Phytoplankton responses to upwelling dynamics under varying iron conditions

Upwelling zones are hotspots of photosynthesis that are very dynamic in space and time. Phytoplankton in these regions must constantly adapt to changes in their chemical and physical environments, including variations in iron concentrations. When upwelled waters move offshore, cells sink out of the illuminated zone, establishing seed populations that remain inactive until the next upwelling event. This process is called the upwelling conveyor belt cycle (UCBC). Through a combination of laboratory and field experiments, we examined how distinct phytoplankton groups respond to each phase of the UCBC as a function of iron status. In the laboratory, the diatom Chaetoceros decipiens and coccolithophore Emiliana huxleyi recently obtained from the California upwelling zone were exposed to simulated UCBC conditions to examine changes in gene expression, growth and photosynthetic characteristics and elemental composition under both iron-replete and iron-limiting conditions. Consistent with recent findings in the field, C. decipiens exhibited frontloading of nitrogen-related genes throughout the UCBC phases. Iron status had distinct influences on each isolate’s ability to respond to the different phases throughout the UCBC. In the field, simulated upwelling experiments on subsurface phytoplankton communities were performed as part of the PUPCYCLE cruise. Within the California upwelling zone, additions of either iron or the iron siderophore, desferrioxamine B were used to induce iron-replete and iron-limiting growth conditions, respectively. Variation in the responses by phytoplankton to upwelling as a function iron status was examined through a combination of physiological and ‘omic approaches. Knowledge of how phytoplankton are affected by UCBC conditions at an integrated molecular, physiological and elemental level under both current and future scenarios is imperative for the proper conservation and management of these critically important ecosystems.