Extant mat microbes synchronize vertical migration to a diel tempo

Animal migrations mark the largest daily movement of biomass on Earth today, but who performed the first diel migration and what are the implications? Modern-day benthic microbial mats resembling those of early Earth inhabit Lake Huron’s low-oxygen, high-sulfur, submerged sinkholes. Over several days during multiple years, we gathered time-lapse images of motile, phototactic, purple-pigmented cyanobacteria capable of oxygenic and anoxygenic photosynthesis, and chemotactic, pigment-free, chemosynthetic sulfur-oxidizing bacteria indulging in diel vertical migration of just 1–2 mm across the mat-sediment interface. As the world turns, alternating waves of vertically migrating photosynthetic and chemosynthetic filaments responded rapidly and non-linearly to daily fluctuating sunlight and chemical gradients. The photosynthesizing cyanobacteria and sulfur oxidizing bacteria exhibited their fastest motility following dawn and dusk, which turned the mat surface visibly purple during day and white at night, respectively. Our high-frequency, time-lapse images visually capture a modern day synchronized diel migration similar to what might have been the first and largest daily mass movement of life during the long Precambrian. During that time, benthic microbial mat motility would have played a critical role in directly optimizing photosynthesis and chemosynthesis, as well as indirectly burying carbon and oxygenating the biosphere. Further studies of these mat ecosystems will add to the expanding knowledge of Earth’s extant biodiversity and physiologies.