An automated pH-controlled culture system for laboratory-based ocean acidification experiments

Using a spectrophotometric pH measurement system, an automated 12-tank culture system was developed, which is capable of maintaining pH levels between 7.51 and 8.00 (local, in-situ pH) within 0.02 pH units at 15.1°C. The precise control of pH in the individual culture tanks was achieved through the addition of CO₂-enriched seawater to unamended seawater. A feed-back system, which automatically adjusted the mixing ratio of unamended and CO₂-enriched seawater, ensured that the pH in each tank was kept within 0.02 pH units of target values. After each tank adjustment, the spectrophotometric pH measurement was combined with a dissolved inorganic carbon measurement to automatically re-calculate carbonate chemistry in each tank. The system was used to study the growth of the geniculate coralline alga 'Arthrocardia corymbosa' at pH levels of 7.52, 7.76, and 7.98.