Glutamate as an extracellular signalling molecule in unicellular protists

The amino acid L-glutamate plays an important role in cell-cell communication, acting as the primary neurotransmitter in the human brain. Glutamate also plays an important signalling role in land plants, enabling long-range signalling in response to wounding¹. However, the role of glutamate as an extracellular signalling molecule in unicellular eukaryotes has received far less attention. We find that ionotropic glutamate receptors are widely distributed in unicellular eukaryotes, including many major phytoplankton lineages. Experimental analyses using diatoms, an important lineage of marine and freshwater algae, revealed that low micromolar quantities of extracellular glutamate induce rapid cytosolic Ca²⁺ elevations, suggesting a sensitive signalling response. The diatom Ca²⁺ elevations were specific to glutamate, with little or no response to a range of other amino acids. Glutamate sensing was partially disrupted by domoic acid, a toxic analogue of glutamate produced by some harmful diatom species. The cellular role of extracellular glutamate sensing in diatoms remains to be elucidated, although the ability to sense this abundant amino acid may represent a mechanism that allows unicellular eukaryotes to detect wounding of neighbouring cells within a population. Our findings support a widespread role for glutamate as an extracellular signalling molecule in unicellular eukaryotes that likely enabled the emergence of complex glutamate-dependent signalling networks in multicellular organisms.