SurWiWa: Understanding the role of mixotrophy for phytoplankton Survival in Southern Ocean Winters under Warming

• Applicant: Dr. Antonia Ahme
• Subject Area: Oceanography
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 561558431

In the Southern Ocean (SO), phytoplankton must survive up to two months of darkness, which represents a major energetic bottleneck and makes overwintering strategies necessary. Phytoplankton can overwinter as resting stages, but many polar phytoplankton can remain morphologically unchanged and physiologically active. Surviving extended darkness in this state seems to be made possible by three different strategies: 1) an ability to use extremely low light; 2) mixotrophy, including phagotrophy and osmotrophy; and 3) the respiration of storage compounds via autophagy, accompanied by the general downregulation of respiration. The three overwintering mechanisms will likely be affected by global warming: respiration could increase, potentially resulting in higher light requirements and an earlier depletion of storage compounds. Mixotrophy might be a key mechanism to compensate for these higher metabolic demands and some functional groups potentially better exploit this strategy than. However, despite the importance of overwintering phytoplankton for the next growing season, data on how SO phytoplankton survive warmer winters are still largely lacking. Warming can also increase the invasion potential of temperate phytoplankton into polar ecosystems, but it is unclear whether the newly introduced species can cope with the strong seasonality in light. Therefore, an assessment of whether phago- or osmotrophy might be linked to specific adaptations to polar nights and potentially affect the invasion potential of temperate organisms is needed. To expand our knowledge of current and future phytoplankton survival during the polar night, this project aims to generate baseline data on the role of mixotrophy for overwintering phytoplankton under warming in the laboratory and combine it with field observations. The project consists of three work packages that will focus on: The potential to use various dissolved organic matter (DOM) sources by several phytoplankton groups (WP1), the overwintering success of SO, Arctic and temperate phytoplankton strains (WP2), and the low light requirements of SO phytoplankton strains (WP3) under different temperatures with and without DOM sources, including an assessment of phagotrophy.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 1158: Infrastructure area - Antarctic Research with Comparative Investigations in Arctic Sea Ice Areas
• International Connection: Australia, Denmark, Spain
• Cooperation Partners: Professor Dr. Per Juel Hansen, Ph.D.; Professorin Dr. María Segovia Azcorra; Dr. Robert Strzepek, Ph.D.