Dissecting the integration of phosphorus and nitrogen nutrition signals in diatoms

Phosphorus (P) and nitrogen (N) are the major nutrients constraining growth of plants and algae. Environmental levels of these nutrients can be highly dynamic. It is therefore critical to understand how photosynthetic eukaryotes optimise growth and P and N acquisition, in conditions with fluctuating supply of these nutrients. Alongside surviving prolonged periods of nutrient deprivation, this requires being able to perceive and rapidly adapt to nutrient replenishment. We will employ an important group of aquatic microalgae, the diatoms, as a model system for investigating nutrient sensing, starvation and crosstalk mechanisms. Diatoms exhibit exquisite sensitivity to changes in P and N availability, and often dominate in coastal and estuarine ecosystems where nutrient supply can vary dramatically. We have recently discovered that diatoms employ the universal second messenger, Ca2+, for rapidly sensing and coordinating early cellular responses to P replenishment. However, the molecular players (genes and proteins) underlying P-Ca2+ signalling, and how they coordinate downstream adaptations are unknown. Furthermore, we do not currently know how diatom P sensing mechanisms are coordinated with known P starvation signalling mechanisms to optimise diatom responses to fluctuating P supply. Finally, the importance of bidirectional cross-talk between P and N signalling for mediating balanced acquisition of these vital nutrients remains to be determined.

The aim of this proposal is to address the above questions in order to better understand how photosynthetic eukaryotes perceive and respond to changes in the supply of P and N, and optimise growth in dynamic nutrient environments. We will employ established expertise in state-of-the-art gene editing approaches (CRISPR-Cas9) to characterise candidate genes of the P-Ca2+ signalling pathway. Examination of mutants will enable us to assess precisely how diatoms generate and coordinate P-Ca2+ signals to regulate diatom recovery responses from P starvation. We will also characterise whether, and what components of P-Ca2+ signaling are controlled by the master regulator of diatom P starvation responses, PSR1. Finally, we will investigate how N availability influences P-signalling and acquisition, to ensure the balanced acquisition of these vital nutrients.

This research will significantly advance understanding of nutrient sensing and crosstalk mechanisms in photosynthetic eukaryotes. As diatoms are evolutionarily distinct from land plants and green algae, the findings will allow us to compare and contrast nutrient signalling mechanisms between diverse photosynthetic taxa. As such, this proposal offers an unprecedented opportunity to gain insight of the evolution of nutrient perception and crosstalk mechanisms in photosynthetic eukaryotes. In doing so, our work will inform research efforts aiming to enhance N and P usage by other photosynthetic eukaryotes, such as important crop species. There is also great interest in using microalgae such as diatoms for removal of P and N from wastewater, to mitigate the harmful impacts of nutrient run-off. This research could therefore inform the optimisation of strains/conditions to maximise P and N recovery. More broadly, the research could also help better manage nutrient run-off and algal blooms, minimising human damage to valuable aquatic ecosystems.

Photosynthetic eukaryotes frequently encounter fluctuations in the availability of the vital nutrients phosphorus (P) and nitrogen (N). A priority research question is thus understanding how plants and algae perceive changes in the supply of these nutrients, to maximise and balance their acquisition in dynamic environments. We recently discovered that diatoms, single-celled, aquatic microalgae that are particularly competitive in fluctuating nutrient environments, employ a novel Ca2+-signalling mechanism for sensing P replenishment. This pathway is activated under P starvation, and coordinates rapid crosstalk with N metabolism to drive recovery from P limitation. However, the molecular components underlying P-Ca2+ signalling, and how they coordinate downstream responses are unknown. We propose to use diatoms as a highly sensitive, genetically tractable model to dissect P signalling and nutrient crosstalk mechanisms. We will employ a cutting-edge toolkit including CRISPR-Cas9 editing and Ca2+ biosensors, alongside lipidomics and stable isotope ratio mass spectrometry, to advance mechanistic insight of P-Ca2+ signalling. Functional characterisation of candidate P-Ca2+ signalling genes, including two Ca2+-dependent protein kinases and Ca2+ channels (PtTRP4 and PtOSCA4) will establish how P-Ca2+ signals coordinate metabolic adaptations to P replenishment. Furthermore, we will determine how P sensing and P starvation signalling mechanisms interact, to coordinate adaptations to fluctuating P supply. Finally, we will determine how N availability drives P regulatory networks and P acquisition. This will provide new insight into the mechanisms enabling diatoms to sense and coordinate P signalling, and balance acquisition of P and N, under fluctuating nutrient regimes. More broadly, the work will advance understanding of nutrient perception and crosstalk mechanisms in plants and algae, and shed light on their evolution in photosynthetic eukaryotes.