decrease calcification? One approach to understanding the basic 

 mechanism is to determine the molecular basis of bleaching, i.e., 

 loss of chlorophyll, that precedes or is concomitant with the loss 

 of symbiotic algae. A model approach can be drawn from the work of 

 Ortiz (1990) and Ortiz et al. (1991) on "heat bleaching" in 

 Euglena . 



The alga Euglena gracilis can be grown indefinitely at 30°C. 

 However, after a few generations at 33°C, cells are found that 

 contain rudimentary plastids and are permanently bleached; this 

 phenomenon has been linked to a progressive failure in the 

 translational activities of chloroplasts (Ortiz, 1990) . The 

 earliest event detected is the abrupt disappearance of normal 

 transcripts probed by psbE , which codes for one of the subunits of 

 cytochrome berg and is one of a cistron of six genes (Price, G.D., 

 pers. comm. ) . In the present case, a candidate for such a process 

 is a switch from one transcriptional start site to another. A 

 second is a change in the processing, e.g., splicing of 

 transcripts. 



Light Limitation 



Light limits photosynthesis in many marine and terrestrial 

 environments. Models of succession in higher plant ecosystems are 

 driven by competition for light between species. In coastal 

 ecosystems, light limitation is the primary factor controlling the 

 rate of carbon fixation by phytoplankton. 



All photosynthetic organisms have some ability to adjust the 

 absorption cross-section of the photosynthetic apparatus in 

 response to changes in irradiance (Falkowski and LaRoche, 1991) . 

 In phytoplankton, for example, the absorption cross-section may 

 increase by a factor of three or more as cells are grown at lower 

 irradiance levels, thus increasing light-harvesting ability 

 (Falkowski and LaRoche, 1991) . Higher plants have a much reduced 

 capacity to photoacclimate (Anderson and Osmond, 1987) . 



The molecular basis for photoacclimation is poorly understood. 

 In the marine green alga, Dunaliella tertiolecta . the abundance of 

 transcript for the light -harvesting chlorophyll proteins increases 

 five-fold within nine hours following a shift to lower irradiance 

 levels (LaRoche et al., in press). Subsequently, the transcripts 



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