A mutualistic bacterium rescues a green alga from an antagonist.

Photosynthetic protists, known as microalgae, are key contributors to primary production on Earth. Since early in evolution, they coexist with bacteria in nature, and their mode of interaction shapes ecosystems. We have recently shown that the bacterium acts algicidal on the microalga It secretes a cyclic lipopeptide and a polyyne that deflagellate, blind, and lyse the algae [P. Aiyar , , 1756 (2017) and V. Hotter , , e2107695118 (2021)]. Here, we report about the bacterium which establishes a mutualistic relationship with and acts as a helper. While enhances algal growth, it receives methionine as needed organic sulfur and the vitamins B, B, and B from the algae. In tripartite cultures with the alga and the antagonistic bacterium , aids the algae in surviving the bacterial attack. By combining synthetic natural product chemistry with high-resolution mass spectrometry and an algal Ca reporter line, we found that rescues the alga from the antagonistic bacterium by cleaving the ester bond of the cyclic lipopeptide involved. The resulting linearized seco acid does not trigger a cytosolic Ca homeostasis imbalance that leads to algal deflagellation. Thus, the algae remain motile, can swim away from the antagonistic bacteria and survive the attack. All three involved genera cooccur in nature. Remarkably, related species of and also act antagonistically against or as helper bacteria in tripartite cultures.