47. Life-long in situ exposure to ocean acidification reduces heterotrophy in the stony coral Galaxea fascicularis

Joy N. Smith (1,2)*, Julia Strahl (2), Sam H.C. Noonan (2), Gertraud M. Schmidt (1), Claudio Richter (1), Katharina E. Fabricius (2)

1 Alfred Wegener Institute, Bremerhaven, D-27568, Germany
2 Australian Institute of Marine Science, Townsville, Queensland, 4810, Australia

Background
Coral heterotrophy provides essential nutrients (nitrogen and phosphorus) for biological processes. For some corals species, this feeding mode becomes increasingly important when stressed from bleaching, high turbidity, and sometimes even ocean acidification. We investigated rates of heterotrophy in a voracious zooplankton consumer, Galaxea fascicularis, after life-long in situ exposure to elevated levels of CO2.

Methods
We conducted complementary field and laboratory feeding experiments on corals that have lived their entire lives under CO2 stress. Colonies of G. fascicularis were studied on a fringing reef in Papua New Guinea where underwater volcanic seeps naturally elevate seawater pCO2. Colony fragments in the field and laboratory were fed a known amount of zooplankton and allowed to feed for one hour before being preserved in 4% formalin for later determination of gut contents. Field and laboratory experiments were repeated for two expeditions. Additional laboratory experiments were conducted to determine if changes in feeding rates might be caused by an interference of the neurotransmitter functioning of the receptor GABAA.

Findings
Results from both excursions and both methods indicate that G. fascicularis colonies eat 2.8 to 4.8 times more zooplankton under normal seawater conditions compared to corals under CO2 stress. Reduced heterotrophy was not caused by any differences in polyp expansion or polyp size since both parameters remained similar across the CO2 gradient. Furthermore, heterotrophy rates were not restored with the addition of gabazine, the GABAA receptor antagonist.

Conclusions
Although increased heterotrophy may help to alleviate the harmful effects of ocean acidification, G. fascicularis consumed less zooplankton after life-long exposure to CO2 stress despite having an equal opportunity to catch food particles (same quantity of food fed to them, similar polyp extension, same polyp size). Reduced heterotrophy will likely have grave consequences for the metabolic processes that require essential nutrients acquired by consuming zooplankton.