Physiological measurements
Productivity and respiration rates were measured using dissolved oxygen probes in customized transparent glass chambers (approx. 4L) to detect oxygen evolution (production), and oxygen uptake (respiration rate). Each chamber had an oxygen sensor spot (PyroScience) glued to the inner chamber wall and a small in-line aquarium pump for water mixing during incubation. Oxygen concentration was monitored using a fiber optic sensor (PyroScience). Colonies were set individually inside the chamber that was bathed in and filled with the filtered, temperature-controlled seawater from the experimental tray table. Dissolved oxygen concentration was measured in the chamber during a 1h-dark period to calculate dark respiration rates (R) and subsequently during a light exposure period of 1h to calculate net productivity (NP). The difference of dissolved oxygen concentration at incubation the start and end of the experiment was normalized over time and colony surface area as described by Schneider & Erez (2006). Surface area was determined by the aluminum foil technique (Marsh, 1970) and image analysis (Image J, US National Institutes of Health) of planar digital images. The dark respiration and photosynthesis rates were analyzed according to their linearity over time for quality control. Gross productivity (GP) was calculated (gross productivity = net productivity + dark respiration). Photosynthesis to respiration (GP:R ratios) were calculated as a ratio of gross productivity (GP) to respiration (R) to estimate if production by theSymbiodiniaceae cells is exceeding maintenance requirements of both the symbiont and the coral host (Coles and Jokiel 1977). Maximum photochemical efficiency of photosystem II (i.e., effective quantum yield) was quantified via pulse-amplitude modulate (PAM) fluorometry (Warner et al. 1996; Jones et al. 1998). After an overnight dark acclimation period, fluorescence (Fm and F0) was measured by saturation pulses at three random spots on each colony to calculate the colony’s average Fv/Fm. Maximum photochemical efficiency of the PSII (Fv/Fm, Fv = Fm – F0) was calculated based on fluorescence measurements using a pulse amplitude modulated fluorometer (DIVING-PAM Walz Inc.).