How does rising atmospheric CO2 affect marine organisms?

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How Temperature Impacts a Tropical Coral and its Symbionts

Paper Reviewed
Winkler, N.S., Pandolfi, J.M. and Sampayo, E.M. 2015. Symbiodinium identity alters the temperature-dependent settlement behavior of Acropora millepora coral larvae before the onset of symbiosis. Proceedings of the Royal Society B: 282: 20142260.

Providing some background for their study, Winkler et al. (2015) note the coral reproductive season "coincides with the season most likely to experience thermal stress," but they report that "many tropical coral species are already near upper thermal limits," citing Fitt et al. (2001). So do these two facts pose a serious dilemma for tropical corals?

Focused on answering this important question, the three Australian researchers conducted a study where they assessed "how temperature and the coral symbionts (Symbiodinium) affect survival, symbiont uptake, settlement success and habitat choice of Acropora millepora larvae." Their work was conducted at Heron Island (Australia), where the coral larvae were exposed to seawater temperatures of 22.5, 24.5, 26.5 and 28.5°C, and where within each treatment the larvae were offered symbionts with a variety of distinct thermal tolerances.

In discussing their findings, Winkler et al. report that lower rather than higher temperatures adversely affected Symbiodinium recruitment by reducing larval survival and settlement. Also, they found that low temperatures reduced Symbiodinium habitat choice and initial symbiont densities, both of which impact post-settlement survival. And even more intriguing, they discovered that at lower temperatures the coral larvae "increasingly settle away from preferred vertical surfaces."

Therefore, just as cold weather is known to lead to the deaths of far more people than is hot weather around the world (see Health Effects (Temperature - Hot vs. Cold Weather) in our Subject Index), and in light of the fact that daily minimum temperatures have historically risen considerably more than have daily maximum temperatures during periods of global warming (see Temperature (Diurnal Range) in our Subject Index), it can be appreciated that both humans and corals -- together with their symbionts -- may well have little to fear if temperatures begin to rise again.

Fitt, W.K., Brown, B.E., Warner, M.E. and Dunne, R.P. 2001. Coral bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical corals. Coral Reefs 20: 51-65.

Posted 1 June 2015