the plants to acclimate to this new light regime for 24 hr and 

 then monitored their pattern of segment production. By the 

 second day, 7 of the 10 transplants were producing segments in 

 the dark (daytime in the large microcosm); none of the 10 control 

 plants in the large microcosm were producing segments at this 

 time. 



Nutritional value and secondary metabolite production 



Nitrogen and organic content. We estimated the nutritional 

 quality of different aged segments of Halimeda incrassata from 

 the sand-plain at St. Croix by evaluating their nitrogen content 

 and organic content as a function of time of day (Fig. 5). 

 During the night and early morning (2100-0800 hr ) , new segments 

 are rich in nitrogen with values ranging from 3.4 to 4.2% of 

 algal dry mass. The maximum concentration of 4.2% occurs as a 

 small nitrogen spike at about 0600 hr (Fig. 5) when the new 

 segments are rapidly becoming pigmented. This spike may result 

 from an increased abundance of chlorophyll molecules and of 

 various enzymes involved in photosynthesis since these contain 

 nitrogen. Our visual observations of segment color, as well as 

 TLC and HPLC analyses, indicated that pigments were rapidly 

 entering the segments at this time and not merely coming to the 

 surface where they would cause an obvious color change. 

 Pigments, and possibly other cellular constituents, appear to 

 move from older to newly developing segments just before and 

 during sunrise. During the first daylight period encountered by 

 the new segment (0800-1800 hr), nitrogen content drops by 22%; by 

 the next morning (0630 hr), nitrogen content has dropped by about 

 43%. Segments that are several days old have only 25% of the 

 nitrogen content of newly forming tips (Fig. 5). 



Almost all of the change in nitrogen content per dry mass 

 of plant shown in Figure 5 resulted from the increasing ash 

 content of the segment and not from changes in the concentration 

 of nitrogen relative to the organic content of the segment. When 

 nitrogen content is expressed as a function of the ash free dry 

 mass of the segments (Fig. 5), nitrogen concentration is 

 relatively constant through time. 



Organic content of segments shows a decline with age that is 

 similar to that seen for nitrogen concentration (Fig. 5). Newly 

 produced tips (1500-0630 hr ) have only 25-45% ash content, new 

 segments (0800-0630 hr ) contain 45-67% ash, and old segments are 

 82% ash. Thus, the newly produced, uncalcified tips that are 

 being expanded at night have nitrogen concentrations 370-450% 

 higher than old segments and concentrations of organic material 

 that are 300-410% higher than old segments. 



379 



