Award: OCE-1334632

Iron is a nutrient required for the growth of phytoplankton (single-celled plants) in the ocean. In large regions of the ocean, the supply of iron -- mostly via airborne dust -- is very low, and iron availability limits the growth of phytoplankton in these areas. Because of the ability of phytoplankton to remove atmospheric carbon dioxide and sequester some of it on the bottom of the ocean, ocean iron limitation has implications for global climate. Hence, it is important to understand how phytoplankton compete for iron in the ocean. In this project we investigated the ability of various species of phytoplankton to store iron for use later, a behavior called 'luxury uptake'. We measured the iron contents of 14 different species of diatoms (a type of phytoplankton) grown in the laboratory at varying concentrations. Our collaborators at University of North Carolina measured levels of the iron-storage protein ferritin in the same species in order to determine if this protein gives cells an enhanced ability to store iron. We also conducted incubation experiments with natural phytoplankton communities collected at various locations along the California coast and further offshore from Vancouver Island to test for luxury iron storage and ferritin gene expression. From the laboratory work we learned that diatoms' ability to store iron is largely independent of the presence of ferritin. For the most part, species that contain ferritin were not significantly better at storing iron than species that lack ferritin. Furthermore, some species were found to contain multiple copies of the ferritin gene, which were expressed (or copied) in different ways. It appears that in some species ferritin may provide short-term 'buffering' of intracellular iron levels, allowing cells to control and maintain iron at beneficial concentrations inside the cell. From the experiments conducted on the research cruises, we documented significant occurrence of luxury iron uptake in natural phytoplankton populations growing along iron gradients. We also found that two groups of diatoms that either contain or lack ferritin did not vary notably in their ability to accumulate iron or respond to altered iron availability. This supports our findings from the laboratory experiments. Our results in this project greatly expand our knowledge of the upper bounds of iron that can be stored by phytoplankton. This information will be used to constrain global climate models that represent iron and phytoplankton behavior in the ocean. This will result in improved accuracy of these models' predictions of future climate conditions. The broader impacts of this work have been transferred to educational communities and the general public in several ways. Two post-doctoral researchers were trained at Bigelow through this research project, the first continuing on to a career in secondary school education. Additionally, two undergraduate researchers were trained by Bigelow scientists through this project. One student participated in the 3-week research cruise along the California Coast. The following year, a second undergraduate spent the summer working at Bigelow conducting diatom growth experiments. Additionally, Twining taught lectures and a laboratory exercise on iron limitation of diatoms during his ocean biogeochemistry course for undergraduates as part of Bigelow's fall semester program. Twining has given multiple outreach talks to the general public during the project, describing this research and the importance of iron and phytoplankton to global climate and ocean ecosystem health. Last Modified: 10/30/2017 Submitted by: Benjamin S Twining