Bio-optical properties and radiative energy budgets in fed and starved scleractinian corals (Pocillopora damicornis) during thermal bleaching

Abstract

16Corals achieve outstanding photosynthetic quantum efficiencies approaching theoretical 17 limits (i.e. 0.125 O2 photon -1 ) and it is unknown how such photosynthetic efficiency varies 18 with environmental stress. In this study, we investigated the combined effects of thermal 19 stress and active feeding on the radiative energy budget and photosynthetic efficiency of the 20 symbiont-bearing coral Pocillopora damicornis by using fiber-optic and electrochemical 21 microsensors in combination with variable chlorophyll fluorescence imaging. At normal 22 temperature (25ºC), the percentage of absorbed light energy used for photosynthesis was 23 higher for fed (~5-6% under low light exposure) compared to unfed corals (4%). Corals from 24 both feeding treatments responded equally to stress from high light exposure (2400 µmol 25 photons m -2 s -1 ), exhibiting a decrease in photosynthetic energy efficiency down to 0.5-0.6%. 26Fed corals showed increased resilience against thermal bleaching compared to unfed corals, 27 as fed corals were able to uphold their high photosynthetic energy efficiency for 5 days longer 28 during thermal stress, as compared to unfed corals, which decreased their photosynthetic 29 energy efficiency almost immediately when exposed to thermal stress. We conclude that 30 active feeding is beneficial to corals by prolonging coral health and resilience during thermal 31 stress as a result of an overall healthier symbiont population. 32 33 Keywords: Radiative energy budget, coral bleaching, coral feeding, symbiosis, microsensors, 34 coral optics, light scattering, bio-optics, photobiology 35 36 PAR irradiance reflectance ( / ) Arbitrary unit Downwelling irradiance in energy units J m -2 s -1 Reflected light energy J m -2 s -1 Absorbed light energy J m -2 s -1 Areal rate of gross primary production nmol O2 cm -2 s -1 Gibbs energy kJ (mol O2) -1 Energy conserved by photosynthesis J m -2 s -1 Thermal conductivity of seawater W m -1 K -1 / Temperature gradient in the TBL K µm -1 − Upward heat flux in energy units J m -2 s -1 − Downward heat flux in energy units J m -2 s -1 Total heat flux in energy units J m -2 s -1 821 822 823