prokaryotes and views eukaryotic cells as the result of ancient symbioses (Carrapiço
2015a; Margulis 1970). This work formed the basis of the Serial Endosymbiotic
Theory (SET), and it constituted the beginning of the rehabilitation and development
of symbiogenic ideas applied not only to the cellular world but also to the construction of a new Biology for the current century (Carrapiço 2015a; Margulis and Fester
1991; Margulis and Sagan 2002). The works of Ris and Plaut (1962) on the detection
of DNA fibers in chloroplasts, and the similar research by Nass and Nass (1963a, b)
in mitochondria, contributed decisively to the acceptance of these new ideas.
Furthermore, the pioneering work of Lynn Margulis and co-workers represents an
important contribution to the development of the post neo-Darwinian approach to
understand biological evolution in an adequate way, as well as to the establishment
of symbiogenesis as an important evolutionary biological mechanism on Earth
(Archibald 2014; Carrapiço 2010a, 2012, 2015a, b).
27.3 Cases of Symbioses and the Role of Symbionts
27.3.1 Azolla as a Polygenomic Entity
A good example of the importance of symbionts to the organism as a whole or
biological unity can be found in the symbiotic system Azolla-Anabaena-bacteria.
Azolla is a small-leafed floating or semi-aquatic heterosporic fern with
overlapping leaves that float or have submersed lobes. The chlorophyllous dorsal
lobe of the leaves contains a prokaryote community (symbionts) in an ovoidal cavity
with filamentous nitrogen fixing cyanobacteria, traditionally assigned to Anabaena
azollae, as well a variety of bacteria strains, mainly assigned to the genera
Arthrobacter, Corynebacterium, and Agrobacterium. However, the cyanobiont’s
identification is still controversial and involves beyond the genus Anabaena, the
genera Nostoc and Trichormus (Carrapiço 2015a, 2017). Also present in the leaf
cavity are three types of trichomes, which show an ultrastructure of transfer cells,
about 20–25 simple hairs, one primary and one secondary branched hair in number
of two (one primary and one secondary) (Carrapiço 2010b, 2015a). Since the Azolla
leaf cavity has no direct connection with the vascular system of the pteridophyte, the
trichomes are involved in the transfer and uptake of metabolites from the fern to the
prokaryote colony and from this one to the plant (Carrapiço 2010b, 2015a).
The leaf cavity behaves as both the physiological and dynamic interface unit of
this symbiotic association, where the main metabolic and energetic flows occur
(Carrapiço 2010b, 2015a; Grilli-Caiola and Forni 1999). In this sense, it can be
considered a natural microcosm, a microecosystem that reveals a self-organization
and an ecological defined structure, where natural selection acts to evolve different
cyanobacteria ecotypes (Carrapiço 2002, 2010b, 2015a; Papaefthimiou et al. 2008).
It can also be considered as a natural photobioreactor (Carrapiço 2010b, 2015a; Shi
and Hall 1988), with millions of years of evolution, where the prokaryote community (cyanobacteria and bacteria) is immobilized and immersed in a fibrillar network
27 The Importance of Being Symbiont and the Role of Symbiosis as a Driving Force. . .
601
2015a; Margulis 1970). This work formed the basis of the Serial Endosymbiotic
Theory (SET), and it constituted the beginning of the rehabilitation and development
of symbiogenic ideas applied not only to the cellular world but also to the construction of a new Biology for the current century (Carrapiço 2015a; Margulis and Fester
1991; Margulis and Sagan 2002). The works of Ris and Plaut (1962) on the detection
of DNA fibers in chloroplasts, and the similar research by Nass and Nass (1963a, b)
in mitochondria, contributed decisively to the acceptance of these new ideas.
Furthermore, the pioneering work of Lynn Margulis and co-workers represents an
important contribution to the development of the post neo-Darwinian approach to
understand biological evolution in an adequate way, as well as to the establishment
of symbiogenesis as an important evolutionary biological mechanism on Earth
(Archibald 2014; Carrapiço 2010a, 2012, 2015a, b).
27.3 Cases of Symbioses and the Role of Symbionts
27.3.1 Azolla as a Polygenomic Entity
A good example of the importance of symbionts to the organism as a whole or
biological unity can be found in the symbiotic system Azolla-Anabaena-bacteria.
Azolla is a small-leafed floating or semi-aquatic heterosporic fern with
overlapping leaves that float or have submersed lobes. The chlorophyllous dorsal
lobe of the leaves contains a prokaryote community (symbionts) in an ovoidal cavity
with filamentous nitrogen fixing cyanobacteria, traditionally assigned to Anabaena
azollae, as well a variety of bacteria strains, mainly assigned to the genera
Arthrobacter, Corynebacterium, and Agrobacterium. However, the cyanobiont’s
identification is still controversial and involves beyond the genus Anabaena, the
genera Nostoc and Trichormus (Carrapiço 2015a, 2017). Also present in the leaf
cavity are three types of trichomes, which show an ultrastructure of transfer cells,
about 20–25 simple hairs, one primary and one secondary branched hair in number
of two (one primary and one secondary) (Carrapiço 2010b, 2015a). Since the Azolla
leaf cavity has no direct connection with the vascular system of the pteridophyte, the
trichomes are involved in the transfer and uptake of metabolites from the fern to the
prokaryote colony and from this one to the plant (Carrapiço 2010b, 2015a).
The leaf cavity behaves as both the physiological and dynamic interface unit of
this symbiotic association, where the main metabolic and energetic flows occur
(Carrapiço 2010b, 2015a; Grilli-Caiola and Forni 1999). In this sense, it can be
considered a natural microcosm, a microecosystem that reveals a self-organization
and an ecological defined structure, where natural selection acts to evolve different
cyanobacteria ecotypes (Carrapiço 2002, 2010b, 2015a; Papaefthimiou et al. 2008).
It can also be considered as a natural photobioreactor (Carrapiço 2010b, 2015a; Shi
and Hall 1988), with millions of years of evolution, where the prokaryote community (cyanobacteria and bacteria) is immobilized and immersed in a fibrillar network
27 The Importance of Being Symbiont and the Role of Symbiosis as a Driving Force. . .
601