Salviniales are an unusual group of ferns. Whereas most ferns are terrestrial or epiphytic, these are freshwater aquatic plants, and whereas most ferns are homosporous (producing only one type of spore), these are heterosporous (they produce microspores and megaspores). Although they are distributed through the tropical and temperate regions of the world, the group includes only about 100 species in five genera. One of these is the exceptional genus Azolla.
Azolla are highly reduced free-floating plants with bilobed leaves only about 1 mm long. There are about 5 species and they are the only nitrogen-fixing ferns. They manage this trick with the help of a cyanobacterial symbiont that lives in a protected area at the shoot apex, and in a cavity in each leaf. As the shoot grows and the leaves develop, the leaves form a cavity that protects the symbiont and allows it to produce biologically available nitrogen from within the safety of the leaf. Genetic studies of this relationship show that some of the housekeeping genes of the cyanobacterium have been exported to the fern, which suggests that this relationships is an old one (Ran et al. 2010; Eily et al. 2019). In Asia, people have been using Azolla as a source of nitrogen-rich fertilizer in rice fields for centuries (Lumpkin and Plucknett, 1980).
Azolla is occasionally in the news since researchers demonstrated that Azolla fossils in middle-Eocene sediments from the Arctic Ocean indicate intervals of freshwater conditions (Brinkhuis et al. 2006). During these intervals, sufficient water drained from North America and Eurasia into the Arctic Ocean to result in sufficiently fresh surface waters, Azolla flourished and and sank, sequestering significant quantities of carbon. The amounts are uncertain, but based on estimates of atmospheric pCO2 for the middle Eocene, and growth rates that would likely have been limited by phosphorous or iron availability, Speelman et al. (2009) estimated that a single Azolla event about 48.5 million years ago may have drawn down CO2 levels 55-470 ppm.
These powerful ferns have a rich fossil record, primarily in the form of dispersed reproductive structures (megaspore apparatuses and microspore massulae). Fossil evidence indicates that the Salviniales are no older than Jurassic, and that Azolla-like plants were present by the Late Cretaceous. Reconstructing the evolutionary history Azolla based on reproductive structures alone is risky because we know that there are extinct plants with similar reproductive structures, but very different vegetative structures (Rothwell and Stockey 1994), but we can get a pretty good idea of the story from those cases where we have both reproductive and vegetative remains associated or attached.
Much of the fossil record of Salviniales comes from paleobotanical work in the Northern Hemisphere, but sampling in the tropics and Southern Hemisphere is improving.
In our recent paper, my colleagues and I described two species of Azolla from the latest Cretaceous and earliest Paleocene deposit of Chubut, Argentina. The specimens were collected as part of a collaboration between Argentine an US scientists to understand how the flora of Patagonia has changed since the Cretaceous period.
These well-preserved fossils consist of stems, leaves, roots, and reproductive structures. The two species, A. coloniensis and A. keuja are the first from the southern hemisphere known from such complete vegetative and reproductive remains. We will continue to explore their significance, but three things that we have learned so far are: 1) Despite being from the same region and within a few million years of each other, the two species are not closely related. This confirms that the diversification of the genus was well underway by ~66 million years ago. 2) One of the two species, A. keuja, does not fit into any of the previously recognized groups used to organize the diversity of living and fossil Azolla species 3) The few extant Azolla are but a shadow of the extinct diversity. A comprehensive understanding the evolution of this group must incorporate that diversity.
In evolutionary biology, evidence from living and extinct species is complementary. Just last year, the complete genome of Azolla filiculoides was published (Li et al. 2018). It is exciting to imagine what new things we will learn about the evolution of this group, the evolution of symbioses in general, and how we might use the N-fixing properties of this plant for agricultural purposes. Although we will likely never get DNA from extinct species of Azolla, we will similarly never understand the scope of morphological and ecological diversity of the group without the fossil record. Keep digging.
View more photos and read our open access paper here:
https://www.journals.uchicago.edu/doi/full/10.1086/704377
P.S. I am very interested in obtaining more photos of living Azolla nilotica, so if you have access to this African species, please don't hesitate to send me a message.