Most ocean acidification (OA) experimental systems rely on pH as an indirect way to
control CO2. However, accurate pH measurements are difficult to obtain and shifts
in temperature and/or salinity alter the relationship between pH and pCO2. Here
we describe a system in which the target pCO2 is controlled via direct analysis of
pCO2 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 pCO2 and conveys this value to a Proportional-Integral-Derivative
(PID) controller. The controller commands a solenoid valve that opens and closes
the CO2 flush that is bubbled into the header tank. This low-cost control system
allows the maintenance of stabilized levels of pCO2 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 CO2 exposure, photosynthesis increased with CO2
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 CO2, such as
temperature, irradiance and nutrients.