Crossed wires: diatom phosphate sensing mechanisms coordinate nitrogen metabolism.

Phytoplankton can encounter dynamic changes in their environment including fluctuating nutrient supply, and therefore require survival mechanisms to compete for such growth-limiting resources. Diatoms, single-celled eukaryotic microalgae, are typically first responders when crucial macronutrients phosphorus (P) and nitrogen (N) enter the marine environment and therefore must have tightly regulated nutrient sensory systems. While nutrient starvation responses have been described, comparatively little is known about diatom nutrient sensing mechanisms. We previously identified that the model diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana use calcium (Ca²⁺) ions as a rapid intracellular signaling response following phosphate resupply. This response is evident only in phosphate deplete conditions, suggesting that it is coordinated in P-starved cells. Rapid increases in N uptake and assimilation pathways observed following phosphate resupply, indicate tight interplay between P and N signaling. To regulate such downstream changes, Ca²⁺ ions must bind to Ca²⁺ sensors following phosphate induced Ca²⁺ signals, yet this molecular machinery is unknown. Here, we describe our findings in relation to known diatom P starvation signaling mechanisms and discuss their implications in the context of environmental macronutrient metadata and in light of recent developments in the field. We also consider the importance of studying phytoplankton nutrient signaling systems in the face of future ocean conditions.