''Loxodes'' is a genus of
karyorelictea
Karyorelictea is a class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, ''Loxodes'', is foun ...
n ciliates, belonging to family
Loxodidae
Loxodidae is a family of karyorelict ciliates.
Loxodidae members possess an elongated, laterally flattened shape. They share two key characters: a beak-like anterior rostrum interrupting the perioral kineties, and peculiar cytoplasmic organelle ...
.
It is the only known karyorelictean ciliate that lives in freshwater habitats.
The term ''Loxodes'' derives from the
ancient greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
(), meaning "oblique, tilted".
Ecology
''Loxodes'' lives in
freshwater habitat
Freshwater ecosystems are a subset of Earth's aquatic ecosystems. They include lakes, ponds, rivers, streams, springs, bogs, and wetlands. They can be contrasted with marine ecosystems, which have a larger salt content. Freshwater habitats can be ...
s such as lakes and ponds, unlike other karyorelictean ciliates such as the other loxodid genus ''
Remanella
''Remanella'' is a genus of karyorelict ciliates, belonging to family Loxodidae. Whereas ''Remanella'' inhabits brackish and marine waters, '' Loxodes'' – the other loxodid genus – is a freshwater taxon.
Etymology
The genus name is a pa ...
'', which live in brackish-water or marine habitats.
They feed on bacteria and protists such as
microalgae
Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist indiv ...
. It is
microaerobic
A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than that are present in the atmosphere (i.e. < 21% O2; typically 2–10% O2) for optimal growth.[ A more r ...]
, preferring low concentrations of oxygen, below 5% atmospheric saturation. It can also survive extended periods in anoxic water, where oxygen is absent. Under such conditions, ''Loxodes'' is able to use
nitrate
Nitrate is a polyatomic ion
A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
instead of oxygen as an electron acceptor for
respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellula ...
.
Nitrate respiration is rare among eukaryotes, and ''Loxodes'' was the first eukaryote known to have this capability. ''Loxodes'' is also sensitive to light.
Geotaxis
Both genera in the family Loxodidae have organelles known as
Müller (or Müllerian) vesicles, which are involved in the sensing of gravity. They are about 7 µm across, and contain a membrane-covered mineral body known as a statolith. In ''Loxodes'', the statolith is mostly composed of
barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
Th ...
salts, compared to ''Remanella'', where they are mostly strontium. Its structure and function resembles the
statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. A similar structure is also found in ''Xenoturbella''. The statocyst cons ...
of some animals.
''Loxodes'' uses its Müller's vesicle to distinguish between up and down (
geotaxis
Gravitaxis (or ''geotaxis'') is a form of taxis characterized by the directional movement of an organism in response to gravity. Gravitaxis is one of the many forms of taxis. It is characterized by the movement of an organism in response to gravita ...
or
gravitaxis
Gravitaxis (or ''geotaxis'') is a form of taxis characterized by the directional movement of an organism in response to gravity. Gravitaxis is one of the many forms of taxis. It is characterized by the movement of an organism in response to gravita ...
), which it uses as a stimulus in addition to the oxygen concentration to orient itself in the water column. When oxygen concentrations are high, ''Loxodes'' tends to swim downwards, and vice versa.
File:Loxodes.png, Species of ''Loxodes'' containing significantly large green-colored algae. Scale bar: 10 µm.cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
Genetic code
''Loxodes'' uses a variant of the standard
genetic code
The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
, where the stop codons UAA and UAG have been reassigned to the amino acid
glutamine
Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral, ...
. This
variant code is also used by other ciliates.
Cell cycle
Unlike other ciliates, the macronuclei of
karyorelictea
Karyorelictea is a class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, ''Loxodes'', is foun ...
ns do not divide. This was first observed in ''Loxodes'' by
Otto Bütschli
Johann Adam Otto Bütschli (3 May 1848 – 2 February 1920) was a German zoologist and professor at the University of Heidelberg. He specialized in invertebrates and insect development. Many of the groups of protists were first recognized by him.
...
in the 1870s.
It was later shown to be a distinctive feature of the class Karyorelictea in general. Experiments on ''Loxodes'' have shown that little or no
DNA synthesis
DNA synthesis is the natural or artificial creation of deoxyribonucleic acid (DNA) molecules. DNA is a macromolecule made up of nucleotide units, which are linked by covalent bonds and hydrogen bonds, in a repeating structure. DNA synthesis occur ...
occurs in their macronuclei, and that the DNA content of a macronucleus is only slightly more than that of a
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
micronucleus ("paradiploid").
References
External links
*
{{Taxonbar, from=Q1981570
Karyorelictea
Ciliate genera