Phillips, WayneMahidol University. Internationa College. Science Division.2015-02-032018-11-202015-02-032018-11-202015-02-032007https://repository.li.mahidol.ac.th/handle/20.500.14594/35031Research Seminar on Research for MUIC Development, Mahidol University International College, Thailand.Objective: To determine whether different faces of a large hemispherical shaped Porites coral head respond differently to ambient light at different times of the day. Methods: A portable underwater Diving-PAM fluorometer was used for chlorophyll fluorescence measurements on distinct areas of a large Porites lobata coral head (diameter >2.5m) oriented in a north, south, east and west direction. Photosynthetic response to irradiance was measured as rapid light curves (RLCs). From these RLCs a number of parameters were derived. Fv/Fm – the maximum photochemical efficiency of photosystem 2 (PS2) and the relative maximum rate of electron transport at saturating light intensities (rETRmax). All measurements were carried out at 1000hrs, 1300hrs and 1600hrs over a three day period. Results: The top surface of the coral showed the lowest Fv/Fm values (e.g. 0.49 on day 1 at 1300hrs) with the north-facing side showing the highest values (e.g. 0.69 day 1 and 2 even at 1300hrs). Although Fv/Fm showed consistent decreases according to orientation and time of day, rETRmax showed less consistent variation with time or orientation. The top surface of the coral and the southfacing side showed significantly lower Fv/Fm than other orientations especially at 1300hrs. Conclusion: Mid-day depression in Fv/Fm with little or no change in rETRmax indicates dynamic photoinhibition.engMahidol UniversityPulse Amplitude Modulated (PAM) fluorometryPorites lobataDynamic photoinhibitionProceeding Book