pH up-regulation in tropical corals: a key mechanism?

Calcifying organisms are the backbone of tropical reefs by constructing its massive three-dimensional framework. Climate change already started and will continue to affect the oceans worldwide by making them not only warmer but also more acidic. This so called ocean acidification (OA) is projected to negatively impact particular ecosystems, whose organisms depend on the formation of a calcium carbonate skeleton. Several organisms were found to locally buffer pH at the sites of skeletal formation by actively H+-pumping to enable calcification. This active regulatory capacity (called up- regulation) was described as key for coral reef resilience in a future ocean. However, only a limited number of studies directly measured the pH of the calcifying fluid revealing generally an elevated pH compared to seawater pH. Most indications for pH up-regulation at the site of calcification derived from indirect data acquisition, e.g. measuring the boron isotopic composition (11B) of the skeleton. Ocean acidification studies revealed that calcifying organisms differ in their sensitivity to changes in ocean pH. These differing physiological responses were so far not addressed by studies investigating calcification pH directly or indirectly. In addition, high resolution observations on 11B showed strong spatial, temporal and inter-specific variability in pH upregulations that needs further evaluation. Hence, the question arises whether coral species sensitivity to OA are related to differences in up- regulation potential, and whether the 11B in the skeleton is actually linked to the external seawater pH. In Papua New Guinea CO2 seeps create a natural gradient in ocean pH and therefore represent a window into the future. Previous studies at this site observed changes in reef community compositions and identified both winners and losers in a future ocean. A recent cruise to the seeps addressed differences in growth rates of non-sensitive and sensitive corals to future pH conditions. Based on this information laboratory studies will be performed using microsensors to investigate the capacity of corals to up-regulate pH at the site of calcification. pH measurements will be conducted under different environmental conditions and by inhibiting physiological processes involved in pH regulation and calcification to study potential drivers for a high spatial variability in measured coral skeletons 11B. The expected outcome is: 1) an improved mechanistic understanding of calcification and 2) an in depth evaluation of direct and indirect pH measurements at the site of calcification. It will allow to predict whether up-regulation potential is the key-innovation for being a winner in a future acidified ocean. Physiological investigations will contribute to a better understanding of the processes that mainly drive high spatial 11B heterogeneity and they will help to better understand and outline the challenges for using 11B as palaeo-pH proxy in corals.

Climate change already started and will continue to affect the oceans worldwide by making them not only warmer but also more acidic. This so called ocean acidification (OA) is projected to negatively impact in particular calcifying organisms like corals. In the course of the project it was assessed whether internal pH regulation represents a key mechanism that allows corals to buffer changing ocean pH. Corals from both tropical and temperate regions were investigated and their ability to buffer external pH changes compared. Uniquely at all sites the corals were growing along natural pH gradients. This allows to test whether livelong exposure allows corals to acclimatize to future pH conditions and improve long-term projects of coral trajectories in a changing ocean. The analysis showed that all corals from both tropical and temperate regions are able to elevate the pH at the site of calcification. The extent of internal pH elevation is not a matter of region and more likely reflect a species- specific and habitat dependent physiological adaptation. In contrast to laboratory studies, the results showed that internal calcification pHcf was declining less strong with changing seawater pH. At the Papua New Guinea CO2 site internal pH was even kept constant for a seawater pH reduction of approx. 0.2. Only beyond internal pH up-regulation could not be maintained and internal pHcf started to declines. The data suggest that corals do have the capacity to acclimate after long-time exposure to end-of-century reduced seawater pH conditions and that strong control over pHcf represents a key mechanism to persistence in future oceans. Only beyond end-of- century pCO2 conditions do they face their current physiological limit of pH homeostasis and pHcf begins to decrease. This underlines that it is imperative to reach targeted climate goals to