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==Introduction==
{{Uncurated}}
[[Image:Phyllosphere b gross.png|thumb|400px|right|]]
=Introduction=


To date, the genus ''Ignicoccus''  is comprised of single cells that are irregularly shaped coccoid ranging in diameter from 1-3 µm. The Achaean genus was first isolated from [http://en.wikipedia.org/wiki/Hydrothermal_vent marine hydrothermal vents] from [http://en.wikipedia.org/wiki/Kolbeinsey Kolbeinsey Ridge] in north Iceland and also off the coast of Mexico [[#References|[1]]] (see Figure 1). They were found to have a novel cell envelope unseen before in other Achaea [[#References|[2]]], and have a very complex and poorly understood symbiotic relationship with [http://en.wikipedia.org/wiki/Nanoarchaeum_equitans ''Nanoarchaeum equitans'' ]  [[#References|[3]]] [[#References|[4]]] [[#References|[5]]] [[#References|[6]]].
[[File:Ignicoccus locations.png|200px|thumb|right| Map showing the discovery locations of  ''I.pacificus '' (A), and  ''I.islandicus '' and '' I.hospitalis '' (B).]]


The [[http://en.wikipedia.org/wiki/Phyllosphere phyllosphere]] is a term used in microbiology to refer to leaf surfaces or total above-ground surfaces of a plant as a habitat for microorganisms. All plants are host to numerous and diverse communities of microorganisms including bacteria, fungi, and yeasts. Some are beneficial to the plant, others function as plant pathogens and may damage the host plant or even kill it (4). However, the majority of bacterial colonists on any given plant have no noticeable effect on plant growth or function.


Conservative estimates indicate that the roughly 1 billion square kilometers of worldwide leaf surfaces host more than 10^26 bacteria, which are the most abundant colonizers of this habitat (2). The overall microbiota in this ecosystem is thus sufficiently large to have an impact on the global carbon and nitrogen cycles. Additionally, the phyllosphere inhabitants influence their hosts at the level of the individual plants.
=Current Species=


With the repeated, rapid alteration of environmental conditions occurring on leaf surfaces, the phyllosphere has been recognized as a hostile environment to bacteria. Leaf surfaces are often dry and temperatures can reach 40–55°C under intense sunlight. During the night, however, leaves are frequently wet with dew and at cool temperatures (5–10°C) (2).
There are three officially recognized ''Ignicoccus''  species: ''Ignicoccus hospitalis'' , '' Ignicoccus pacificus '' and '' Ignicoccus islandicus'' . The three species were initially identified by 16S rRNA gene analysis from the hydrothermal vent samples obtained from Kolbeinsey Ridge and the coast of Mexico[[#References|[1]]] . All three species have been characterized as hyperthermophiles that are also [http://en.wikipedia.org/wiki/Obligate_anaerobe obligate anaerobes] which explains the presence of ''Ignicoccus'' species near hydrothermal vents[[#References|[1]]] . None of the members of the ''Ignicoccus''  genus have been found to be [http://en.wikipedia.org/wiki/Pathogenic] pathogenic to humans.


Microbes that live in the Phyllosphere are called [[http://en.wikipedia.org/wiki/Epiphyte Epiphytes]]
=Morphology=


==Physical environment==
The members of the ''Ignicoccus''  genus are motile irregular coccoid cells that range in diameter from 1 to 3 µm. The motility observed is due to the presence of flagella, but unfortunately the polarity of the flagella is not yet fully elucidated. They are known to have an outer-membrane but no [http://en.wikipedia.org/wiki/S-layer S-layer]. This is a novel characteristic for these [http://en.wikipedia.org/wiki/Archaea Archaea] because''Ignicoccus'' are the only known Archaea that have been shown to possess an outer-membrane[[#References|[2]]] [[#References|[10]]] .  
The leaf surface has long been considered a hostile environment for bacterial colonists. The leaf surface is exposed to rapidly fluctuating temperature and relative humidity, as well as repeated alternation between presence and absence of free moisture due to rain and dew (3,6). The leaf also provides limited nutrient resources to bacterial colonists. Such rapid and extreme fluctuations in the physical conditions of above ground plant surfaces yields a hostile microbe environment.


Several factors may influence the microhabitat experienced by bacteria on leaves. First, the leaf itself is surrounded by a very thin laminar layer in which moisture emitted through stomata may be sequestered, thereby alleviating the water stress to which epiphytes are exposed (4).
[[File:Huber-abb2.jpg|300px|thumb|left| Ultrathin section of an ''Ignicoccus hospitalis'' cell.]]


Second, some cells in a leaf bacterial population, particularly in [[http://en.wikipedia.org/wiki/Plant_pathology plant-pathogenic]] populations, may not reside in exposed sites on the leaf surface but instead may at least locally invade the interior of the leaf, avoiding the stresses on the exterior of the leaf by residing in substomatal chambers or other interior locations. Thus, while some phytopathogens may have the option of avoiding stresses, most other epiphytes apparently must tolerate them in some way.
[[Image:Leaf surface1.jpg|thumb|300px|right| "Under the microscope, aerial plant leaves resemble eerie landscapes, with deep gorges, tall peaks and gaping pits that riddle the waxy surface." -Leveau, J. (2009)]]
The surfaces of most plants are very tortuous at the small scales at which interactions with bacteria will occur. Epidermal cells produce bulges and troughs that will determine the shape and size of low areas on the surface, which in turn will influence the shape and spread of water droplets on the plant.  The first contact between immigrating bacteria and a leaf normally occurs at the plant cuticle. This waxy layer, which has different three-dimensional crystalline structures on different plant species and can change as leaves age, presumably in part due to microbial modifications, limits passive diffusion of nutrients and water vapor from the plant interior onto the surface and defines the hydrophobicity of the leaf. Thick waxy cuticles have thus been thought to interfere with bacterial colonization of plants by limiting diffusion of nutrients and inhibiting the wetting of the leaf surface.


Because the Phyllosphere is a hostile environment for the residing microorganisms  physical parameters contribute to stressful conditions, such as UV radiation, temperature shifts, and the presence of reactive oxygen species. Adaptation to stressful conditions was reflected by the detection of various proteins, assigned to diverse bacterial genera and detected in all analyzed samples. Among these proteins were superoxide dismutase, catalase, DNA protection proteins, chaperones, and proteins involved in the formation of the osmoprotectant trehalose (1).
==Outer-Membrane==


==Biological interactions==
The outer-membrane of ''Ignicoccus''  species was found to be composed of various derivatives of the typical lipid [http://en.wikipedia.org/wiki/Archaeol archaeol], including the derivative known as [http://en.wikipedia.org/wiki/Caldarchaeol caldarchaeol] [[#References|[5]]] . The outer-membrane is dominated by a pore composed of the Imp1227 protein (''Ignicoccus''  outer membrane protein 1227). The Imp1227 protein forms a large nonamer ring with a predicted pore size of 2nm[[#References|[7]]] .
Certain bacteria in the Phyllosphere can increase the wettability of leaves by producing compounds with surfactant properties. Studies have found that this ability occurred in 50% of the Pseudomonas strains tested. Because of the hydrophobic nature of the cuticle, it is likely that increased wettability of these habitats allows solubilization and diffusion of substrates, making them more readily available to epiphytic bacteria (7,8). Alternatively, biosurfactants may facilitate the movement of bacteria on the phylloplane, as was suggested for tolaasin, a toxin produced by Pseudomonas tolaasi. The water film created by the surfactant could spread the bacteria across the leaf surface to areas where nutrients are more abundant. Thus, the production of biosurfactants may be one trait by which bacteria can alter their habitat to exploit it more efficiently (3).


===Subsection 2===
=Metabolism=
[[Image:Bio interatcions.gif|thumb|right|350px|"Schematic diagram representing various hypothetical bacterial-habitat modifications in the phyllosphere, such as the release of nutrients from plant cells and bacterial cell dispersal effected by the production of syringomycin, which may act both as a phytotoxin and as a surfactant (A); the release of saccharides from the plant cell wall, caused by bacterial secretion of auxin (B); and protection from environmental stresses via production of EPS in bacterial aggregates(C)"]]


Communities in the phyllosphere are thought to be limited by carbon availability, and it may be expected that access to carbon compounds on leaves is a major determinant of epiphytic colonization. There is evidence that small amounts of nutrients, such as simple sugars including glucose, fructose, and sucrose, leach from the interior of the plant.
''Ignicoccus'' species are [http://en.wikipedia.org/wiki/Chemolithoautotroph chemolithoautotrophs] that use molecular hydrogen as the inorganic electron donor and elemental sulphur as the inorganic terminal electron acceptor[[#References|[1]]] . The reduction of the elemental sulphur results in the production of hydrogen sulphide gas.  


==Microbial processes==
''Ignicoccus'' are autotrophs in that they fix their own carbon dioxide into organic molecules. The carbon dioxide fixation process they use is a novel process called [http://www.pnas.org/content/105/22/7851.full a dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle] that involves 14 different enzymes[[#References|[8]]] .  
There is viable evidence that bacteria form large and heterogeneous aggregates on plant surfaces. Microscopic examinations of colonized leaves revealed that on plant surfaces, many epiphytes occur in large mixed-bacterial-species aggregates that also harbor fungi . While large numbers of solitary bacterial cells occur on plants, a few large masses of apparently mixed bacterial species can be found.  
===Subsection 1===
Such aggregates can constitute between 30 and 80% of the total bacterial population on certain plant species. These assemblages, with an extent and structure similar to those of biofilms that develop in aquatic habitats, are probably found only on long-lived leaves in moist climates such as the tropics or wet temperate regions such as the Pacific Northwest.


===Subsection 2===
Members of the ''Ignicoccus''  genus are able to use ammonium as a nitrogen source.
The conglomerates on most other plants, while still sizable, are best known as aggregates.  The formation of aggregates by bacteria on plants has major implications for the ability of these microbes to colonize and survive the harsh environment of the phyllosphere (8,9);  it may provide them with a means to modify their immediate environment in the habitat . The production of extracellular polysaccharide (EPS), which is considered to form a major part of the bacterial aggregate matrix, may benefit epiphytes in the phyllosphere


to see a photo of phyllospheric bacteria click the link [[http://aem.asm.org/cgi/content/full/69/4/1875/F1 microbial interactions]]
==Growth Conditions==


==Subsection 2a==
Because members of the ''Ignicoccus''  genus are [http://en.wikipedia.org/wiki/Hyperthermophile hyperthermophiles] and obligate anaerobes, it is not surprising that their growth conditions are very complex. They are grown in a liquid medium known as ½ SME ''Ignicoccus''  which is a solution of synthetic sea water which is then made anaerobic.
Given that water availability is likely one of the most highly fluctuating factors on leaf surfaces, the heavy EPS slime within aggregates may shield the bacteria from desiccation stress by buffering the matric and osmotic potentials of their surroundings. Additionally, EPS has a role in protecting plant-associated bacteria from reactive oxygen species, which are often encountered on plants.  It has been demonstrated that aggregated bacteria resist oxidative stress better than planktonic bacteria (1,7).


==Key Microorganisms==
Grown in this media at their optimal growth temperature of 90C, the members of the ''Ignicoccus''  genus typically reach a cell density of ~4x107cells/mL[[#References|[1]]] .


The study of bacterial colonizers of leaves has been restricted mostly to aerobic culturable bacteria and also driven by the importance of investigating the ecology of plant-pathogenic bacteria because of their deleterious effect on plant productivity. Thus, the microbial ecology of the phyllosphere has been viewed mainly through the biology of gram-negative bacteria such as Pseudomonas syringae and Erwinia (Pantoea) spp., two of the most ubiquitous bacterial participants of phyllosphere communities.
The addition of [http://en.wikipedia.org/wiki/Yeast_extract yeast extract] to the ½ SME media has been shown to stimulate the growth and increase maximum cell density achieved. The mechanism by which this is achieved is not known[[#References|[1]]] .


In recent years, the association of multiple outbreaks of food-borne illness with fresh fruits and vegetables has raised concern about the possible preharvest contamination of plants with human pathogens. Surveys of the occurrence of enteric pathogens on produce showed that Salmonella spp. and Shigella spp. were detected in up to 4% of the samples (2).
=Symbiosis=


Two independent studies have shown that [[http://en.wikipedia.org/wiki/Salmonella_enterica Salmonella enterica]] and [[http://en.wikipedia.org/wiki/Escherichia_coli Escherichia coli]] have the ability to colonize corn, bean, and cilantro plants under humid conditions, albeit to lower population levels than those of common bacterial epiphytes.  Unlike P. syringae, they failed to grow on leaves under dry conditions.  However, S. enterica survived dry conditions on cilantro leaves and recovered to achieve significant population sizes under subsequent wet conditions (4,6).  The relative fitness of some human enteric pathogens in the phyllosphere, as well as the wide distribution on plants of Enterococcus spp. and of common opportunistic pathogens such as Pseudomonas aeruginosa and Burkholderia cepacia, encourage technological innovation in the field of crop harvesting and food safety with regards to virus's.
''Ignicoccus hospitalis''  is the only member of the genus ''Ignicoccus'' that has been shown to have an extensive [http://en.wikipedia.org/wiki/Symbiosis symbiotic relationship] with another organism.


There are several microbes that live on the Phyllosphere which are of great importance in regards to humans.  These microbes account for a large amount of the food borne illnesses in the United States: Campylobacter,Escherichia coli O157:H7, Salmonella, [[http://en.wikipedia.org/wiki/Cryptosporidiosis Cryptosporidium]], [[http://en.wikipedia.org/wiki/Toxoplasma_gondii Toxoplasma]] and [[http://www.cdc.gov/ncidod/dpd/parasites/cyclospora/default.htm Cyclospora]]. A large amount of the food crop regulatory commissioning focus's on eliminating these bacteria and protozoa.
''Ignicoccus hospitalis''  has been shown to engage in symbiosis with ''Nanoarchaeum equitans'' . ''Nanoarchaeum equitans'' is a very small coccoid species with a cell diameter of 0.4 µm[[#References|[9]]] . Genome analysis has provided much of the known information about this species.


Phyllosphere bacteria can promote plant growth and both
To further complicate the symbiotic relationship between both species, it’s been observed that the presence of ''Nanoarchaeum equitans'' on the surface of ''Ignicoccus hospitalis'' somehow inhibits the cell replication of ''Ignicoccus hospitalis'' . How or why this occurs has not yet been elucidated[[#References|[3]]] .  
suppress and stimulate the colonisation and infection of tissues by plant pathogens. Fungal endophytes of leaves may deter herbivores, protect against pathogens and increase drought tolerance. Interactions in the phyllosphere zone determine the extent to which human pathogens are able to colonise and survive on plant tissues, an area of greatimportance with the rise in cases of human disease associated with consumption of fresh salad, fruit and vegetable produce.


==Examples of organisms within the group==
[[File:Urzwerg.jpg|300px|thumb|right| ''Ignicoccus hospitalis'' with two attached  ''Nanoarchaeum equitans'' cells.]]


There are many organisms which live in the Phyllosphere, they vary from microscopic to several inches in length. The most Common organism in the Phyllosphere is bacteria, mostly bacilli and are not detrimental to the plant, although there are around 100 species that are classified as pathogens and are harmful to the plant or primary consumers.
[[File:IhNeRelationship2 jpeg.jpg|250px|thumb|left| Epifluoroscence micrographs of an ''Ignicoccus hospitalis''and ''Nanoarchaeum equitans'' coculture stained with BacLight at various time points. Living cells stain green while dead cells stain red. (A) Exponential growth phase 3.25 hours after inoculation. (B) Transition into the stationary phase 7.5 hours after inoculation. (C) Stationary phase 10 hours after inoculation. (D) Stationary phase 23 hours after inoculation.]]


Fungi, Oomycetes, phytoplasma, nematodes, protozoa and even small parasitic plants have been found in the Phyllosphere.


==Current Research==
==''Nanoarchaeum equitans''==
Research into the characteristics of microbial life in the phyllosphere is of great commercial importance to the agricultural industry for two reasons. First, understanding the survival of plant disease-causing bacteria and fungi is vital for developing new ways to control their spread. Second, there has been a recent rise in the number of food poisoning cases associated with fruit and vegetables contaminated with bacteria such as Salmonella and E. coli O157:H7. This is particularly true of fresh fruits and salads which are not cooked prior to consumption. Preventing these outbreaks by developing better decontamination strategies is important to protect public health.


Immediate-term information is needed to guide growers, Cooperative Extension, the diagnostic service industry, shippers, and processors in the development of on-the-farm
''Nanoarchaeum equitans'' has the smallest non-viral genome ever sequenced at 491kb[[#References|[9]]] . Analysis of the genome sequence indicates that 95% of the predicted proteins and stable RNA molecules are somehow involved in repair and replication of the cell and its genome[[#References|[3]]] .
management practices to prevent these microbial pathogens from being introduced during productionand at harvest. Areas researchers are beginning to address include (4);


· Information on sources and persistence
Analysis of the genome also showed that ''Nanoarchaeum equitans''  lacks nearly all genes known to be required in amino acid, nucleotide, cofactor and lipid metabolism. This is partially supported by the evidence that ''Nanoarchaeum equitans''  has been shown to derive its cell membrane from its host ''Ignicoccus hospitalis''  cell membrane. The direct contact observed between ''Nanoarchaeum equitans'' and ''Ignicoccus hospitalis''  is hypothesized to form a pore between the two organisms in order to exchange metabolites or substrates (likely from ''Ignicoccus hospitalis''  towards ''Nanoarchaeum equitans'' due to the parasitic relationship). The exchange of periplasmic vesicles is not thought to be involved in metabolite or substrate exchange despite the presence of vesicles in the periplasm of ''Ignicoccus hospitalis'' .


· Manure management and compost process control
These analyses of the ''Nanoarchaeum equitans'' genome support the fact of the extensive symbiotic relationship between ''Nanoarchaeum equitans'' and ''Ignicoccus hospitalis''. However, it has not yet been proven that it is a strictly parasitic relationship and further research may prove that there is a commensal relationship between the two species.


· Timing of incorporation of animal manures relative to crop seeding and harvest
=References=


· Depth of incorporation into soil to minimize persistence or transfer
(1) Burggraf S., Huber H., Mayer T., Rachel R., Stetter K.O. and Wyschkony I. ” Ignicoccus gen. nov., a novel genus of hyperthermophilic, chemolithoautotrophic Archaea, represented by two new species, Ignicoccus islandicus sp. nov. and Ignicoccus pacificus sp. nov.” International Journal of Systematic and Evolutionary Microbiology, 2000, Volume 50.


· Potential for establishment of key pathogens on plant parts during production
(2) Naether D.J. and Rachel R. “The outer membrane of the hyperthermophilic archaeon Ignicoccus: dynamics, ultrastructure and composition.” Biochemical Society Transactions, 2004, Volume 32, part 2.


· Postharvest prevention programs
(3) Giannone R.J., Heimerl T., Hettich R.L., Huber H., Karpinets T., Keller M., Kueper U., Podar M. and Rachel R. “Proteomic Characterization of Cellular and Molecular Processes that Enable the Nanoarchaeum equitans- Ignicoccus hospitalis Relationship.” PLoS ONE, 2011, Volume 6, Issue 8.


==References==
(4) Eisenreich W., Gallenberger M., Huber H., Jahn U., Junglas B., Paper W., Rachel R. and Stetter K.O. “Nanoarchaeum equitans and Ignicoccus hospitalis: New Insights into a Unique, Intimate Association of Two Archaea.” Journal of Bacteriology, 2008, DOI: 10.1128/JB.01731-07.


(1) http://aem.asm.org/cgi/content/full/69/4/1875#F1
(5) Grosjean E., Huber H., Jahn U., Sturt H, and Summons R. “Composition of the lipids of Nanoarchaeum equitans and their origin from its host Ignicoccus sp. strain KIN4/I.” Arch Microbiol, 2004, DOI: 10.1007/s00203-004-0725-x.


(2) http://www.pnas.org/content/106/38/16428.full
(6) Briegel A., Burghardt T., Huber H., Junglas B., Rachel R., Walther P. and Wirth R. “Ignicoccus hospitalis and Nanoarchaeum equitans: ultrastructure, cell–cell interaction, and 3D reconstruction from serial sections of freeze-substituted cells and by electron cryotomography.”  Arch Microbiol, 2008, DOI 10.1007/s00203-008-0402-6.


(3) http://www.iripz.pl/ftp/09_biotic.pdf
(7) Burghardt T., Huber H., Junglas B., Naether D.J. and Rachel R. “The dominating outer membrane protein of the hyperthermophilic Archaeum Ignicoccus hospitalis: a novel pore-forming complex.” Molecular Microbiology, 2007, Volume 63.


(4) http://ucce.ucdavis.edu/files/filelibrary/5453/6558.PDF
(8) Berg I.A., Eisenreich W., Eylert E., Fuchs G., Gallenberger M., Huber H.,Jahn U. and Kockelkorn D. “A dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle in the hyperthermophilic Archaeum Ignicoccus hospitalis.” PNAS, 2008, Volume 105, issue 22.


(5) http://en.wikipedia.org/wiki/Plant_pathogen#Oomycetes
(9) Brochier C., Gribaldo S.,  Zivanovic Y.,  Confalonieri F. and Forterre P. “Nanoarchaea: representatives of a novel archaeal phylum or a fast-evolving euryarchaeal lineage related to Thermococcales?” Genome Biology 2005, DOI:10.1186/gb-2005-6-5-r42.


(6) Sylvia, D. 2007. Principles and applications of soil microbiology
(10) Huber H., Rachel R., Riehl S. and Wyschkony I. “The ultrastructure of Ignicoccus: Evidence for a novel outer membrane and for intracellular vesicle budding in an archaeon.” Archaea, 2002, Volume 1.
 
(7) hhttp://www.microbiologyprocedure.com/growth-of-microorganisms/rhizosphere-and-phyllosphere.html
 
(8) http://images.google.com/imgres?imgurl=http://howplantswork.files.wordpress.com/2009/10/leaf_surface1.jpg&imgrefurl=http://howplantswork.wordpress.com/2009/10/11/life-in-the-phyllosphere-what-microbes-commonly-dwell-on-the-surface-of-leaves/&usg=__Hk7V6LLkcnEEEg0rCRkEfOPOv1k=&h=250&w=375&sz=209&hl=en&start=15&sig2=MGLBm_RbXYRxha04e5sfDw&um=1&itbs=1&tbnid=tm4dw-W3uEH3WM:&tbnh=81&tbnw=122&prev=/images%3Fq%3Dphyllosphere%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26sa%3DN%26rls%3Dorg.mozilla:en-US:official%26tbs%3Disch:1&ei=LDS-S86dCofaNvz92MYJ
 
(9) http://wrap.warwick.ac.uk/449/1/WRAP_Bending_JAM_review_revised_4_April_2008.pdf
 
Edited by student of Angela Kent at the University of Illinois at Urbana-Champaign.
 
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Introduction

To date, the genus Ignicoccus is comprised of single cells that are irregularly shaped coccoid ranging in diameter from 1-3 µm. The Achaean genus was first isolated from marine hydrothermal vents from Kolbeinsey Ridge in north Iceland and also off the coast of Mexico [1] (see Figure 1). They were found to have a novel cell envelope unseen before in other Achaea [2], and have a very complex and poorly understood symbiotic relationship with Nanoarchaeum equitans [3] [4] [5] [6].

Map showing the discovery locations of I.pacificus (A), and I.islandicus and I.hospitalis (B).


Current Species

There are three officially recognized Ignicoccus species: Ignicoccus hospitalis , Ignicoccus pacificus and Ignicoccus islandicus . The three species were initially identified by 16S rRNA gene analysis from the hydrothermal vent samples obtained from Kolbeinsey Ridge and the coast of Mexico[1] . All three species have been characterized as hyperthermophiles that are also obligate anaerobes which explains the presence of Ignicoccus species near hydrothermal vents[1] . None of the members of the Ignicoccus genus have been found to be [1] pathogenic to humans.

Morphology

The members of the Ignicoccus genus are motile irregular coccoid cells that range in diameter from 1 to 3 µm. The motility observed is due to the presence of flagella, but unfortunately the polarity of the flagella is not yet fully elucidated. They are known to have an outer-membrane but no S-layer. This is a novel characteristic for these Archaea becauseIgnicoccus are the only known Archaea that have been shown to possess an outer-membrane[2] [10] .

Ultrathin section of an Ignicoccus hospitalis cell.


Outer-Membrane

The outer-membrane of Ignicoccus species was found to be composed of various derivatives of the typical lipid archaeol, including the derivative known as caldarchaeol [5] . The outer-membrane is dominated by a pore composed of the Imp1227 protein (Ignicoccus outer membrane protein 1227). The Imp1227 protein forms a large nonamer ring with a predicted pore size of 2nm[7] .

Metabolism

Ignicoccus species are chemolithoautotrophs that use molecular hydrogen as the inorganic electron donor and elemental sulphur as the inorganic terminal electron acceptor[1] . The reduction of the elemental sulphur results in the production of hydrogen sulphide gas.

Ignicoccus are autotrophs in that they fix their own carbon dioxide into organic molecules. The carbon dioxide fixation process they use is a novel process called a dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle that involves 14 different enzymes[8] .

Members of the Ignicoccus genus are able to use ammonium as a nitrogen source.

Growth Conditions

Because members of the Ignicoccus genus are hyperthermophiles and obligate anaerobes, it is not surprising that their growth conditions are very complex. They are grown in a liquid medium known as ½ SME Ignicoccus which is a solution of synthetic sea water which is then made anaerobic.

Grown in this media at their optimal growth temperature of 90C, the members of the Ignicoccus genus typically reach a cell density of ~4x107cells/mL[1] .

The addition of yeast extract to the ½ SME media has been shown to stimulate the growth and increase maximum cell density achieved. The mechanism by which this is achieved is not known[1] .

Symbiosis

Ignicoccus hospitalis is the only member of the genus Ignicoccus that has been shown to have an extensive symbiotic relationship with another organism.

Ignicoccus hospitalis has been shown to engage in symbiosis with Nanoarchaeum equitans . Nanoarchaeum equitans is a very small coccoid species with a cell diameter of 0.4 µm[9] . Genome analysis has provided much of the known information about this species.

To further complicate the symbiotic relationship between both species, it’s been observed that the presence of Nanoarchaeum equitans on the surface of Ignicoccus hospitalis somehow inhibits the cell replication of Ignicoccus hospitalis . How or why this occurs has not yet been elucidated[3] .

Ignicoccus hospitalis with two attached Nanoarchaeum equitans cells.
Epifluoroscence micrographs of an Ignicoccus hospitalisand Nanoarchaeum equitans coculture stained with BacLight at various time points. Living cells stain green while dead cells stain red. (A) Exponential growth phase 3.25 hours after inoculation. (B) Transition into the stationary phase 7.5 hours after inoculation. (C) Stationary phase 10 hours after inoculation. (D) Stationary phase 23 hours after inoculation.


Nanoarchaeum equitans

Nanoarchaeum equitans has the smallest non-viral genome ever sequenced at 491kb[9] . Analysis of the genome sequence indicates that 95% of the predicted proteins and stable RNA molecules are somehow involved in repair and replication of the cell and its genome[3] .

Analysis of the genome also showed that Nanoarchaeum equitans lacks nearly all genes known to be required in amino acid, nucleotide, cofactor and lipid metabolism. This is partially supported by the evidence that Nanoarchaeum equitans has been shown to derive its cell membrane from its host Ignicoccus hospitalis cell membrane. The direct contact observed between Nanoarchaeum equitans and Ignicoccus hospitalis is hypothesized to form a pore between the two organisms in order to exchange metabolites or substrates (likely from Ignicoccus hospitalis towards Nanoarchaeum equitans due to the parasitic relationship). The exchange of periplasmic vesicles is not thought to be involved in metabolite or substrate exchange despite the presence of vesicles in the periplasm of Ignicoccus hospitalis .

These analyses of the Nanoarchaeum equitans genome support the fact of the extensive symbiotic relationship between Nanoarchaeum equitans and Ignicoccus hospitalis. However, it has not yet been proven that it is a strictly parasitic relationship and further research may prove that there is a commensal relationship between the two species.




References

(1) Burggraf S., Huber H., Mayer T., Rachel R., Stetter K.O. and Wyschkony I. ” Ignicoccus gen. nov., a novel genus of hyperthermophilic, chemolithoautotrophic Archaea, represented by two new species, Ignicoccus islandicus sp. nov. and Ignicoccus pacificus sp. nov.” International Journal of Systematic and Evolutionary Microbiology, 2000, Volume 50.

(2) Naether D.J. and Rachel R. “The outer membrane of the hyperthermophilic archaeon Ignicoccus: dynamics, ultrastructure and composition.” Biochemical Society Transactions, 2004, Volume 32, part 2.

(3) Giannone R.J., Heimerl T., Hettich R.L., Huber H., Karpinets T., Keller M., Kueper U., Podar M. and Rachel R. “Proteomic Characterization of Cellular and Molecular Processes that Enable the Nanoarchaeum equitans- Ignicoccus hospitalis Relationship.” PLoS ONE, 2011, Volume 6, Issue 8.

(4) Eisenreich W., Gallenberger M., Huber H., Jahn U., Junglas B., Paper W., Rachel R. and Stetter K.O. “Nanoarchaeum equitans and Ignicoccus hospitalis: New Insights into a Unique, Intimate Association of Two Archaea.” Journal of Bacteriology, 2008, DOI: 10.1128/JB.01731-07.

(5) Grosjean E., Huber H., Jahn U., Sturt H, and Summons R. “Composition of the lipids of Nanoarchaeum equitans and their origin from its host Ignicoccus sp. strain KIN4/I.” Arch Microbiol, 2004, DOI: 10.1007/s00203-004-0725-x.

(6) Briegel A., Burghardt T., Huber H., Junglas B., Rachel R., Walther P. and Wirth R. “Ignicoccus hospitalis and Nanoarchaeum equitans: ultrastructure, cell–cell interaction, and 3D reconstruction from serial sections of freeze-substituted cells and by electron cryotomography.” Arch Microbiol, 2008, DOI 10.1007/s00203-008-0402-6.

(7) Burghardt T., Huber H., Junglas B., Naether D.J. and Rachel R. “The dominating outer membrane protein of the hyperthermophilic Archaeum Ignicoccus hospitalis: a novel pore-forming complex.” Molecular Microbiology, 2007, Volume 63.

(8) Berg I.A., Eisenreich W., Eylert E., Fuchs G., Gallenberger M., Huber H.,Jahn U. and Kockelkorn D. “A dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle in the hyperthermophilic Archaeum Ignicoccus hospitalis.” PNAS, 2008, Volume 105, issue 22.

(9) Brochier C., Gribaldo S., Zivanovic Y., Confalonieri F. and Forterre P. “Nanoarchaea: representatives of a novel archaeal phylum or a fast-evolving euryarchaeal lineage related to Thermococcales?” Genome Biology 2005, DOI:10.1186/gb-2005-6-5-r42.

(10) Huber H., Rachel R., Riehl S. and Wyschkony I. “The ultrastructure of Ignicoccus: Evidence for a novel outer membrane and for intracellular vesicle budding in an archaeon.” Archaea, 2002, Volume 1.

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