Interrelated influence of iron, light, and CO₂ on carbon fixation in a Southern Ocean diatom

The productivity of diatoms in the Southern Ocean plays a key role in the transfer of carbon (C) from the atmosphere to the ocean's interior, which impacts climate. However, diatom growth in the Southern Ocean is limited by several environmental factors including iron (Fe) and light. Ongoing increases in ocean CO2 concentrations may increase diatom carbon fixation, but it is uncertain how this will interact with extant Fe and light limitation. Here we grew the Southern Ocean diatom Fragilariopsis cylindrus under a matrix of growth sufficient and limiting Fe and light levels and current and elevated CO2 concentrations, and found that a decrease in Fe concentration at high light, or in light intensity at high Fe, caused a similar 28-35% decrease in growth rate. Combined low Fe and low light caused a much larger (71-75%) decrease in growth rate than occurred with low Fe or low light alone, indicating Fe and light co-limitation. At a given concentration of bioavailable dissolved inorganic Fe (Fe '), increasing pCO(2) from 400 to 750 mu atm had no significant effect on growth or C-fixation rates under any Fe and light conditions. These results suggest that unlike previous measurements in Fe- and light-limited temperate diatoms, increased CO2 should have little effect on C-fixation rates in Southern Ocean diatoms. The different physiological responses of cold-water and temperate diatoms to the changing environment warrant further investigation for understanding and predicting changes in the efficiency of the biological carbon pump and the associated potential feedback to the climate change.