. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. SYMBIONT PHOTOSYNTHESIS IN HYDRA 89 TABLE I Kinetics of oxygen evolution as a function of light intensity for algae o/H. viridis. Slope is that of initial region of curves in Figures 4 and 5. O, Cone. Symbiont location pmax mM O2 min ' mg chl~' ^E m 2 s' Slope mg O2 1 ' in situ in vitro 125 280 13 mg O2 1 ' in situ in vitro 125 520 13 Thus, photosynthetic response to light decreased in algal symbionts isolated from their hosts, and this decrease was greater at higher O2 concentrat
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. SYMBIONT PHOTOSYNTHESIS IN HYDRA 89 TABLE I Kinetics of oxygen evolution as a function of light intensity for algae o/H. viridis. Slope is that of initial region of curves in Figures 4 and 5. O, Cone. Symbiont location pmax mM O2 min ' mg chl~' ^E m 2 s' Slope mg O2 1 ' in situ in vitro 125 280 13 mg O2 1 ' in situ in vitro 125 520 13 Thus, photosynthetic response to light decreased in algal symbionts isolated from their hosts, and this decrease was greater at higher O2 concentrations. In contrast, the photosynthetic response to light of symbionts in situ was relatively insensitive to oxygen. Oxygen inhibition of photosynthetic oxygen evolution As the light intensity-photosynthesis data suggested, whether the symbionts were in situ or in vitro strongly influenced the inhibition of photosynthetic oxygen evolution by O2 (Fig. 6). Increasing environmental O2 concentration from low to near air-saturation strongly inhibited O2 evolution by symbionts in vitro. In contrast, symbionts in situ were only slightly affected, and only at the higher O2 concen- trations. This is evident from percent inhibition of O2 evolution as a function of O2 concentration (Fig. 7). For example, at 8 mg O2 I"1 symbionts in vitro were inhibited 110%, but symbionts in situ only 11%. DISCUSSION The Rank Brothers polargraphic oxygen electrode has been used to study oxygen evolution by a coral and its symbiotic zooxanthellae (Crossland and Barnes, 1977) and by cell suspensions of photosynthetic organisms (Findenegg, 1976; Burris, 1977; Takabe and Akazawa, 1977). Our use of the Plexiglas light guide avoids the len- 3 4 5,6 [DISSOLVED 02]MG L FIGURE 6. Oxygen evolution by H. viridis symbionts as a function of oxygen concentration. Open circles = algal symbionts in situ, hydra incubated at 400 ^E-m~2-s~' in M solution + 1 mM KHCO3. Closed circles = algal symbionts in vitro in ph
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