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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.

A direct CO2control system for ocean acidification experiments: testing effects on the coralline red algaePhymatolithon lusitanicum

A direct CO₂control system for ocean acidification experiments: testing effects on the coralline red algaePhymatolithon lusitanicum