The importance of microgradients for marine calcifiers

This thesis describes the importance of microgradients around organic tissue of calcifiers in comparison to bulk seawater conditions. It is shown that microenvironmental acidification around calcifiers can result from various causes, such as calcification (Chapter 1), microbial mat or sediment exposure (Chapter 2), respiration (Chapter 2) or ocean acidification conditions (Chapter 3). Some calcifiers, like sediment dwelling foraminifera and bivalves, are naturally adapted to low pH and hypoxic conditions (Chapter 1, 2). Yet, microenvironmental low pH, hypoxia and high levels of sulphide resulted in tissue necrosis of corals, but only if those conditions were trapped close to the tissue of corals for extended periods of time (Chapter 2). The performance of some calcifiers under low pH and hypoxic conditions thus depends upon the duration of the exposure, as well as the diffusional resistance between the bulk seawater and their tissues. If diffusivity around their organic tissues is significantly reduced, some calcifiers may not be able to maintain pH homeostasis and thus severely suffer from extended exposure to low pH, hypoxia and high levels of sulphide (Chapter 2).