Photosynthesis is presently the main process fuelling life on Earth, and considered the virtually sole source of primary production in most ecosystems. However, 3.8 billion years ago, early life derived its energy mainly from chemosynthesis, a process still fuelling life in the deep sea and other sunlight-deprived systems. Here, we hypothesize that, in addition to photosynthesis, chemosynthetic primary production is a major, but overlooked, secondary pathway fuelling shallow sea food webs. We propose to begin to test this paradigm-challenging idea in six temperate to tropical seagrass ecosystems around the world.