Hamus (archaea)
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Archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
, one of the three domains of life, are a highly diverse group of
prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s that include a number of extremophiles. One of these
extremophile An extremophile () is an organism that is able to live (or in some cases thrive) in extreme environments, i.e., environments with conditions approaching or stretching the limits of what known life can adapt to, such as extreme temperature, press ...
s has given rise to a highly complex new
appendage An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...
known as the hamus (: hami). In contrast to the well-studied prokaryotic appendages pili and fimbriae, much is yet to be discovered about
archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
l appendage such as hami. Appendages serve multiple functions for cells and are often involved in attachment, horizontal conjugation, and movement. The unique appendage was discovered at the same time as the unique community of archaea that produces them. Research into the structure of hami suggests their main function aids in attachment and
biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
formation. This is accomplished due to their evenly placed prickles, helical structure, and barbed end. These appendages are heat and
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
resistant, aiding in the cell's ability to live in extreme environments.


Archaeal background

In 1977, archaea, then known as archaebacteria, were first discovered when Carl Woese and George Fox published their findings in the ''Proceedings of the National Academy of Sciences,'' stating that these organisms were distantly related to
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
. This revolutionized biology into the three domains of life known today; Bacteria, Eukarya, and Archaea. By checking the ratios of biogenic
isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
s that are unique to different metabolisms, scientists have dated archaea as far back as 2,500 million years. Due to oxygen being a trace element in the atmosphere at this time, archaea
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 ...
methanotrophy is believed to have preceded bacterial
aerobic Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cellu ...
methanotrophy. When studying
phylogenetic tree A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...
s, Bacteria are evolved from the last universal common ancestor or
LUCA Luca or LUCA may refer to: People * Luca (masculine given name), including a list of people * Luca (feminine given name), including a list of people * Luca (surname), including a list of people Places * The ancient name of Lucca, an Etruscan ...
, while Archaea and Eukarya are considered sister lineages because they share a last common ancestor that is more recent than LUCA.


Hami function

Archaea, much like other microorganisms, possess a variety of extracellular appendages to facilitate important functions such as motility,
cell adhesion Cell adhesion is the process by which cells interact and attach to neighbouring cells through specialised molecules of the cell surface. This process can occur either through direct contact between cell surfaces such as Cell_junction, cell junc ...
, and
DNA transfer In molecular biology and genetics, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material from its surroundings through the cell membrane(s). For transformation to ta ...
. Unlike fimbriae and pili, whose composition and function(s) are well defined among bacterial species, hami belong to a relatively new class of filamentous cell appendages unique to archaea. Archaeal cells may have as many as 100 hami, which are largely composed of 120
kDa The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted f ...
subunits. Each hamus is helical in shape with many hook-like projections at the distal end, which are hypothesized to aid in attachment to surfaces within the environment, or in the formation of biofilms.


Hami producers

Archaeal cells possessing hami appear to grow only in relatively cold aquatic environments around 10 degrees Celsius, which could be suggestive of a particular function that has not yet been defined. One possible explanation for this observation could be the relationship archaeal cells – ''SM1 euryarchaeon,'' possessing hami – have with '' Thiothrix,'' a type of sulfur-oxidizing bacterium typically found within similar conditions. Hamus-bearing archaeal cells sometimes form macroscopically visible communities with ''Thiothrix'' or ''IMB1 ε-proteobacterium,'' called a string-of-pearls. Thiothrix and ''IMB1 ε- proteobacterium'' are filamentous bacteria that appear to form the outer shell of the pearl as well as the strings that connect these pearls together. Within the pearls, it appears the archaea ''SM1 euryarchaeon'' forms the majority of the core. Research has shown the ''SM1 euryarchaeon'' use the hamus to aid in biofilm formation. The formation of string-of-pearls communities suggests a mutual dependency for nutrient exchange, though the entirety of this unique relationship has yet to be established. Another hami producing biofilm was discovered that was dissimilar from the string pearl formation. This biofilm consists almost entirely of SM1 archaea making it the first biofilm found of this nature as no other biofilm with a nearly pure composition of archaea has been found. This biofilm has a highly organized structure with distances between cells being exceptionally consistent. Scientists speculate the hami are not only responsible for the strong attachments found in the biofilm formation but also this highly intricate and specific structure. It is possible that other archaeal cells possessing hami have not yet been discovered or cultured.


Hami special abilities

Archaeal appendages serve a variety of purposes and provide the archaeal cells with multiple unique and essential abilities. Hami play a large role in cellular attachment. These appendages allow the cells to adhere to each other, as well as their surroundings. When the hami filaments of one cell come into contact with a neighboring cell, the hami are able to entangle and produce a web like structure between the cells. This helps to form and maintain the biofilm. Hami are also used by the cells in biofilms or individually to adhere to external environmental surfaces. They have been proven to attach to substances with varying chemical compositions including those of an inorganic nature. Hami are also capable of contributing to the
EPS An extended play (EP) is a Sound recording and reproduction, musical recording that contains more tracks than a Single (music), single but fewer than an album. Contemporary EPs generally contain up to eight tracks and have a playing time of 1 ...
of the cell as part of the main protein component of the EPS. One interesting facet of these hami is that their 120 kDa protein allows them to remain stable over a broad range of temperatures. One research experiment found hami to be stable at 70 degrees C and noted the finding curious as the only currently known hami-producing cells live in 10 degrees C. These hami were also noted to be stable over a significant pH range of 0.5-11.5. Archaea are known as extremophiles and live in extreme environments, but this capacity to remain stable over a large range of both pH and temperature makes hami very unique structures. Similarly, this lends to the possibility that archaeal hami may exist in other yet to be discovered biofilms outside of the 10 degree C temperature range and in various pH ranges.


See also

*
Cannula (archaea) Cannula (: cannulae) is the term to describe hollow structures which resemble tubes that are only found in certain archaeal cells. Specifically identifying family members of the '' Pyrodictium'' genus, many examples have been examined from hydr ...
*
Pilus A pilus (Latin for 'hair'; : pili) is a hair-like cell-surface appendage found on many bacteria and archaea. The terms ''pilus'' and '' fimbria'' (Latin for 'fringe'; plural: ''fimbriae'') can be used interchangeably, although some researchers ...


References

{{Reflist Archaea biology Prokaryotic cell anatomy Molecular biology stubs Organelles