Botanica Marina (1990) 33, 31-45
Studies on Halimeda
III. A Daily Cycle of Chloroplast Migration within Segments*
Edward A Drew and Kay M Abel
Australian Institute of Marine Science, PMB No 3, MSO, Townsville,
Queensland 4810, Australia
*Contribution No 473 from the Australian Institute of Marine Science
Abstract
Under natural conditions of irradiance, daylength and water temperature, the segments of Halimeda plants rapidly become very pale after dark, remain so for most of the night, but are again green enough by dawn to permit rapid photosynthesis as soon as light is available. This diurnal paling phenomenon affects the entire plant and has been observed in the field, in a laboratory seawater cascade system and in laboratory experiments under controlled conditions (25^C, 350 uE/m2/sec, 12 h light/12 h dark). Plants also pale dramatically when placed in the dark at any time of day. These changes in surface pigmentation are not due to changes in chlorophyll concentration, which remains above 45 ug CHL/cm2 throughout the day and night. However, microscopic examination showed that dramatic migrations of chloroplasts occur within segments, whilst time-lapse photography has allowed quantification of the resulting changes in surface pigmentation. Immediately after dark, chloroplasts begin to migrate from the surface utricles, through the secondary utricles, and into the internal medullary filaments which are beneath the calcium carbonate exo-skeleton. This movement is reversed before dawn the next day, suggesting that, although the inward migration of the chloroplasts may be directly triggered by the onset of darkness, initiation of movement in the opposite direction is not directly controlled by light.
More than 100 chloroplasts 3 to 5 um wide must pass through the 5 to 6 um wide constrictions at the base of each surface utricle during only about one hour after dark, so chloroplast migration must be highly organised to avoid blocking these narrow openings. The chloroplasts were associated with, and appeared to move along a network of cytoplasmic striations which are probably bundles of microtubules. Several of these striations pass into each surface utricle through its basal constriction, thus providing a structural basis for the rapid, co-ordinated movement of so many organelles.
The phenomenon of chloroplast withdrawal from the plant surface, a feature necessarily restricted to coenocytic algae without crosswalls, is discussed in terms of observations on related genera, its relationship with large-scale re-arrangement of organelles during sexual reproduction in Halimeda, and its possible roles both in minimising damage from surface grazers and in the rapid development of new segments.