Measurements of photosynthesis and respiration of Ulva lactuca occurred at the end of 3 weeks of acclimation to various levels of ocean acidification at 15°C. Small circular sections of U. lactuca thalli used for measurements were sampled with a cork borer the evening before measurement and returned to culture conditions overnight to allow for wound healing. Plant sections used to estimate rates of photosynthesis and respiration from oxygen exchange measurements were placed in small white, mesh threaded bags designed to fit the diameter of the cuvette chamber so that the plant material was oriented perpendicularly to the light source provided in the Qubit Systems Dissolved Oxygen package. Prior to introducing plants to the cuvette chamber, oxygen exchange between the stirred seawater and electrode system (i.e., oxygen consumption by the electrode) was monitored to adjust for its effect on subsequent measurements in the presence of plant material. Sections were transferred to the cuvette containing a magnetic stirring bar and 3 ml of seawater corresponding to the average pCO2 level in their particular culture pot and placed in darkness for 10-15 minutes prior to beginning measurement. Once measurements began, output of oxygen exchange was monitored until a steady state of oxygen exchange was attained (typically within 10 minutes) in either darkness or light. Photosynthesis and respiration were measured at 15°C and ~550, and 0, µmole photons m-2 s-1, respectively; the same temperature and light conditions as in growth culture. Respiration was measured both following a ≥10 minute acclimation period in darkness prior to light exposure (dark respiration) and again in darkness immediately following exposure to the saturating light intensity (as an estimate of respiration that occurs in the light). Following these measurements in seawater at their culture pCO2 level, the seawater in the cuvette was replaced with standard air-saturated seawater at 15°C and the measurement process described above was repeated for measurements of respiration in the dark and ‘light’ and photosynthesis. After measurements of gas exchange, the mass of plant tissue was measured on an analytical balance.

Note: Trial 1 was a pilot test of culture system and methodological procedures so was not used for data collection.