 The climate system is a perpetual energy balancing act in which temperature, carbon dioxide or CO2, sunlight and ocean currents all interact in complex ways. Marine microorganisms called phytoplankton play a crucial role in this system by converting sunlight and inorganic nutrients into organic matter. This organic matter, the result of a process called primary production, is an important food source for marine life and its creation also helps regulate the global carbon cycle by consuming CO2. For the first time, scientists have addressed how temperature and CO2 levels in the European Arctic Ocean interact and jointly affect primary production. CO2 levels were found to enhance primary production by as much as 10 times, though only at low temperatures. With the Arctic Ocean predicted to become warmer and enriched with carbon dioxide, the influence of temperature on primary production is expected to hold significant implications for the energy balance of the Arctic ecosystem. To determine whether CO2 enhanced primary production is modulated by temperature, the researchers monitored the primary production of phytoplankton in seawater samples set to various temperature and CO2 levels. Below 6 degrees Celsius, primary production could increase by as much as 10 fold over the range of CO2 concentrations tested in the lab. And even over the more moderate range of CO2 levels measured in the European Arctic, primary production doubled. Above 6 degrees Celsius, however, the amount of CO2 bubbled into the samples had no significant effect, suggesting that CO2 controls primary production mainly at low temperatures. In fact, when temperatures were dropped from 6 degrees Celsius to 1 degree Celsius, primary production increased exponentially, confirming that the effect of CO2 on primary production is temperature dependent. Separate on-site measurements showed that the joint effect of temperature and CO2 concentration on primary production only occurs at low CO2 concentrations, which might explain why the phenomenon of CO2-controlled primary production has been largely ignored. Outside of the Arctic, seawater generally contains large amounts of dissolved carbon, so the productivity of microorganisms like phytoplankton is not limited by the amount of CO2 available. But in the European Arctic, where the world's lowest CO2 concentrations and near-freezing temperatures converge, primary production does vary with CO2 levels. The research team is cautious not to extrapolate their findings to regions beyond the European Arctic, but they do stress the implications of their work for a part of the ocean that is critical to balancing energy conversion processes in the Arctic ecosystem and regulating the global carbon cycle. In addition, they suggest that current biogeochemical models could be improved by incorporating the feedback effects they observed into future scenarios of the Arctic.