A direct CO 2 control system for ocean acidification experiments: testing effects on the coralline red algae Phymatolithon lusitanicum

• Published in: PeerJ
• Main Authors: Sordo, Laura, Santos, Rui, Reis, Joao, Shulika, Alona, Silva, Joao
• Other Authors: Fundação para a Ciência e a Tecnologia (FCT), FCT doctoral grant
• Format: Article in Journal/Newspaper
• Language: English
• Published: PeerJ 2016
• Subjects: Ocean acidification
• Online Access: http://dx.doi.org/10.7717/peerj.2503; https://peerj.com/articles/2503.pdf; https://peerj.com/articles/2503.xml; https://peerj.com/articles/2503.html

Most ocean acidification (OA) experimental systems rely on pH as an indirect way to control CO 2 . However, accurate pH measurements are difficult to obtain and shifts in temperature and/or salinity alter the relationship between pH and p CO 2 . Here we describe a system in which the target p CO 2 is controlled via direct analysis of p CO 2 in seawater. This direct type of control accommodates potential temperature and salinity shifts, as the target variable is directly measured instead of being estimated. Water in a header tank is permanently re-circulated through an air-water equilibrator. The equilibrated air is then routed to an infrared gas analyzer (IRGA) that measures p CO 2 and conveys this value to a Proportional-Integral-Derivative (PID) controller. The controller commands a solenoid valve that opens and closes the CO 2 flush that is bubbled into the header tank. This low-cost control system allows the maintenance of stabilized levels of p CO 2 for extended periods of time ensuring accurate experimental conditions. This system was used to study the long term effect of OA on the coralline red algae Phymatolithon lusitanicum . We found that after 11 months of high CO 2 exposure, photosynthesis increased with CO 2 as opposed to respiration, which was positively affected by temperature. Results showed that this system is adequate to run long-term OA experiments and can be easily adapted to test other relevant variables simultaneously with CO 2 , such as temperature, irradiance and nutrients.