Marine microbial symbiosis

The most notable display of marine symbiotic relationship would be coral. Coral reefs are home to a variety of dinoflagellate symbiont,[10] 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[10]

Figure 1 shows Osedax rubiplumus with ovisac (red colored projections) which houses symbiont bacteria

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.[11]

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.

Figure 2 shows sagittal section of bobtail squid Euprymna scolopes. light organ. The crypts house symbiont bacteria Vibrio fischeri. They emit light during night time to camouflage themselves against the moon and star light coming down the ocean. It helps them to avoid predators.

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.[12] 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.[13]

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.[14] 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.[14] 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.[14]  

Elysia rufescens grazes on Bryopsis sp., an alga that defends itself from predators by using peptide toxins with fatty acids, called kahalalides.[15] 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.[15] 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.

The sponge holobiont

The sponge holobiont is an example of the concept of nested ecosystems. Key functions carried out by the microbiome (coloured arrows) influence holobiont functioning and, through cascading effects, subsequently influence community structure and ecosystem functioning. Environmental factors act at multiple scales to alter microbiome, holobiont, community, and ecosystem scale processes. Thus, factors that alter microbiome functioning can lead to changes at the holobiont, community, or even ecosystem level and vice versa, illustrating the necessity of considering multiple scales when evaluating functioning in nested ecosystems.[16]

DOM: dissolved organic matter; POM: particulate organic matter; DIN: dissolved inorganic nitrogen

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.[17] 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.[18] 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.[18]

This type of bacteria was first described in 2007.[19] 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.[19] 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.[19]