Coral Calcification and Ocean Acidification

Coral Calcification and Ocean Acidification

Anthropogenic CO2 emissions represent a massive reorganization of global carbon.  As this carbon invades the ocean it changes the acid-base chemistry of seawater impacting a wide range of biological and chemical processes.  Many marine organisms reduce skeletal growth in response to this ocean acidification.  However, the detailed mechanism responsible for this sensitivity is poorly understood.

During biomineralization, organisms choreograph the flow of ions, manipulate aqueous speciation, and modulate surface chemistry through organic-mineral interactions.  Due to differences in the intricate balance between these processes, marine calcifiers exhibit divergent sensitivities to ocean acidification.  Furthermore, this response is often modulated by secondary environmental and biological factors, making the impact of ocean acidification on skeletal growth difficult to predict.   A mechanistic understanding of biomineralization would resolve these problems, dramatically transforming our ability to predict and mange the impacts of ocean acidification.

We know that coral are particularly sensitive to ocean acidification through previous experiments on reef communities, mesocosms, and individual cultured specimens.  Furthermore, we know that coral actively elevate the pH of the calcifying microenvironment, the extracellular space where skeletal growth occurs.  Given that this biological control can override the chemistry of the surrounding seawater, how does ocean acidification impact coral growth?  Furthermore, what key factors modulate this sensitivity?  In the Gagnon Lab we address these questions through a geochemical approach to coral biomineralization.

The geochemical approach harnesses compositional signatures recorded in skeletal material to as a tool to generate and test biomineralization models.  From skeletal chemistry we can infer the dynamics of ion transport, how seawater chemistry impacts calcifying-fluid pH regulation, and identify the basic chemical rules governing calcification.  To resolve these distinctive compositional signatures we  use high-precision mass spectrometry coupled with aquarium-based coral culture.  (For example, the aquarium pictured at the top of the page is a coral culture system built by our collaborator, Jonathan Erez of Hebrew University, Jerusalem. A complementary tropical culture aquarium will be constructed at UW!).

Organisms control when and where skeletal growth occurs by manipulating the energetic barriers to nucleation. This SEM image shows skeletal nuclei that were precipitated by coral across a glass window. Using this culture method together with micro-characterization techniques, we can map the relationships between nucleation kinetics, skeletal composition, and crystal morphology in vivo. This energy landscape determines the response of new skeletal growth to ocean acidification and may explain why skeletal density and skeletal growth rates respond to acidification with different sensitivities in Great Barrier Reef corals.

Through the use of new culture techniques we will also isolate specific stages of skeletal development, quantifying the impact of ocean acidification on nucleation.

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