title: A direct CO2control system for ocean acidification experiments: testing effects on the coralline red algaePhymatolithon lusitanicum.
abstract: 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 andpCO2. Here we describe a system in which the targetpCO2is controlled via direct analysis ofpCO2in 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 measurespCO2and conveys this value to a Proportional-Integral-Derivative (PID) controller. The controller commands a solenoid valve that opens and closes the CO2flush that is bubbled into the header tank. This low-cost control system allows the maintenance of stabilized levels ofpCO2for extended periods of time ensuring accurate experimental conditions. This system was used to study the long term effect of OA on the coralline red algaePhymatolithon lusitanicum. We found that after 11 months of high CO2exposure, photosynthesis increased with CO2as 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.
authors.affiliation: CCMAR-Centre of Marine Sciences, Universidade do Algarve , FARO , Portugal.
authors.forename: Laura
authors.initials: L
authors.lastname: Sordo
authors.affiliation: CCMAR-Centre of Marine Sciences, Universidade do Algarve , FARO , Portugal.
authors.forename: Rui
authors.initials: R
authors.lastname: Santos
authors.affiliation: CCMAR-Centre of Marine Sciences, Universidade do Algarve , FARO , Portugal.
authors.forename: Joao
authors.initials: J
authors.lastname: Reis
authors.affiliation: CCMAR-Centre of Marine Sciences, Universidade do Algarve , FARO , Portugal.
authors.forename: Alona
authors.initials: A
authors.lastname: Shulika
authors.affiliation: CCMAR-Centre of Marine Sciences, Universidade do Algarve , FARO , Portugal.
authors.forename: Joao
authors.initials: J
authors.lastname: Silva
keywords: CO2 bubbling; Control system; Coralline algae; Ocean acidification (OA)