You are on a remote sandy beach, swimming in pristine, crystal-clear water, unaware that you are immersed in a soup of environmental DNA. A single litre of seawater contains the genetic traces of hundreds of organisms: bacteria, archaea, microscopic algae, fungi, and countless others that have no name, no image, and no description in any scientific catalogue. We know they exist only because we can read their DNA. Welcome to the world of environmental DNA and to a revolution in how we understand life on Earth.
For centuries, our vision of biodiversity was shaped by what we could see and classify. From Aristotle’s first systematic descriptions of animals to Linnaeus’s binomial nomenclature, from Haeckel’s astonishing drawings to Whittaker’s recognition of fungi as a kingdom distinct from plants, life was whatever the eye could catch. Step by step, the map of life expanded. The DNA revolution of the late twentieth century showed us that this map still contained enormous blank spaces and that the organisms filling them were not exotic creatures living in remote places, but mostly microbes found everywhere around us, including in every drop of ocean water.
Environmental DNA (eDNA) is the genetic material that organisms shed into their surroundings: skin cells, mucus, faeces, gametes, and cellular debris. By filtering seawater samples, extracting this DNA, and reading it as we would read a book written in a language of four letters and sixty-four words, spoken by nearly all life on Earth, we can identify who lives in a given place without ever seeing them directly. One small filter, a sequencer, and a powerful bioinformatics pipeline: that is all it takes today to produce a census of an entire ecosystem.
This tool is already revealing remarkable stories. It allows us to detect endangered, cryptic, and rare species without relying on labour-intensive trapping methods, as demonstrated by the detection of the dwarf sperm whale around Malpelo Island, Colombia, where it had not been recorded since 2006. Environmental DNA can also serve as an early warning system for invasive aquatic species, detecting their presence long before populations become large enough to be observed directly. At the same time, it enables us to map the genetic signatures of thousands of organisms simultaneously, creating comprehensive inventories of entire ecosystems. Global expeditions such as Tara Oceans have revealed that most microbial genes found in the ocean do not match anything in existing databases, pointing to an immense reservoir of biological diversity that we are only beginning to explore.
The next time you swim in the sea, consider this: the water around you contains the genetic signatures of everything that lives there. You are not simply swimming in the ocean. You are swimming through its memory.
I am a biologist fascinated by two big questions: how biodiversity arises through evolution and how interactions among species shape the structure and dynamics of ecosystems. I mostly explore these questions using genetic tools, though occasionally I swap the lab for scuba gear and look for answers underwater.