The effect of CO₂ on the photosynthetic physiology of phytoplankton in the Gulf of Alaska

In the high-nutrient, low-chlorophyll waters of the Gulf of Alaska, microcosm manipulation experiments were used to assess the effect of CO ₂ on growth and primary production under iron-limited and iron-replete conditions. As expected, iron had a strong effect on growth and photosynthesis. A modest and variable stimulation of growth and biomass production by CO₂ (high CO₂: 77-122 Pa; low CO ₂: 11-17 Pa) was observed under both iron-replete and iron-limited conditions, though near the limit of precision of our measurements in slow-growing low-iron experiments. Physiological acclimations responsible for the changes in growth were assessed. Under iron-limited conditions, growth stimulation at high CO₂ appeared to result from an increase in photosynthetic efficiency, which we attribute to energy savings from down-regulation of the carbon concentrating mechanisms. In some cases, iron-rich photosynthetic proteins (PsbA, PsaC, and cytochrome b₆) were down-regulated at elevated CO₂ in iron-limited controls. Under iron-replete conditions, there was an increase in growth rate and biomass at high CO₂ in some experiments. This increase was unexpectedly supported by reductions in cellular carbon loss, most likely decreased respiration. We speculate that this effect may be due to acclimation to decreased pH rather than high CO₂. The variability in responses to CO₂ among experiments did not appear to be caused by differences in phytoplankton community structure and may reflect the sensitivity of the net response of phytoplankton to antagonistic effects of the several parameters that co-vary with CO₂.