Symbiotic capacity in a changing ocean: emerging questions around phytoplankton interactions.

Phytoplankton fix half of global CO2 annually and drive key global nutrient cycles. They depend on interactions with bacteria, viruses, and protists. Yet our understanding of these interactions remains fragmented. We introduce symbiotic capacity-an organism's ability to initiate, maintain, and modulate partnerships-as a quantifiable trait fundamental to phytoplankton ecology and resilience in changing oceans. By adapting Tinbergen's four questions from ethology, we reveal critical gaps in our knowledge regarding phytoplankton symbiotic capacity. While functional benefits, molecular mechanisms, and evolutionary origins of phytoplankton symbioses are increasingly documented, their developmental dynamics remain almost entirely unexplored. Bridging molecular mechanisms to planetary-scale biogeochemical processes requires a fifth question and suitable model systems, such as Chaetoceros diatoms, that integrate laboratory tractability with ecological relevance.