What is Azolla?

Division: Pteridophyta
Class: Filicopsida
Order: Salviniales
Family: Azollaceae
Genus: Azolla

Azolla is an aquatic fern growing in various fresh-water environments as lakes and ponds in tropical, subtropical and warm temperate regions. The geographical distribution of the genus ranges from South-, Central- and North America to Africa, India, Ceylon, Japan, Australia, New Zealand and Hawaii (Tryon, 1990).

Fig. 1 Azolla filiculoides

It is a small and free floating fern, with leaves that are clustered, attached to a root-bearing rhizome (Fig. 1). The leaves are overlapping, with floating and submerged lobes. Sporocarps develop on submerged lobes. They contain either several microsporangia (microsporocarps) or one single megasporan-gium (megasporocarps). 

Each microsporangium contains numerous microspores that are clumped together to so called microspore massulae (Fig.2).

Fig. 2 Azolla microspore massulae recovered from mid-Eocene Arctic sediments (Brinkhuis et al, 2006).

The megasporangium, on the other hand, contains just one megaspore, which is arranged in a so called megaspore apparatus together with numerous floats (Fig.3). At maturity, the sporangia break open and the massulae that are released into the water get attached to the megaspore apparatus. 



Fig. 3 Light-microscope image of a megaspore apparatus. The section (right) shows the megaspore and the floats


Azolla is living in symbiosis with the nitrogen fixing cyanobacterium Anabaena azollae and several genera of bacteria that are living in the leaf cavity. During the life-cycle of Azolla the symbionts are transferred from the parent plant to the new sporophyte (Fig.4). This symbiotic association is thought to be a co-evolved system (Carrapiço, 2006). 


Fig. 4 Azolla’s life cycle, showing the permanent presence of bacteria and cyanobacteria throughout the fern’s life cycle (Carrapiço, 2006).

Anabaena azollae supplies Azolla with sufficient organic nitrogen for its growth, which makes phosphorus the most important (limiting) nutrient for Azolla. It is known that high phosphorous concentrations in the water can cause big blooms of this fern. For example in Portugal in 1993 high phosphorus concentrations were causing an enormous bloom of Azolla filiculoides in the Guadiana River (Fig.5).

Fig. 5 Azolla filiculoides bloom on the Guadiana River in Portugal.
(Carrapiço et al., 1996) 

Nowadays, seven extant species of Azolla are known, which are arranged in three different subgenera: A. filiculoides, A. rubra, A. caroliniana, A. microphylla and A. mexicana belong to the subgenus Azolla, A. pinnata belongs to Rhizosperma and A. nilotica belongs to Tetrasporocarpia (Saunders & Fowler, 1993). Furthermore, more than fifty fossil species are known, the oldest one dating back to the late-Cretaceous (Collinson, 1980).

References:

Brinkhuis et al., 2006. Episodic fresh surface waters in the Eocene Arctic Ocean. Nature 441: 606-609.

Carrapiço, F. 2006. Is the Azolla-Anabaena symbiosis a co-evolution case? Proceedings of the International conference ' General Botany: Traditions and Perspective', Department of Botany of the Kazan University.

Carrapiço, F. et al., 1996. The uncontrolled growth of Azolla in the Guadiana River. Aquaphyte Newsletter 16: Nr.2.

Collinson, M.E. 1980. A new multiple-floated Azolla from the Eocene of Britain with a brief review of the genus. Palaeontology, 23: 213-229.

Saunders, R.M.K and Fowler, K.1993. The supraspecific taxonomy and evolution of the fern genus Azolla (Azollaceae). Plant Systematics and Evolution 184: 175-193.

Tryon, A.F. 1990. Spores of the Pteridophyta. Springer-Verlag New York.