The cyanobacteria UCYN-A (Candidatus Atelocyanobacterium thalassa) provides an important part of the usable nitrogen in the oceans. Analysis of UCYN-A's genome has revealed that around 91 million years ago in the late Cretaceous, it formed a symbiotic relationship with single-celled algae and helped restore the abundance of ocean nutrients after a great shortage during the Jurassic.
Nitrogen is essential to life, but few life forms have the capacity to use the molecular nitrogen that makes up most of Earth's atmosphere. Consequently, other species rely on the few that can “fix” molecular nitrogen and turn it into ammonia (NH3), which the other species then take up. On land, this process is dominated by rhizobia, which form a symbiotic relationship with legumes, enriching the soil they live in with nitrogen.
Some bioavailable nitrogen is washed into the oceans from land, but some comes from marine cyanobacteria. Despite their essential role in the web of life, we know very little about these nitrogen-fixing organisms.
A paper in Nature Communications fills part of this gap. The Tara Oceans circumnavigation expedition, a philanthropically funded mission to study the marine environment, has demonstrated that UCYN-A operates in symbiosis with two species of single-celled alga prymnesiophytes. UCYN-A needs this symbiosis because it cannot photosynthesize, an unusual deficit for a cyanobacteria, and relies on the energy the prymnesiophytes provide instead.
"This is a very important symbiotic system in marine environments because they are globally distributed, playing a significant role in today's nitrogen and carbon marine cycles,” senior author Dr. Silvia Acinas of the Institut de Ciències del Mar in Barcelona said in a statement.
Acinas and her colleagues showed that two lineages of UCYN-A diverged approximately 91 million years ago, and they think that this is around the time the symbiotic relationship formed. Eventually, UCYN-A lost the genes that enable photosynthesis in its relatives and became a specialist nitrogen fixer.
The alliance appears to have been driven by necessity. “In the Jurassic, between 190 and 100 million years ago, nutrient availability in the ocean was lower than at any point during the last 550 million years,” the authors noted. Consequently, they consider it likely that a common ancestor of the UCYN-As entered into a symbiotic relationship with an ancestral prymnesiophyte to fix some much-needed nitrogen, leading to the Cretaceous nutritional rebound.
Once the symbiosis had formed, UCYN-A no longer needed to photosynthesize for itself, and gradually underwent what the authors call “purifying selection” as unwanted genes disappeared until it could no longer survive without its symbiotic partners. A similar pattern has happened with two freshwater diatom species that have also acquired nitrogen-fixing symbionts.
Each of the two UCYN-A lineages studied has formed a partnership with a different prymnesiophyte, and these have come to fill distinct ecological niches. One of the prymnesiophytes is familiar to marine biologists as Braarudosphaera bigelowii, but the other smaller species has yet to be scientifically described, emphasizing how little we know about the species that help give life to the oceans.