Design of a low-cost pH-Stat to study effects of ocean acidification on growth and nutrient consumption of diatoms

Increasing CO2 emissions has modified oceanic pH levels. These pH changes affect phytoplankton growth and composition. Diatom cells constitute almost 50% of phytoplankton, and they have significant importance in the ocean food chains and biotechnology industries. Therefore, knowledge of their response to pH changes could be useful for conservation and aquaculture of these species. There are different pH-Stat systems to supply CO2 gas to the culture medium, however, it is common to use one unit or pH probe for each culture. In this study, we designed a low-cost pH-stat to regulate the pH level in fifteen simultaneous cultures. It was evaluated with Phaeodactylum tricornutum at three pH setpoints:7.5 and 7.8 as acid treatments and 8.1 as control; each experiment lasted seven days, and growth rates, latency phases and nutrient consumption rates were determined. The accuracy and precision of the pH regulated was in an acceptable level compared with other systems. The growth rate and consumption of nitrate were higher at pH 8.1, moreover differences were observed in the duration of the latency phase, suggesting a longer acclimation process at lower pH. Changes in phosphate and iron consumption indicated a higher availability in acid treatments, however they did not enhance the growth. These denoted unfavorable effects of ocean acidification on diatoms growth.