and terminates at hour 3 in the light, whereas DNA synthesis initiates 3 hours prior to cell division and terminates at hour 10 in the dark. Synchronous replication of the cell's numerous chloroplasts begins at hour 10 in the light and terminates almost 8 hours before cell division is completed. The average number of chloroplasts found in an exponentially growing synchronous culture is rather stringently maintained at 20 to 21 plastids per cell, although a large variability in plastid complement (4-50) is observed within individual cells of the population. A change in the physiological condition of an Olisthodiscus cell may cause an alteration of this chloroplast complement. For example, during the linear growth period, chloroplast number is reduced to 14 plastids per cell. In addition, when Olisthodiscus cells are grown in medium lacking vitamin B12, plastid replication continues in the absence of cell division thereby increasing the cell's plastid complement significantly.