Bio-optical properties and radiative energy budgets in fed and unfed scleractinian corals (Pocillopora sp.) during thermal bleaching

© 2019 The authors. Corals live in symbiosis with algal dinoflagellates, which can achieve outstanding photo - synthetic energy efficiencies in hospite approaching theo retical limits. However, how such photosynthetic efficiency varies with environmental stress remains poorly known. Using fiber-optic and electrochemical microsensors in combination with variable chlorophyll fluorescence imaging, we investigated the combined effects of thermal stress and active feeding on the radiative energy budget and photosynthetic efficiency of the symbiotic coral Pocillopora sp. At ambient temperature (25°C), the percentage of ab sorbed light energy used for photosynthesis under low irradiance was higher for fed (∼5-6%) compared to unfed corals (4%). Corals from both feeding treatments responded equally to stress from high light ex posure (2400 μmol photons m-2 s-1), exhibiting a de crease in photosynthetic efficiency, down to 0.5-0.6%. Fed corals showed increased resilience to thermal-induced bleaching (loss of symbionts) compared to unfed corals. In addition, while unfed corals decreased their photosynthetic efficiency almost immediately when exposed to thermal stress, fed corals maintained a constant and high photosynthetic efficiency for 5 more days after onset of thermal stress. We conclude that active feeding is beneficial to corals by prolonging coral health and resilience during thermal stress as a result of an overall healthier symbiont population

Elevated CO<inf>2</inf> leads to enhanced photosynthesis but decreased growth in early life stages of reef building coralline algae

© 2019 Ordoñez, Wangpraseurt, Lyndby, Kühl and Diaz-Pulido. Crustose coralline algae (CCA) are key organisms in coral reef ecosystems, where they contribute to reef building and substrate stabilization. While ocean acidification due to increasing CO2can affect the biology, physiology and ecology of fully developed CCA, the impacts of elevated CO2on the early life stages of CCA are much less explored. We assessed the photosynthetic activity and growth of 10-day-old recruits of the reef-building crustose coralline alga Porolithon cf. onkodes exposed to ambient and enhanced CO2seawater concentration causing a downward shift in pH of ~0.3 units. Growth of the CCA was estimated using measurements of crust thickness and marginal expansion, while photosynthetic activity was studied with O2microsensors. We found that elevated seawater CO2enhanced gross photosynthesis and respiration, but significantly reduced vertical and marginal growth of the early life stages of P. cf. onkodes. Elevated CO2stimulated photosynthesis, particularly at high irradiance, likely due to increased availability of CO2, but this increase did not translate into increased algal growth as expected, suggesting a decoupling of these two processes under ocean acidification scenarios. This study confirms the sensitivity of early stages of CCA to elevated CO2and identifies complexities in the physiological processes underlying the decreased growth and abundance in these important coral reef builders upon ocean acidification