2:30 PM - 2:45 PM
[BPT03-04] Light response of photosymbiotic planktonic foraminifera under culturing experiments
Keywords:Planktonic foraminifera, Light response, Culture experiments, Photosymbiosis, Symbionts
Planktonic foraminifera species with symbiotic algae are mainly distributed in the sun-lit surface layer due to their need for light. However, they are sometimes collected from depths well below the photic layer. Such deeper distribution of photosymbiotic foraminifera species may be related to the difference in the type of symbiotic algae, or its adaptability to various light environments if the symbiont species is the same. We hypothesized that this adaptation is achieved by photohysiological responses of the symbiotic algae, changing the light-harvesting system of their photosystems in response to the light environment. In this study, we examined this hypothesis through light-controlling culture experiments of a single species and time-series measurements of photophysiological parameters.
The target species was Trilobatus sacculifer which is known to harbor a single dinoflagellate species. The culturing experiments were conducted under three irradiance levels; 10–20 μmol m-2 s-1 (LL group), 70–100 μmol m-2 s-1 (ML group), and 200–250 μmol m-2 s-1 (HL group), and continued for one week. A total of 91 individuals were used in the experiments. The photosystem II parameters representing the physiological state of photosynthesis were measured daily for each individual.
In the time-series profiles of photophysiological parameters, Fv/Fm, an index of PSII health, tended to decrease in the HL group, and σPSII, an index of light absorption efficiency, tended to increase slightly in the LL group. 1/τQA, an index of the rate of electron transfer downstream of the photosystem, remained consistently high in all groups. To clarify the influence of light intensity, we set the ML group’s results as a reference and evaluated the differences between the other two groups. The only significant difference was obtained for Fv/Fm in HL, showing that photosynthetic activity decreased under high light. It indicates photosynthetic inhibition by excess light. On the other hand, differences in σPSII were not significant, indicating that no clear changes occurred in the photosynthetic light-harvesting system even when the light environment changed significantly. This result means that the hypothesis of this study that “light adaptation is achieved by changing the light-harvesting system by symbionts” is not likely, and a different mechanism is suggested.
Observations of foraminifera individuals during the culture experiments showed that the symbionts of the HL group were not deployed much outside the shell, whereas those of the LL group were outside the shell and often evenly distributed along the spines. This suggests that the host foraminifera may physically control the light level irradiated to the symbionts by changing the distribution of symbionts inside or outside the shell in response to the light environment.
The target species was Trilobatus sacculifer which is known to harbor a single dinoflagellate species. The culturing experiments were conducted under three irradiance levels; 10–20 μmol m-2 s-1 (LL group), 70–100 μmol m-2 s-1 (ML group), and 200–250 μmol m-2 s-1 (HL group), and continued for one week. A total of 91 individuals were used in the experiments. The photosystem II parameters representing the physiological state of photosynthesis were measured daily for each individual.
In the time-series profiles of photophysiological parameters, Fv/Fm, an index of PSII health, tended to decrease in the HL group, and σPSII, an index of light absorption efficiency, tended to increase slightly in the LL group. 1/τQA, an index of the rate of electron transfer downstream of the photosystem, remained consistently high in all groups. To clarify the influence of light intensity, we set the ML group’s results as a reference and evaluated the differences between the other two groups. The only significant difference was obtained for Fv/Fm in HL, showing that photosynthetic activity decreased under high light. It indicates photosynthetic inhibition by excess light. On the other hand, differences in σPSII were not significant, indicating that no clear changes occurred in the photosynthetic light-harvesting system even when the light environment changed significantly. This result means that the hypothesis of this study that “light adaptation is achieved by changing the light-harvesting system by symbionts” is not likely, and a different mechanism is suggested.
Observations of foraminifera individuals during the culture experiments showed that the symbionts of the HL group were not deployed much outside the shell, whereas those of the LL group were outside the shell and often evenly distributed along the spines. This suggests that the host foraminifera may physically control the light level irradiated to the symbionts by changing the distribution of symbionts inside or outside the shell in response to the light environment.