. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 482 R. G. STEEN AND L. MUSCATINE Daily net carbon fixed Figure 3 shows the measured irradiance at the Bermuda Biological Station on 8 September 1983, a clear cloudless day. This curve of light available for photosynthesis on a "representative day" was used in conjunction with the P/I curves to calculate the total daily increment of carbon fixed by zooxanthellae photosynthesis. Our estimate of absolute values of net daily photosynthesis is strictly valid only for the day on which the diel irradiance curve was record
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 482 R. G. STEEN AND L. MUSCATINE Daily net carbon fixed Figure 3 shows the measured irradiance at the Bermuda Biological Station on 8 September 1983, a clear cloudless day. This curve of light available for photosynthesis on a "representative day" was used in conjunction with the P/I curves to calculate the total daily increment of carbon fixed by zooxanthellae photosynthesis. Our estimate of absolute values of net daily photosynthesis is strictly valid only for the day on which the diel irradiance curve was recorded, but it reflects the potential for any day. The method also assumes that photosynthesis at dawn and dusk, at levels below Pmax, does not significantly contribute to total diel C fixation. Table II shows net hourly C fixation at Pmax normalized to 10^ zooxanthellae and to an average polyp of each species. The data show that P. variabilis fixed twice as much carbon per polyp per day as Z. sociatus. Cell specific growth rate of zooxanthellae Figure 4 shows the average number of dividing zooxanthellae per 200 zooxanthellae examined for each host. Zooxanthellae in both hosts were characterized by a low specific growth rate with an average of algal cells in division. Zooxanthellae of Z. sociatus showed no phasing of cell division, but P. variabilis zooxanthellae showed slight phasing of cell division beginning at 11 PM. The number of dividing cells among P. variabilis zooxanthellae is significantly higher (two sample /-test; P < ) than for Z. sociatus zooxanthellae in several of the night time samples. The growth rate of Z sociatus zooxanthellae was day"' (doubling time = 66 days) and for P. variabilis n = day"' (doubling time = 41 days) (Table III). I500r 1000- w O E o. •o o UJ X u. o. p. variabilis r= A^l^ Z. sociatus ." r = 20 60 100 150 200 250 300 350 400 IRRADIANCE (/u Ein-m"^-sec"') Figure 2. The photosynthetic
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