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Microbial symbiosis in marine animals was not discovered until 1981. In the time following, symbiotic relationships between marine invertebrates and chemoautotrophic bacteria have been found in a variety of ecosystems, ranging from shallow coastal waters to deep-sea hydrothermal vents. Symbiosis is a way for marine organisms to find creative ways to survive in a very dynamic environment. They are different in relation to how dependent the organisms are on each other or how they are associated. It is also considered a selective force behind evolution in some scientific aspects. The symbiotic relationships of organisms has the ability to change behavior, morphology and metabolic pathways. With increased recognition and research, new terminology also arises, such as holobiont, which the relationship between a host and its symbionts as one grouping. Many scientists will look at the hologenome, which is the combined genetic information of the host and its symbionts. These terms are more commonly used to describe microbial symbionts.
The type of marine animal vary greatly, for example, sponges, sea squirts, corals, worms, and algae all host a variety of unique symbionts. Each symbiotic relationship displays a unique ecological niche, which in turn can lead to entirely new species of host species and symbiont.
It is particularly interesting that it took so long to discover the marine microbial symbiosis because nearly every surface submerged in the oceans becomes covered with biofilm, including a large number of living organisms. Many marine organisms display symbiotic relationships with microbes. Epibiotic bacteria have been found to live on crustacean larvae and protect them from fungal infections. Other microbes in deep-sea vents have been found to prevent the settlement of barnacles and tunicate larvae.
Various mechanisms are utilized in order to facilitate symbiotic relationships and to help these associates evolve alongside one another. By using horizontal gene transfer, certain genetic elements are able to pass from one organisms to another. In non-mating species, this helps with genetic differentiation and adaptive evolution. An example of this is the sponge Astroclera willeyana which has a gene that is used in expressing spherulite-forming cells which has an origin in bacteria. Another example is the starlet sea anemone, Nematostella vectensis, which has genes from bacteria that have a role in producing UV radiation protection in the form of shikimic acid. Another way for symbiotic relationships to co-evolve is through genome erosion. This is a process where genes that are typically used during free-living periods aren't necessary because of the symbioses of the organisms. Without that gene, the organism is able to decrease the energy necessary for cell maintenance and replication.
Among various types of symbiotic relationships, mutualism is where partners mutually benefit. Commensalism is a relationship where one partner receives a benefit while the other is not affected. There is parasitism, where one partner benefits while it is at the expense of the host. And amensalism is a less common type of relationship where one organisms receives no benefit but the host still has negative ramifications. The relationships can be an ectosymbiont, a symbiont that survives by being attached to the surface of the host which includes areas such as the inner surfaces of the gut cavity or even the ducts of endocrine glands. Or it can be an endosymbiont which is a symbiont that lives within its host and can be known as an intracellular symbiont. They are further classified by their dependence on their host and can be a facultative symbiont that can exist in a free living condition and is not dependent on its host. Or it can be an obligate symbiont which has adapted in such a way that it is not able to exit without the benefit it receives from its host. An example of an obligate symbioses is the relationship between microalgae and corals. The microalgae provides a large source of the coral diet
The most notable display of marine symbiotic relationship would be coral. Coral reefs are home to a variety of dinoflagellate symbiont, these symbionts give coral its bright coloring and are vital for the survival of the reef. The symbionts provide the coral with food in exchange for protection. If the waters warm or become too acidic, the symbionts are expelled, the coral bleaches and if conditions persist the coral will die. This in turn leads to the collapse of the entire reef ecosystem
Osedax, also called the bone eating worm is a siboglinid worm from polychaete genus. It was discovered in a whalefall community on the surface of bones, in the axis of Monterey Canyon, California, in 2002. Osedax lacks a mouth, a functional gut and a trophosome. But female osedax have a vascularized root system originating from their ovisac which contains heterotrophic endosymbiotic bacterial community dominated by γ-proteobacteria clade. They use the vascularized root system to access the whale bones. The endosymbionts help the host utilize nutrients from the whale bones.
Hawaiian sepiolid squid Euprymna scolopes and bacterium Vibrio fischeri also show symbiosis. In this symbiosis, symbiont not only serve the host for defense, but also shapes the host morphology. Bioluminescent V. fischeri can be found in epithelial lined crypts of the light organ of the host. Symbiosis begins as soon as a newly hatched squid finds and houses V. fischeri bacteria.
The symbiosis process begins when Peptidoglycan shed by the sea water bacteria comes in contact to the ciliated epithelial cells of the light organ. It induces mucus production in the cells. Mucus entraps bacterial cells. Antimicrobial peptides, nitric oxide and sialyted mucins in the mucus then selectively allow only V. fischeri which encode gene rscS to adhere and win over gram positive and other gram negative bacteria. The symbiotic bacteria are then guided up to the light organ via chemotaxis. After successful colonization, symbionts induce loss of mucus and ciliated sites to prevent further attachment of bacterial cells via MAMP (microbe associated molecular pattern) signalling. Also, they induce changes in protein expression in the host symbiotic tissues and modify both physiology and morphology of light organs. After bacterial cells divide and increase in population, they begin expressing enzyme luciferase as a result of quorum sensing. Luciferase enzymes produce bioluminescence. Squids can then emit the luminescence from the light organ. Because Euprymna scolopes emerges only during night time, it helps them avoid predation. Bioluminescence allows them to camouflage with the light coming from moon and stars to ocean and avoid predators.
Alvinella pompejana, the Pompeii worm is a polychaete, found in the far depths of the sea, typically found near hydrothermal vents. They were originally discovered by French researchers in the early 1980s. They can grow as large as 5 inches long and are normally described as having pale gray coloring with red "tentacle-like" gills protruding from their heads. Their tails are most likely found in temperatures as high as 176 degrees Fahrenheit, while their heads, which stick out from the tubes they live in are only exposed to temperatures as high as 72 degrees Fahrenheit. Its ability to survive the termpatures of hydrothermal vents lies in its symbiotic relationship with the bacteria that resides on its back. It forms a "fleece-like" protective covering. Mucous is secreted from glands on the back of the Pompeii worm in order to provide nutrients for the bacteria. Further study of the bacteria led to the discovery that they are chemolithotrophic.
Elysia rufescens grazes on Bryopsis sp., an alga that defends itself from predators by using peptide toxins with fatty acids, called kahalalides. A bacterial obligate symbiont produces many defensive molecules, including kahalalides, in order to protect the alga. This bacteria is able to use substrates derived from the host in order to synthesize the toxins. The Hawaiian Sea Slug grazes on the alga in order to accumulate kahalalide. This uptake of the toxin, which the slug is immune to, allows it to also become toxic to predators. This shared ability, both originating from the bacteria, provide protection within the marine ecosystems.
Besides a one to one symbiotic relationship, it is possible for a host to become symbiotic with a microbial consortia. In the case of the sponge (phylum Porifera), they are able to host a lot of wide range of microbial communities that can also be very specific. The microbial communities that form a symbiotic relationship with the sponge can actually comprise up to 35% of the biomass of its host. The term for this specific symbiotic relationship, where a microbial consortia pairs with a host is called a holobiotic relationship. The sponge as well as the microbial community associated with it will produce a large range of secondary metabolites that help protect it against predators through mechanisms such as chemical defense. Some of these relationships include endosymbionts within bacteriocyte cells, and cyanobacteria or microalgae found below the pinacoderm cell layer where they are able to receive the highest amount of light, used for phototrophy. They can host approximately 52 different microbial phyla and candidate phyla, including Alphaprotoebacteria, Actinobacteria, Chloroflexi, Nitrospirae, Cyanobacteria, the taxa Gamma-, and the candidate phylum Poribacteria, and Thaumarchaea.
This type of bacteria was first described in 2007. It is able to form symbiotic relationships with a wide range of hosts in the marine environment such as cnidarians, poriferans, molluscs, annelids, tunicates, and fish. They are distributed through various marine zones from extreme depths to warm photic zones. Endozoicomonas is thought to acquisition nutrients from nitrogen/carbon recycling, methane/sulfur recycling, and synthesize amino acids and various other molecules necessary for life. It was also found that it has a correlation to photosymbionts which provide carbon and sulfur to the bacteria from dimethylsulfopropionate (DMSP). They are also suspected to help regulate bacterial colonization of the host by using bioactive secondary metabolites or even probiotic mechanisms like limiting pathogenic bacteria by means of competitive exclusion. When Endozoicomonas is removed from the host, there are often signs of lesions on corals and disease.
Marine environment consists of a large number of chemosynthetic symbioses in different regions of the ocean: shallow-water coastal sediments, continental slope sediments, whale and wood falls, cold seeps and deep-sea hydrothermal vents. Organisms from seven phyla (ciliophora, porifera, platyhelminthes, nematoda, mollusca, annelida and arthropoda) are known to have chemosynthetic symbiosis till now. Some of them include nematode, tube worms, clam, sponge, hydrothermal vent shrimp, worms mollusc, mussels and so on. The symbionts can be ectosymbionts or endosymbionts. Some ectosymbionts are: symbionts of polychaete worm Alvinella which occur in their dorsal surface and symbionts occurring on the mouthparts and gill chamber of the vent shrimp Rimicaris. Endosymbionts include symbionts of gastropod snails which occur in their gill tissues. In the siboglinid tube worms of the groups Monilifera, Frenulata and Vestimentifera, symbionts can be found in an interior organ called trophosome.
Most of the animals in deep-sea hydrothermal vents exist in a symbiotic relationship with chemosynthetic bacteria. These chemosynthetic bacteria are found to be methane or sulphur oxidizers.
Marine invertebrates are the hosts of a wide spectrum of bioactive metabolites, which have vast potential as drugs and research tools. In many cases, microbes aid in or are responsible for marine invertebrates natural products. Certain marine microbes can provide insight into the biosynthesis mechanisms of natural products, which in turn could solve the current limitations on marine drug development.
An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living".) Examples are nitrogen-fixing bacteria, which live in the root nodules of legumes; single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10–15% of insects.
Symbiosis is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. The organisms, each termed a symbiont, must be of different species. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms". The term was subject to a century-long debate about whether it should specifically denote mutualism, as in lichens. Biologists have now abandoned that restriction.
In biochemistry, chemosynthesis is the biological conversion of one or more carbon-containing molecules and nutrients into organic matter using the oxidation of inorganic compounds or ferrous ions as a source of energy, rather than sunlight, as in photosynthesis. Chemoautotrophs, organisms that obtain carbon from carbon dioxide through chemosynthesis, are phylogenetically diverse. Groups that include conspicuous or biogeochemically-important taxa include the sulfur-oxidizing gamma and epsilon proteobacteria, the Aquificae, the methanogenic archaea and the neutrophilic iron-oxidizing bacteria.
Riftia pachyptila, commonly known as the giant tube worm, is a marine invertebrate in the phylum Annelida related to tube worms commonly found in the intertidal and pelagic zones. R. pachyptila lives on the floor of the Pacific Ocean near hydrothermal vents, and can tolerate extremely high hydrogen sulfide levels. These worms can reach a length of 3 m, and their tubular bodies have a diameter of 4 cm (1.6 in). Ambient temperature in their natural environment ranges from 2 to 30°C.
Symbiotic bacteria are bacteria living in symbiosis with another organism or each other. For example, Zoamastogopera, found in the stomach of termites, enable them to digest cellulose.
Horizontal transmission is the transmission of organisms between biotic and/or abiotic members of an ecosystem that are not in a parent-progeny relationship. This concept has been generalized to include transmissions of infectious agents, symbionts, and cultural traits between humans.
The Class Gammaproteobacteria belongs to the Proteobacteria phylum and contains about 250 genera, which makes it the most genera-rich taxa of the Prokaryotes. Several medically, ecologically, and scientifically important groups of bacteria belong to this class. It is composed by all Gram-negative microbes and is the most phylogenetically and physiologically diverse class of Proteobacteria.
Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plants; as well as, algal and fungal species. They can reside within extracellular or intracellular structures of the host. In order for a cyanobacterium to successfully form a symbiotic relationship, it must be able to exchange signals with the host, overcome defense mounted by the host, be capable of hormogonia formation, chemotaxis, heterocyst formation, as well as possess adequate resilience to reside in host tissue which may present extreme conditions, such as low oxygen levels, and/or acidic mucilage. The most well-known plant-associated cyanobionts belong to the genus Nostoc. With the ability to differentiate into several cell types that have various functions, members of the genus Nostoc have the morphological plasticity, flexibility and adaptability to adjust to a wide range of environmental conditions, contributing to its high capacity to form symbiotic relationships with other organisms. Several cyanobionts involved with fungi and marine organisms also belong to the genera Richelia, Calothrix, Synechocystis, Aphanocapsa and Anabaena, as well as the species Oscillatoria spongeliae. Although there are many documented symbioses between cyanobacteria and marine organisms, little is known about the nature of many of these symbioses. The possibility of discovering more novel symbiotic relationships is apparent from preliminary microscopic observations.
A trophosome is a highly vascularised organ found in some animals that houses symbiotic bacteria that provide food for their host. Trophosomes are located in the coelomic cavity in the vestimentiferan tube worms and in symbiotic flatworms of the genus Paracatenula.
The hologenome theory of evolution recasts the individual animal or plant as a community or a "holobiont" – the host plus all of its symbiotic microbes. Consequently, the collective genomes of the holobiont form a "hologenome". Holobionts and hologenomes are structural entities that replace misnomers in the context of host-microbiota symbioses such as superorganism, organ, and metagenome. Variation in the hologenome may encode phenotypic plasticity of the holobiont and can be subject to evolutionary changes caused by selection and drift, if portions of the hologenome are transmitted between generations with reasonable fidelity. One of the important outcomes of recasting the individual as a holobiont subject to evolutionary forces is that genetic variation in the hologenome can be brought about by changes in the host genome and also by changes in the microbiome, including new acquisitions of microbes, horizontal gene transfers, and changes in microbial abundance within hosts. Although there is a rich literature on binary host–microbe symbioses, the hologenome concept distinguishes itself by including the vast symbiotic complexity inherent in many multicellular hosts. For recent literature on holobionts and hologenomes published in an open access platform, see the following reference.
Olavius algarvensis is a species of gutless oligochaete worm in the family Tubificidae which depends on symbiotic bacteria for its nutrition.
Paracatenula is a genus of millimeter sized free-living marine gutless catenulid flatworms.
The word microbiome was first used by J.L. Mohr in 1952 in The Scientific Monthly to mean the microorganisms found in a specific environment. It was defined in 1988 by Whipps et al. as "a characteristic microbial community occupying a reasonably well-defined habitat which has distinct physio-chemical properties. The term thus not only refers to the microorganisms involved but also encompasses their theatre of activity".
A holobiont is an assemblage of a host and the many other species living in or around it, which together form a discrete ecological unit, though there is controversy over this discreteness. The components of a holobiont are individual species or bionts, while the combined genome of all bionts is the hologenome. The holobiont concept was initially introduced by German theoretical biologist Adolf Meyer-Abich, and then independently by Dr. Lynn Margulis in her 1991 book Symbiosis as a Source of Evolutionary Innovation. Moreover, the concept has evolved since the original definitions. Holobionts include the host, virome, microbiome, and other members, all of which contribute in some way to the function of the whole. Well-studied holobionts include reef-building corals and humans.
Hologenomics is the omics study of hologenomes. A hologenome is the whole set of genomes of a holobiont, an organism together with all co-habitating microbes, other life forms, and viruses. While the term hologenome originated from the hologenome theory of evolution, which postulates that natural selection occurs on the holobiont level, hologenomics uses an integrative framework to investigate interactions between the host and its associated species. Examples include gut microbe or viral genomes linked to human or animal genomes for host-microbe interaction research. Hologenomics approaches have also been used to explain genetic diversity in the microbial communities of marine sponges.
Stilbonematinae is a subfamily of the nematode worm family Desmodoridae that is notable for its symbiosis with sulfur-oxidizing bacteria.
The hydrothermal vent microbial community includes all unicellular organisms that live and reproduce in a chemically distinct area around hydrothermal vents. These include organisms in the microbial mat, free floating cells, or bacteria in an endosymbiotic relationship with animals. Chemolithoautotrophic bacteria derive nutrients and energy from the geological activity at Hydrothermal vents to fix carbon into organic forms. Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.
A symbiosome is a specialised compartment in a host cell that houses an endosymbiont in a symbiotic relationship.
All animals on Earth form associations with microorganisms, including protists, bacteria, archaea, fungi, and viruses. In the ocean, animal–microbial relationships were historically explored in single host–symbiont systems. However, new explorations into the diversity of marine microorganisms associating with diverse marine animal hosts is moving the field into studies that address interactions between the animal host and a more multi-member microbiome. The potential for microbiomes to influence the health, physiology, behavior, and ecology of marine animals could alter current understandings of how marine animals adapt to change, and especially the growing climate-related and anthropogenic-induced changes already impacting the ocean environment.
The holobiont concept is a renewed paradigm in biology that can help to describe and understand complex systems, like the host-microbe interactions that play crucial roles in marine ecosystems. However, there is still little understanding of the mechanisms that govern these relationships, the evolutionary processes that shape them and their ecological consequences. The holobiont concept posits that a host and its associated microbiota with which it interacts, form a holobiont, and have to be studied together as a coherent biological and functional unit to understand its biology, ecology, and evolution.