Capturing Southern Ocean diatom community response to Fe enrichment and potential interactions with bacteria using metatranscriptomics

Diatoms are important marine community members and major contributors to global biogeochemical cycles. In high nutrient, low chlorophyll (HNLC) areas such as the Southern Ocean (SO), diatoms are growth limited by the micronutrient iron (Fe) and respond rapidly to sudden changes in dissolved Fe, but the response is not uniform amongst species. The rapid growth response often results in a large bloom event and subsequent carbon export, depending on the species response. How individual diatom taxa respond genetically and physiologically in a community setting, however, is not well understood. During an early austral spring cruise in the Western Antarctic Peninsula region of the SO, Fe amendment experiments were conducted with low Fe, HNLC phytoplankton communities. Metatranscriptomic sequencing was then used to determine the diatom community response to the Fe amendments. Particular emphasis was given to teasing apart responses from different genera. The diatom taxa Pseudo-nitzschia, Chaetoceros, Thalassiosira and Fragilariopsis showed dynamic responses to Fe addition. Upregulation of photosynthetic pathways, nitrogen metabolism, and fatty acid biosynthesis and downregulation of canonical Fe stress response genes were observed. The relative contribution to these regulatory patterns by the different genera will be discussed. In addition, analysis of potential diatom interactions with bacteria to uptake ligand-bound Fe versus bioavailable Fe is ongoing. The investigation of these different Fe enrichment responses and potential diatom-bacteria interactions is important for understanding SO biogeochemical cycles and for linking in situ diatom metabolism with carbon export potential.