Award: OCE-1041038

Will high CO2 conditions affect production, partitioning and fate of organic matter? This project investigated specific questions relating to the response of the ocean to increased atmospheric carbon dioxide (CO2). Since the industrial revolution, the ocean has taken up about 25% to 30% of all human produced CO2. Atmospheric CO2 concentrations would be much higher without this uptake by the ocean. However, we do not know if the ocean will be able to continue to take up additional CO2 in the future, or if the environmental changes in the ocean, such as increased temperature and increased concentrations of CO2 and its dissolved forms (e.g. increased pCO2 = partial pressure of CO2), will also change the potential of the ocean to take up carbon. Phytoplankton, the tiny algae found floating at the surface where light penetrates, and marine bacteria play important roles in the uptake of carbon by the ocean. Phytoplankton use CO2 and light to generate energy-rich organic carbon, such as carbohydrates, lipids, proteins. If the phytoplankton are then eaten, some of this organic carbon is used by the animals that eat them for energy and the carbon is turned back into CO2 when the animals exhale. Nothing changes. However, a small portion of the fixed organic carbon may sink into the deep ocean, as dead phytoplankton, animals or feces. Once the carbon reaches the deep ocean (e.g. sinks to below 1000 m depths) it is removed from the atmosphere and stored in the ocean for 100 to 1000?s of years. One question we asked was if phytoplankton will fix more carbon in the future when atmospheric CO2 concentrations are higher. We conducted experiments using diatoms, a ubiquitous group of phytoplankton common in all the world's oceans and responsible for a large fraction of all carbon fixation. Like land plants phytoplankton also need nutrients (fertilizers) to grow well. In our experiments some diatoms initially grew with their rate of carbon fixation unchanged at elevated pCO2, but once nutrients like nitrogen or phosphorous became scarce, they were able to fix more carbon under high pCO2 conditions. They thus removed more CO2 from the water under future conditions. However, we also found that this response depends on other environmental conditions, like light intensity or temperature. In some cases, for example if the temperature was too hot or too cold for the algae, the increased pCO2 even led to lower carbon fixation. Thus each specific case needs to be evaluated separately, as there is no general answer to our question. This makes predictions very complicated. We also investigated the response of bacteria that consume organic carbon and respire it similar to animals. Our results indicate that as the concentration of CO2 in the atmosphere continues to rise, bacteria become less efficient, meaning less of the carbon they consume is used to grow and more is released as CO2. The costs of growth are higher! Because only particles, like dead algae, animals or feces can sink to the deep ocean, the CO2 released within the surface of the ocean by bacteria (or animals) may easily escape back into the atmosphere. The reduced efficiency in growth of bacteria may therefore indicate that less carbon can be transported to depth. If this is generally true, then the future ocean might take up less carbon than presently, because more is needed to fuel bacteria activity. By itself this would lead to a more rapid increase in atmospheric CO2 concentrations in the future, even if human production of CO2 remains the same. The overall question remains whether the ocean will continue to be able to take up atmospheric CO2 produced by human activity. The results of our project provide essential puzzle pieces to help answer this question by determining how several environmental factors influence the mechanism of carbon fixation by phytoplankton and bacteria. Last Modified: 11/15/2016 Submitted by: Uta D E Passow