Most Dinobionta species thrive in marine and freshwater
plankton. About half of the species are photosynthetic.
A number of photosynthetic species live in mutualistic symbiosis with a variety of hosts: e.g. metazoans (sponges, jellyfish, anemones, mollusks, scleractinian corals, gorgonians),
ciliates, radiolarians, and foraminifera. Symbiodinum, in particular, is associated with scleractinian corals; it provides
them with most of the products of photosynthesis and they
in return receive waste (nitrogen, phosphorus); this mutualism, which keeps the nutrients in a closed circuit, is the key to
the extraordinary richness of coral reef ecosystems in very
oligotrophic* waters, i.e. very poor in nutrients. Although
photosynthetic, many species are mixotrophic. This is the
case of Karlodinium armiger; it has a chloroplast, but only
in the presence of light, growth is low; it is capable of preying
on a wide variety of unicellular species; it extracts the
contents of prey through a feeding tube, but may also ingest
whole prey cells; in the dark, predation is not enough to
ensure the growth and survival; thus, K. armiger is an omnivorous obligate mixotrophic species (Berge et al. 2008).
Finally, many species of Dinobionta are obligate
heterotrophs. Haplozoon axiothella lives in the intestine of
an annelid, Axiothella rubrocincta (Fig. 5.6e); Noctiluca spp.
are predators that engulf their food, e.g. diatoms, other
Dinobionta, metazoan eggs, and bacteria, by phagocytosis;
Pfiesteria spp. are parasites of teleosts which are claimed to be
responsible for large fish kills (Vogelbein et al. 2002);
Dissodinium pseudolunula parasites the eggs of planktonic
copepods (Drebes 1981).
A number of species of Dinobionta may proliferate to
the point where they generate what is known as “red tides”
(more correctly named “discolored waters”). The cell density can reach 20 million per liter. The color of the water
can become red, but sometimes brown, yellow, etc. Many
of these species produce toxins, so that red tides are often
associated with mass mortalities of marine and lagoon
animals (e.g., fish) or problems related to human health.
Generally speaking, blooms of planktonic unicellular
organisms that cause negative impacts to other organisms
via production of natural toxins, mechanical damage to
other organisms, or by other means, are named “Harmful
Algal Blooms” (HABs). Although HABs can be caused by
taxa other than Dinobiontes, the latter are responsible for
75 % of HABs (Genovesi-Giunti 2006). In tropical and
subtropical waters, shallow benthic Dinobionta belonging to
a complex of cryptic species (e.g., Gambierdiscus toxicus,
G. caribaeus, and G. carolinianus; Litaker et al. 2009)
produce toxins (e.g., maitotoxin) transmitted through the
food chain with magnification via bioaccumulation; the consumption of predator teleosts, near the top of the food chain,
causes ciguatera poisoning; hallmark symptoms of ciguatera
in humans include gastrointestinal and neurological effects;
severe case of ciguatera can result in long-term disability and
even death.
7.8.4 Apicomplexa
The Apicomplexa
19 (also referred to as Apicomplexia and
Sporozoa) are unicellular intracellular parasites of metazoans.
There are about 5,000 known species, but this is almost
certainly a gross underestimate of the actual number.
The cell of Apicomplexa is characterized by an apical complex (a structure involved in penetrating the host’s cell) which is
constituted in particular by a conoid (a set of spirally arranged
microtubules) and secretory bodies (rhoptry and micronemes)
which produce enzymes allowing penetration into the host cell.
The name of the taxon is derived from this apical complex. The
centriole, where present, has an unconventional structure; it
consists of a central single microtubule surrounded by nine
singlet microtubules, a deviation from the classical structure in
eukaryotes, characterized by nine sets of microtubules triplets.
The size of the genome is very small (3,800 genes in Cryptosporidium parvum, 5,300 in Plasmodium falciparum); it is
among the smallest in eukaryotes, which is a consequence of
parasitism (Abrahamsen et al. 2004). Flattened alveoli are present below the plasmalemma and mitochondria have tubular
cristae, which are shared characters in Alveolata.
Most Apicomplexa have an apicoplast, an organelle
surrounded by 3–4 membranes which is the non-photosynthetic
relict of a former chloroplast (McFadden and Waller 1997;
McFadden et al. 2001). The ancestors of the present
Apicomplexa were therefore photosynthetic organisms. Photosynthesis originated from a secondary endosymbiosis with a
Rhodobionta (Archaeplastida) (Bhattacharya et al. 2003). The
discovery of Chromera velia and Vitrella brassicaformis
(Chromerida; Fig. 7.28), sort of ‘living fossils’ close to the
supposed ancestor of the Apicomplexa, which have chloroplasts,
confirms the ancestry of photosynthesis in Apicomoplexa
(Keeling 2008; Moore et al. 2008; Obornı ´k et al. 2012).
The life cycle is often extremely complex and involves two
successive hosts: an intermediate host and a definitive host, in
which sexual reproduction occurs. In the case of Plasmodium
falciparum, a species causing malaria in humans, the intermediate host is the man and the definitive host is a mosquito of
the genus Anopheles (Fig. 7.37). The fertilization is of the
fucogamy type and the life cycle can be interpreted as digenetic
monophasic, the single phase being a haplophase (Fig. 7.38);
however, it can also be interpreted as showing an alternation
between a haplophase and diplophase (Lecointre and Le
Guyader 2006). In Toxoplasma gondii, that causes the disease toxoplasmosis, rodents are the intermediate hosts while
felids, e.g. domestic cats, are the definitive hosts; it can
reproduce sexually only within the intestines of members of
the felid family. T. gondii appears to manipulate the behavior
of rodents in ways that would increase their predation by cats
19 The name Apicomplexa is derived from le Latin apex (top) and
complexus (infolds).
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
227
plankton. About half of the species are photosynthetic.
A number of photosynthetic species live in mutualistic symbiosis with a variety of hosts: e.g. metazoans (sponges, jellyfish, anemones, mollusks, scleractinian corals, gorgonians),
ciliates, radiolarians, and foraminifera. Symbiodinum, in particular, is associated with scleractinian corals; it provides
them with most of the products of photosynthesis and they
in return receive waste (nitrogen, phosphorus); this mutualism, which keeps the nutrients in a closed circuit, is the key to
the extraordinary richness of coral reef ecosystems in very
oligotrophic* waters, i.e. very poor in nutrients. Although
photosynthetic, many species are mixotrophic. This is the
case of Karlodinium armiger; it has a chloroplast, but only
in the presence of light, growth is low; it is capable of preying
on a wide variety of unicellular species; it extracts the
contents of prey through a feeding tube, but may also ingest
whole prey cells; in the dark, predation is not enough to
ensure the growth and survival; thus, K. armiger is an omnivorous obligate mixotrophic species (Berge et al. 2008).
Finally, many species of Dinobionta are obligate
heterotrophs. Haplozoon axiothella lives in the intestine of
an annelid, Axiothella rubrocincta (Fig. 5.6e); Noctiluca spp.
are predators that engulf their food, e.g. diatoms, other
Dinobionta, metazoan eggs, and bacteria, by phagocytosis;
Pfiesteria spp. are parasites of teleosts which are claimed to be
responsible for large fish kills (Vogelbein et al. 2002);
Dissodinium pseudolunula parasites the eggs of planktonic
copepods (Drebes 1981).
A number of species of Dinobionta may proliferate to
the point where they generate what is known as “red tides”
(more correctly named “discolored waters”). The cell density can reach 20 million per liter. The color of the water
can become red, but sometimes brown, yellow, etc. Many
of these species produce toxins, so that red tides are often
associated with mass mortalities of marine and lagoon
animals (e.g., fish) or problems related to human health.
Generally speaking, blooms of planktonic unicellular
organisms that cause negative impacts to other organisms
via production of natural toxins, mechanical damage to
other organisms, or by other means, are named “Harmful
Algal Blooms” (HABs). Although HABs can be caused by
taxa other than Dinobiontes, the latter are responsible for
75 % of HABs (Genovesi-Giunti 2006). In tropical and
subtropical waters, shallow benthic Dinobionta belonging to
a complex of cryptic species (e.g., Gambierdiscus toxicus,
G. caribaeus, and G. carolinianus; Litaker et al. 2009)
produce toxins (e.g., maitotoxin) transmitted through the
food chain with magnification via bioaccumulation; the consumption of predator teleosts, near the top of the food chain,
causes ciguatera poisoning; hallmark symptoms of ciguatera
in humans include gastrointestinal and neurological effects;
severe case of ciguatera can result in long-term disability and
even death.
7.8.4 Apicomplexa
The Apicomplexa
19 (also referred to as Apicomplexia and
Sporozoa) are unicellular intracellular parasites of metazoans.
There are about 5,000 known species, but this is almost
certainly a gross underestimate of the actual number.
The cell of Apicomplexa is characterized by an apical complex (a structure involved in penetrating the host’s cell) which is
constituted in particular by a conoid (a set of spirally arranged
microtubules) and secretory bodies (rhoptry and micronemes)
which produce enzymes allowing penetration into the host cell.
The name of the taxon is derived from this apical complex. The
centriole, where present, has an unconventional structure; it
consists of a central single microtubule surrounded by nine
singlet microtubules, a deviation from the classical structure in
eukaryotes, characterized by nine sets of microtubules triplets.
The size of the genome is very small (3,800 genes in Cryptosporidium parvum, 5,300 in Plasmodium falciparum); it is
among the smallest in eukaryotes, which is a consequence of
parasitism (Abrahamsen et al. 2004). Flattened alveoli are present below the plasmalemma and mitochondria have tubular
cristae, which are shared characters in Alveolata.
Most Apicomplexa have an apicoplast, an organelle
surrounded by 3–4 membranes which is the non-photosynthetic
relict of a former chloroplast (McFadden and Waller 1997;
McFadden et al. 2001). The ancestors of the present
Apicomplexa were therefore photosynthetic organisms. Photosynthesis originated from a secondary endosymbiosis with a
Rhodobionta (Archaeplastida) (Bhattacharya et al. 2003). The
discovery of Chromera velia and Vitrella brassicaformis
(Chromerida; Fig. 7.28), sort of ‘living fossils’ close to the
supposed ancestor of the Apicomplexa, which have chloroplasts,
confirms the ancestry of photosynthesis in Apicomoplexa
(Keeling 2008; Moore et al. 2008; Obornı ´k et al. 2012).
The life cycle is often extremely complex and involves two
successive hosts: an intermediate host and a definitive host, in
which sexual reproduction occurs. In the case of Plasmodium
falciparum, a species causing malaria in humans, the intermediate host is the man and the definitive host is a mosquito of
the genus Anopheles (Fig. 7.37). The fertilization is of the
fucogamy type and the life cycle can be interpreted as digenetic
monophasic, the single phase being a haplophase (Fig. 7.38);
however, it can also be interpreted as showing an alternation
between a haplophase and diplophase (Lecointre and Le
Guyader 2006). In Toxoplasma gondii, that causes the disease toxoplasmosis, rodents are the intermediate hosts while
felids, e.g. domestic cats, are the definitive hosts; it can
reproduce sexually only within the intestines of members of
the felid family. T. gondii appears to manipulate the behavior
of rodents in ways that would increase their predation by cats
19 The name Apicomplexa is derived from le Latin apex (top) and
complexus (infolds).
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
227