Azolla Lam., Encycl.

Kessler, Michael & Smith, Alan R., 2017, Prodromus of a fern flora for Bolivia. XVI. Salviniaceae, Phytotaxa 329 (1), pp. 97-100 : 97-98

Azolla Lam., Encycl. 1(1): 343. 1783.

Recognized by the small, alternate leaves 0.4–2 mm long, each 2-lobed with a greenish or reddish upper lobe and a membranaceous, translucent, submerged lobe. Most often mistaken for duckweed ( Lemna L. and allies, Araceae ) among which it frequently grows. It has megaspores with floating structures and microspores that are contained in 4-clumps (massulae) which often have hooked projecting structures (glochidia). Identification of species is based mainly on microscopic characters of these reproductive organs, whether the glochidia on the microsporangia are septate or not, megaspore surface ornamentation, zonation of the perine, and number of cells in epidermal trichomes ( Fowler & Stennet-Willson 1978, Perkins et al. 1985). Unfortunately, most specimens (>70%) are sterile and cannot be reliably identified. Even if fertile material is available, species identification may not be possible with a dissecting microscope (40 × magnification); a compound microscope and even the scanning electron microscope may be necessary. Species boundaries are also unclear, and the number of species, their distribution, and ecology are unclear ( Svenson 1944, Dunham 1986, Zimmermann et al. 1991, Saunders & Fowler 1992, 1993, Evrard & Van Hove 2004). We here apply a broad species concept, recognizing only two species in the Neotropics; four additional species occur in the Palaeotropics.

Azolla is an ancient genus with a rich fossil record (e.g., Hall 1974, Foster & Harris 1981), and estimated divergence times between species that are much older than usual among ferns ( Reid et al. 2006, Metzgar et al. 2007). The spongy mesophyll of the leaves of Azolla contains colonies of the nitrogen-fixing cyanobacterium Anabaena azollae Strassburger , making this one of the economically and ecologically most important fern genera ( Lumpkin & Plucknett 1980, Watanabe & van Hove 1996, Wagner 1997). Because of the resulting capability for nitrogen enrichment, Azolla is used to fertilize flooded rice fields ( Talley & Rains 1980), while also being used to extract nutrients from waste water ( Forni et al. 2001). Perhaps most astonishingly, it is thought that a bloom of Azolla on the Arctic Ocean during the Eocene (about 49 million years ago), when this was still a warm ocean, led to such a depletion of atmospheric carbon dioxide as to reduce the greenhouse effect and thus trigger a global cooling phase ( Brinkhuis et al. 2006, Speelman et al. 2009).