Osteoporic sea urchin skeletons in a high CO2 world

Chair: Janice Lough

Roberta Johnson(1), Januar Harianto(2), Cecelia J. Brothers(3), Sergio Torres Gabarda(2), Murray Thomson(1), Maria Byrne(
1,2)

1 School of Biological Sciences, The University of Sydney, NSW, 2006, Australia
2 School of Medical Sciences, The University of Sydney, NSW, 2006, Australia
3
Department of Biology, University of Alabama at Birmingham, Birmingham, AL 35294, USA

Background
By 2100, ocean pH is expected to drop by 2.5-4.5 units due to increasing atmospheric CO2. Marine calcifiers are particularly vulnerable to ocean acidification (OA) due to negative effects on their skeleton. It may also affect species’ ability to maintain body fluid pH, which can be energetically costly. We measured coelomic fluid pH and investigated the effect of OA on skeleton morphology with advanced imaging techniques in Heliocidaris erythrogramma. This study is the first to use micro-computed tomography (µCT) to investigate the effect of high CO2 on the sea urchin skeleton.

Methods
Heliocidaris erythrogramma were gradually introduced (0.1 pH units/week) to OA (pH 7.6NIST) conditions and acclimated to these conditions for 10 months in flow-through sea water. Scanning electron microscopy (SEM) and µCT were used to investigate the skeleton and to construct 3-dimensional models of young apical plates of urchins from control and high CO2 environments. Coelomic fluid pH was measured with a microelectrode.

Findings
SEM revealed an increase in skeletal pore size on the inner surface of the apical plates in the high CO2 group. The results from µCT showed that the apical plates were more porous in a reduced pH environment. The large increase in void space indicated a decrease in the amount of skeleton produced. Coelomic fluid pH did not differ between groups.

Conclusions
This study indicates that in a high CO2 world, the skeletal structures of Heliocidaris erythrogramma may become more porous, similar to the bones of mammals suffering from osteoporosis, which are mechanically weaker. The cost of maintaining coelomic fluid pH in a high CO2 ocean may increase, compromising physiological functions such as skeletogenesis. For H. erythrogramma, the increase in porosity of their skeleton may mean that they will be less able to defend themselves from predation or protect themselves against physical damage.