Development of an economical, autonomous pHstat system for culturing phytoplankton under steady state or dynamic conditions

- The design for an autonomous pHstat system is presented.
- This pHstat system can maintain static or dynamic pH regimes for extended periods.
- Gases or reagents can be used to maintain pH treatments.
- pH can be controlled ± 0.03 pH units from the target pH using reagents, or ± 0.10 pH units using gases.

Laboratory investigations of physiological processes in phytoplankton require precise control of experimental conditions. Chemostats customized to control and maintain stable pH levels (pHstats) are ideally suited for investigations of the effects of pH on phytoplankton physiology, for example in context of ocean acidification. Here we designed and constructed a simple, flexible pHstat system and demonstrated its operational capabilities under laboratory culture conditions. In particular, the system is useful for simulating natural cyclic pH variability within aquatic ecosystems, such as diel fluctuations that result from metabolic activity or tidal mixing in estuaries. The pHstat system operates in two modes: (1) static/set point pH, which maintains pH at a constant level, or (2) dynamic pH, which generates regular, sinusoidal pH fluctuations by systematically varying pH according to user-defined parameters. The pHstat is self-regulating through the use of interchangeable electronically controlled reagent or gas-mediated pH-modification manifolds, both of which feature flow regulation by solenoid valves. Although effective pH control was achieved using both liquid reagent additions and gas-mediated methods, the liquid manifold exhibited tighter control (± 0.03 pH units) of the desired pH than the gas manifold (± 0.10 pH units). The precise control provided by this pHstat system, as well as its operational flexibility will facilitate studies that examine responses by marine microbiota to fluctuations in pH in aquatic ecosystems.