Award: OPP-1443646

The potential for the micronutrient iron to limit primary productivity in the major high nutrient, low chlorophyll (HNLC) regions of the ocean has been well established, with the Southern Ocean being the largest HNLC region. As part of this project, we performed two sets of incubation experiments with surface seawater collected from the southern boundary of the Drake Passage, near the Western Antarctic Peninsula in early Austral Spring 2016. One experiment involved five amended treatments and a control. The amended treatments included adding iron at three different amounts (+1 nM, +4 nM), and +10 nM), a vitamin B12 (600 pM) addition, and a vitamin B12 (600 pM)/iron (4nM) co-addition. A second incubation at this location two weeks later tested whether additions of high iron water sourced from a coastal region could alleviate iron stress. Measurements of extracted chlorophyll were used to track phytoplankton growth in incubated waters. Samples were also collected from the incubations to analyze the community composition of diatoms in the different treatments, as diatoms are an important phytoplankton group that are known to dominate the spring bloom phytoplankton communities in this region. Additionally, numerous diatom species were isolated from these incubations and brought back to the laboratory for physiological experiments. Isolates that matched diatom species that were found to be abundant under the different treatment conditions were grown under a variety of iron and temperature conditions. RNA was extracted and sequenced from multiple diatom species grown under both high and low iron conditions with multiple purposes. One was to add to the database of sequence data from different diatom species, which is a valuable resource for the broader oceanographic community. Another important reason for the sequencing was to examine how the gene expression of multiple species differs in response to iron stress, which will hopefully shed light on why one species is more or less successful at growing in the Southern Ocean under varying iron conditions. In spite of the two-week separation of the two incubations, the starting diatom community composition in both sets of incubated waters was similar and they shifted in similar ways between the initial and final time points in the control treatments. Our results from the incubation experiments include that iron additions of at least 4 nM resulted in significantly higher chlorophyll (phytoplankton) growth when compared with no addition controls or the + vitamin B12 condition. In the +4 nM and +10 nM iron treatments the diatom communities were very similar by the end of the incubation and also different from the control (no addition), +vitamin B12, and +1 nM iron treatments. Addition of filtered coastal waters also led to a significant increase in chlorophyll over control treatments and resulted in diatom communities similar to those where > 4 nM iron was added. Unlike other Southern Ocean HNLC areas that have been found to be iron/vitamin B12 co-limited in summer, vitamin B12 additions in this early austral spring season had no impact on diatom community composition or in phytoplankton biomass. These results confirm hypotheses that iron is the important limiting nutrient in the Southern Drake Passage and that coastal iron advected offshore has the potential to alleviate iron stress of phytoplankton communities. We successfully isolated >100 diatoms and through ribosomal DNA sequence screening, reduced that number of isolates down to ~25 different diatom species. After evaluating the diatom community composition from the various incubation treatments, we selected ~10 species for experiments to determine the iron tolerance of a number of different species and found varying tolerances to low iron in addition to a variety of absolute growth rates. We also extracted RNA from select species grown under both high and low iron conditions to continue to build the global diatom sequence database and to explore how different species are able to thrive under various iron concentrations. Last Modified: 09/28/2019 Submitted by: Phoebe Chappell