Dynamic photoinhibition of photosynthesis of a large porites lobata colony, Samae San, Thailand.

Abstract

The north, south, east and west facing sides of a Porites lobata colony, at a depth of 4.5m, experience differing intensities of light relative to each other and to the top, upward facing surface of the coral which is at a depth of 2m. Photosynthetic response to irradiance was measured as rapid light curves (RLCs) using the underwater Diving-PAM fluorometer (Walz Gmbh, Germany). Measurements were carried out at 1000hrs, 1300hrs and 1600hrs over a three day period. Photochemical efficiency of photosystem 2 at ambient light intensity (Fq′/Fm′), maximum relative rate of electron transport (rETRmax), the initial slope of the RLC (α) and the minimum saturating irradiance (Ek) were derived from RLCs. The top surface of the coral showed the lowest values of Fq′/Fm′ (eg 0.49 on day 1 at 1300hrs) with the north-facing side showing highest values (eg 0.69 on day 1 and 2 at 1300hrs). Although Fq′/Fm′ showed consistent decreases according to orientation and time of day, with lower values at 1300hrs compared to 1000hrs and 1600hrs, rETRmax, α and Ek showed less consistent variation with time or orientation. The top surface of the coral was investigated further by measuring the same parameters every hour from 0700 – 1800hrs over a 2 day period. Values ranged from a maximum 0.59 at 0700hrs to a minimum 0.35 at 1200hrs. Values recovered in the late afternoon, indicating dynamic photoinhibition. This will have implications for corals exposed to warmer waters because of climate change.