Stygiellidae is a family of free-living marine

flagellates A flagellate is a cell or organism with one or more whip-like appendages called flagellum, flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryote ...

belonging to the order Jakobida, a deep-branching lineage within the eukaryotic supergroup

Discoba Excavata is an obsolete, extensive and diverse Paraphyly, paraphyletic group of unicellular Eukaryote, Eukaryota. The group was first suggested by Simpson and Patterson in 1999 and the name latinized and assigned a rank by Thomas Cavalier-Smit ...

. They are

unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...

organisms that commonly inhabit

anoxic Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

-rich and

ammonium Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...

-rich marine habitats worldwide.

Cell morphology and behavior

Members of Stygiellidae are genetically diverse but morphologically similar

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...

jakobid Jakobida is an order (sole order in the class Jakobea) of free-living, heterotrophic, flagellar eukaryotes in the clade Discoba. They are small (less than 15 μm), and can be found in aerobic and anaerobic environments. The order Jakobida, b ...

s. The unifying aspect of their appearance is their

crista A crista (; : cristae) is a fold in the inner mitochondrial membrane, inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large a ...

e-lacking

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

due to the

secondary loss In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

aerobic metabolism Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cellu ...

. They resemble jakobid cells, and form two morphotypes: grooved cells, attached to the substrate, with a conspicuous groove; and swimming cells, with a less distinct, narrower groove. Both morphotypes move in a spiral motion, although the swimming cells are faster. It is difficult to distinguish morphological aspects between species due to the variability among cells within the same strain and the similarity between grooved and swimming cells.

Ecology and distribution

Stygiellidae are

bacterivorous A bacterivore is an organism which obtains energy and nutrients primarily or entirely from the consumption of bacteria. The term is most commonly used to describe free-living, heterotrophic, microscopic organisms such as nematodes as well as many s ...

nanoflagellates almost exclusively found in marine,

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

-poor (

or microoxic) environments, often in presence of

sulfides Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families of ...

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

. However, some environmental sequences of Stygiellidae have been reported in oxic waters in

Saanich Inlet Saanich Inlet (also Saanich Arm) is a body of salt water that lies between the Saanich Peninsula and the Malahat, British Columbia, Malahat highlands of Vancouver Island, British Columbia, Canada. Located just northwest of Victoria, British Colum ...

, without sulfide or ammonium. They are also detected in other non-marine saline environments such as

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuary ...

s and inland

salt springs A brine spring or salt spring is a saltwater spring. Brine springs are not necessarily associated with halite deposits in the immediate vicinity. They may occur at valley bottoms made of clay and gravel which became soggy with brine seeped down ...

, but never in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

habitats.

Evolution and classification

Evolution of the anaerobic metabolism

The anaerobic Stygiellidae is one of two

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

s of the suborder Andalucina, the other clade being the aerobic Andaluciidae. Andalucina, along with Histionina and Ophirinina, are the two lineages of Jakobida, an important group in the supergroup

. Stygiellidae, unlike the rest of jakobids, have evolved to adopt an obligatory

lifestyle, and possess

mitochondrion A mitochondrion () is an organelle found in the cell (biology), cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosine tri ...

-related

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s (MROs) that lack

e. They appear to lack all components of the

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

and the

TCA cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of chemical reaction, biochemical reactions that release the energy stored in nutrients through acetyl-Co ...

, except for two

complex I Respiratory complex I, (also known as NADH:ubiquinone oxidoreductase, Type I NADH dehydrogenase and mitochondrial complex I) is the first large protein complex of the respiratory chains of many organisms from bacteria to humans. It catalyzes th ...

subunits, and no

mitochondrial genome Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in ...

has been found. Instead, the MROs possess components of the

pyruvate metabolism Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic ac ...

similar to the

hydrogenosome A hydrogenosome is a membrane-enclosed organelle found in some Anaerobic organism, anaerobic Ciliate, ciliates, Flagellate, flagellates, Fungus, fungi, and three species of Loricifera, loriciferans. Hydrogenosomes are highly variable organelles t ...

s seen in Metamonada, another basal anaerobic group of eukaryotes. Overall, Stygiellidae appear to have retained a wider group of genes for the mitochondrial aminoacid metabolism than

metamonad The metamonads are a large group of flagellate amitochondriate microscopic eukaryotes. They include the retortamonads, diplomonads, parabasalids, oxymonads, and a range of more poorly studied taxa, most of which are free-living flagellates. ...

Internal phylogeny

Stygiellidae is a monophyletic group containing all jakobids detected in

habitats. It splits into six well-supported clades: '' Stygiella'', '' Velundella'' and four

environmental Environment most often refers to: __NOTOC__ * Natural environment, referring respectively to all living and non-living things occurring naturally and the physical and biological factors along with their chemical interactions that affect an organism ...

clades (named EC I–IV), all of which appear strongly monophyletic.

Classification

The family contains 6 species distributed in 2 genera. *'' Stygiella'' (4 species) **'' Stygiella incarcerata''
= ''Andalucia incarcerata''
= ''Jakoba incarcerata'' **'' Stygiella adhaerens'' **'' Stygiella agilis'' **'' Stygiella cryptica'' *'' Velundella'' (2 species) **'' Velundella trypanoides'' **'' Velundella nauta''

References

External links

Stygiellidae
on the UniEuk taxonomy map. {{Discoba Discoba families Taxa described in 2015