Mats of microbes containing photosynthetic bacteria create an oasis of oxygen-rich sediments in otherwise low-oxygen lagoons that is exploited by small animals, suggests a paper published online in Nature Geoscience. These conditions may be comparable to those found 555 million years ago when mobile animals first appeared in the fossil record.

Murray Gingras and colleagues analysed microbial mats found in high-salinity, low-oxygen lagoons off the coast of Venezuela. The mats cover patches of the sea floor in the lagoons. They found that oxygen levels were much higher in the mats than in the mat-free areas, although they declined dramatically at nightfall. The lagoon was largely devoid of seafloor animals, with the exception of small burrowing shore crabs and insect larvae that lived only in the mats.

The authors suggest that early mobile animals that lived in seafloor sediments may also have exploited the oxygen-rich conditions associated with microbial mats.