cells in some Magnoliophyta, in favor of the sporogen
(Table 7.4). This trend of reduction in the gametogenous
generation results, in Dasycladophyceae (Chlorobionta), in
its complete disappearance and therefore a monogenetic life
cycle whose single generation is the diploid sporogen. This
trend is also found in Chromobionta (cf. Sect. 7.9.3).
Unicellular Viridiplantae have colonized a wide variety
of habitats, terrestrial, freshwater, and marine. They often
play a major role in the functioning of ecosystems. (i) Most
species are photosynthetic. In freshwater habitats, e.g.
Chlamydomonas, Volvox, Scenedesmus (Chlorophyceae),
Micrasterias, and Closterium (Zygnematophyceae) can
be found. Chaetosphaeridium minus (Chaetosphaeridiophyceae) is epibiontic of freshwater MPOs.
11 In coastal
lagoons and the open sea, the photosynthetic eukaryotes
of the picoplankton (Box 5.2) mainly belong to
Prasinophyceae: Crustomastix, Dolichomastix, Halosphaera
Mamiella, Micromonas, Ostreococcus, and Pyramimonas
(Rodriguez et al. 2005; S ˇ lapeta et al. 2005a; Viprey et al.
2008). In the Thau Lagoon (France, Mediterranean Sea), a
density of 90–250 millions cells per liter of Ostreococcus
tauri was observed in summer (Courties et al. 1998). Photosynthetic eukaryotic picoplankton also consists of
Trebouxiophyceae: Chlorella, Nannochloris, Picochlorum
(Fuller et al. 2006; Viprey et al. 2008). Trebouxia
(Trebouxiophyceae) is the green dust that covers tree
trunks and walls. Parachloroidium thrives in corticolous
biofilms (Neustupa et al. 2013). Chlorokybus atmophyticus
(Chlorokybophyceae) lives in the soil surface of the terrestrial realm. (ii) A number of photosynthetic species are
actually facultative heterotrophs; this is the case of
Chlamydomonas reinhardtii (Chlorophyceae), common in
fresh water and soil, which is able to grow in the dark with
acetate as a carbon source. (iii) Coccomyxa parasitica
(Chlorophyceae) is a unicellular parasite of the mantle of
the sea scallop Placopecten magellanicus and the mussel
Mytilus edulis, that has not lost its chloroplasts; Coccomyxa
ophiurae and C. astericola are parasites of the brittle star
Ophiura texturata and the sea star Hippasteria phrygiana,
respectively; in the brittle star, Coccomyxa ophiurae is
responsible for spine malformations (Stevenson and South
1974, 1976; Gray et al. 1999). Prototheca probably derived
from Chlorella (Trebouxiophyceae) by loss of photosynthesis; members of this genus cause cutaneous infections
in humans and cattle (Lass-Flo ¨rl and Mayr 2007). In
Viridiplantae, the absence of photosynthesis results from a
secondary loss; this is a very rare trait in unicellular taxa. (iv)
Finally, some species have established mutualistic relations
with other species. Trebouxia (Trebouxiophyceae) species
are involved in the lichen mutualistic symbiosis, where they
constitute the photosynthetic partner of the mushroom
(Fungi), e.g. in Xanthoria and Cladonia (Feldmann 1978;
Kroken and Taylor 2000). In the lichen family
Verrucariaceae, a number of trebouxiophycean (mainly
Diplosphaera, but also Asterochloris, Myrmecia, Prasiola,
etc.), ulvophycean (e.g., Dilabifilum), and trentepohliophycean genera (e.g., Trentepohlia) are involved in the
lichenic mutualistic symbiosis (Thu ¨s et al. 2011).
Pedinomonas and Tetraselmis (Prasinophyceae) are mutualistic symbionts of Noctiluca (Dinobionta) and the flatworms
Convoluta (Platyhelminthes, Metazoa), respectively.
7.6
Kingdom Rhizaria
7.6.1 General Remarks
Rhizaria have sometimes been named Cercozoa. However,
Cercozoa (here as Cercobionta) constitutes a subkingdom
within the kingdom Rhizaria. The kingdom Rhizaria is fairly
well characterized at the genetic level (Cavalier-Smith and
Chao 2003). However, its limits are uncertain. From a cytological, biochemical, and biological perspective, it is difficult to identify characters shared by all Rhizaria. The
phylogenetic diversity of Rhizaria is probably considerable,
and this diversity is likely to increase when new taxa are
sequenced. In addition, some taxa currently included within
Rhizaria may prove to belong to other kingdoms (Lecointre
and Le Guyader 2006).
The Rhizaria are mostly unicellular, though a multicellular form has been described (Brown et al. 2012). Most
Rhizaria are amoeboid with peudopods. Many produce
“shells,” more or less complex in structure.
In this chapter, we have chosen to illustrate the phylogenetic diversity of Rhizaria by a small number of
taxa particularly emblematic of this kingdom: Radiolaria,
Chlorarachniobionta, Phytomyxea, and Foraminifera.
7.6.2 Radiolaria
Radiolaria are unicellular, but some species forming
colonies are known. They usually measure between 0.1 and
2 mm in diameter. Colonial species can exceed 10 cm; a
giant species, Collozoum caudatum, up to 2 m long, is
known from equatorial and Gulf Stream waters in the Atlantic Ocean (Swanberg and Anderson 1981). Radiolaria, due to
their siliceous skeleton, fossilize well; the oldest fossils date
back to 600 Ma ago. Thousands of species have been
described.
The cytoplasm consists of two parts. The inner part
(endoplasm) contains the nucleus, most mitochondria, and
the Golgi apparatus; it is limited by a capsular wall
11 MPOs ¼ Multicellular Photosynthetic Organisms.
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
215
(Table 7.4). This trend of reduction in the gametogenous
generation results, in Dasycladophyceae (Chlorobionta), in
its complete disappearance and therefore a monogenetic life
cycle whose single generation is the diploid sporogen. This
trend is also found in Chromobionta (cf. Sect. 7.9.3).
Unicellular Viridiplantae have colonized a wide variety
of habitats, terrestrial, freshwater, and marine. They often
play a major role in the functioning of ecosystems. (i) Most
species are photosynthetic. In freshwater habitats, e.g.
Chlamydomonas, Volvox, Scenedesmus (Chlorophyceae),
Micrasterias, and Closterium (Zygnematophyceae) can
be found. Chaetosphaeridium minus (Chaetosphaeridiophyceae) is epibiontic of freshwater MPOs.
11 In coastal
lagoons and the open sea, the photosynthetic eukaryotes
of the picoplankton (Box 5.2) mainly belong to
Prasinophyceae: Crustomastix, Dolichomastix, Halosphaera
Mamiella, Micromonas, Ostreococcus, and Pyramimonas
(Rodriguez et al. 2005; S ˇ lapeta et al. 2005a; Viprey et al.
2008). In the Thau Lagoon (France, Mediterranean Sea), a
density of 90–250 millions cells per liter of Ostreococcus
tauri was observed in summer (Courties et al. 1998). Photosynthetic eukaryotic picoplankton also consists of
Trebouxiophyceae: Chlorella, Nannochloris, Picochlorum
(Fuller et al. 2006; Viprey et al. 2008). Trebouxia
(Trebouxiophyceae) is the green dust that covers tree
trunks and walls. Parachloroidium thrives in corticolous
biofilms (Neustupa et al. 2013). Chlorokybus atmophyticus
(Chlorokybophyceae) lives in the soil surface of the terrestrial realm. (ii) A number of photosynthetic species are
actually facultative heterotrophs; this is the case of
Chlamydomonas reinhardtii (Chlorophyceae), common in
fresh water and soil, which is able to grow in the dark with
acetate as a carbon source. (iii) Coccomyxa parasitica
(Chlorophyceae) is a unicellular parasite of the mantle of
the sea scallop Placopecten magellanicus and the mussel
Mytilus edulis, that has not lost its chloroplasts; Coccomyxa
ophiurae and C. astericola are parasites of the brittle star
Ophiura texturata and the sea star Hippasteria phrygiana,
respectively; in the brittle star, Coccomyxa ophiurae is
responsible for spine malformations (Stevenson and South
1974, 1976; Gray et al. 1999). Prototheca probably derived
from Chlorella (Trebouxiophyceae) by loss of photosynthesis; members of this genus cause cutaneous infections
in humans and cattle (Lass-Flo ¨rl and Mayr 2007). In
Viridiplantae, the absence of photosynthesis results from a
secondary loss; this is a very rare trait in unicellular taxa. (iv)
Finally, some species have established mutualistic relations
with other species. Trebouxia (Trebouxiophyceae) species
are involved in the lichen mutualistic symbiosis, where they
constitute the photosynthetic partner of the mushroom
(Fungi), e.g. in Xanthoria and Cladonia (Feldmann 1978;
Kroken and Taylor 2000). In the lichen family
Verrucariaceae, a number of trebouxiophycean (mainly
Diplosphaera, but also Asterochloris, Myrmecia, Prasiola,
etc.), ulvophycean (e.g., Dilabifilum), and trentepohliophycean genera (e.g., Trentepohlia) are involved in the
lichenic mutualistic symbiosis (Thu ¨s et al. 2011).
Pedinomonas and Tetraselmis (Prasinophyceae) are mutualistic symbionts of Noctiluca (Dinobionta) and the flatworms
Convoluta (Platyhelminthes, Metazoa), respectively.
7.6
Kingdom Rhizaria
7.6.1 General Remarks
Rhizaria have sometimes been named Cercozoa. However,
Cercozoa (here as Cercobionta) constitutes a subkingdom
within the kingdom Rhizaria. The kingdom Rhizaria is fairly
well characterized at the genetic level (Cavalier-Smith and
Chao 2003). However, its limits are uncertain. From a cytological, biochemical, and biological perspective, it is difficult to identify characters shared by all Rhizaria. The
phylogenetic diversity of Rhizaria is probably considerable,
and this diversity is likely to increase when new taxa are
sequenced. In addition, some taxa currently included within
Rhizaria may prove to belong to other kingdoms (Lecointre
and Le Guyader 2006).
The Rhizaria are mostly unicellular, though a multicellular form has been described (Brown et al. 2012). Most
Rhizaria are amoeboid with peudopods. Many produce
“shells,” more or less complex in structure.
In this chapter, we have chosen to illustrate the phylogenetic diversity of Rhizaria by a small number of
taxa particularly emblematic of this kingdom: Radiolaria,
Chlorarachniobionta, Phytomyxea, and Foraminifera.
7.6.2 Radiolaria
Radiolaria are unicellular, but some species forming
colonies are known. They usually measure between 0.1 and
2 mm in diameter. Colonial species can exceed 10 cm; a
giant species, Collozoum caudatum, up to 2 m long, is
known from equatorial and Gulf Stream waters in the Atlantic Ocean (Swanberg and Anderson 1981). Radiolaria, due to
their siliceous skeleton, fossilize well; the oldest fossils date
back to 600 Ma ago. Thousands of species have been
described.
The cytoplasm consists of two parts. The inner part
(endoplasm) contains the nucleus, most mitochondria, and
the Golgi apparatus; it is limited by a capsular wall
11 MPOs ¼ Multicellular Photosynthetic Organisms.
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
215