User:Alastairroberts/Alteromonas macleodii

Alteromonas macleodii is a widespread marine bacterium found in both surface and deep waters across temperate and tropical regions. First discovered in a survey of aerobic bacteria in 1972, A. macleodii has since been placed within the phylum Pseudomonadota and is recognised as a prominent component of surface and deep waters. Alteromonas macleodii has a single circular DNA chromosome which varies in size between strains but is typically between 4.4 to 4.6 million base pairs. Variable genomic regions confer functional diversity to even closely related strains of A. macleodii and facilitate different lifestyles and strategies. Certain A. macleodii strains are currently being explored for their industrial uses, including in cosmetics, bioethanol production and rare earth mining.

Morphology
Alteromonas macleodii is an encapsulated gram-negative heterotrophic γ-proteobacterium commonly found in marine waters. It is an aerobic, motile bacterium with a singular unsheathed polar flagellum. Isolates of A. macleodii are between 0.6 to 0.8 μm width and 1.4 to 2.0 μm length, and do not show signs of fermentation, luminescence or pigmentation. Colonies grown in glucose media can be up to 0.9 cm in diameter with irregular edges. As a result of phenotypic variability and differences between strains, competitiveness in cultures varies both amongst co-cultures and between strains from different geographical areas.

Due to A. macleodii being r-strategists, large cells with high nucleic acid content are commonly seen, with a high dividing frequency and high carbon production rate. As it is capable of using glucose as its only carbon and energy source, A. macleodii grows in environments with high sodium concentrations. A copiotroph, A. macleodii blooms under high-nutrient conditions where it is able to out-compete other organisms, while low temperatures and a lack of available carbon generally impede growth.

Distribution
Alteromonas macleodii are ubiquitous in the global oceans, constituting a significant proportion of bacterial abundance in the North Atlantic and Mediterranean , and are also present in deep waters of the Northeastern Pacific and subtropical Atlantic.

Two ecotypes of Alteromonas macleodii have been described, as niche differentiation has caused two distinct strains of the bacterium to occupy different water depth profiles. The “deep ecotype” adheres to larger organic particles and sinks rapidly into the pelagic zone. The “surface ecotype” adheres to smaller organic particles, sinks more slowly, and therefore occupies waters closer to the surface. The abundance of A. macleodii in deeper waters is higher than in surface waters, and the deep ecotype strains are more commonly found attached to particles greater than 5 mm in diameter than as free living entities.

Physiology
Physiological variation in Alteromonas macleodii leads to specific adaptive strategies in terms of carbon and iron metabolism, cellular communication, and nutrient acquisitions. Some strains are specialised in the association with prochlorococcus species via enhanced phenol degradation, while others have a unique capacity to metabolise sugars from specific algae species. The production of homoserine, lactones and siderophores are also strain-specific, with different nutrient acquisition and cellular communication strategies between strains under different ecological conditions.

The physiology of Alteromonas macleodii can influence iron concentrations and recalcitrant dissolved organic matter (DOM) production in the oceans. These bacteria utilise unique Ton-B dependent transporters to acquire iron and carbon substrates. As a result, some strains of A. macleodii are able to more efficiently regulate the uptake of glucose, tryptophan, and tyrosine during growth and metabolism. An ATPase-independent mechanism is involved in the transport of the siderophores which scavenge iron for the bacterium, iron which is then used by enzymes to facilitate carbon metabolism.

Depending on the ecotype, A. macleodii exhibits different physiological responses to carbon acquisition. In deep sea ecotypes, the majority of A. macleodii were found to express a single alginate lyase, while most surface strains were found to express five alginate lyases, likely due to greater exposure to algal biomass.

The physiological responses of A. macleodii depend on the type of amino acid intake. The D-amino acids[15](D-Alanine, D-Serine, and D-Glutamic acid) have an effect of reducing metabolic activity, also inhibiting the production of exopolysaccharide (EPS). EPS is responsible for cell aggregation in marine bacterial species as they enhance the composition and structure of biofilms. Therefore, the uptake of D-amino acids by Alteromonas macleodii impedes the formation of single-celled planktonic behavior, while promoting aggregation growth factors via alternative cellular mechanisms despite the inhibition of EPS production.

The extracellular membrane vesicles in Alteromonas macleodii play a crucial role in algae degradation and habitat colonization. It is discovered that the vesicles contain hydrolytic enzymes such as lipases, protease, nucleases…etc. These enzymes are responsible for the degradation of cell walls and inner components of red algae like  kappaphycus . The degradation of the polysaccharides and proteins is crucial for nutrient acquisition and establishing biomass of the bacteria. This also has a means of preventing overgrowth of red algae, stabilizing the local ecosystem.

Niche differentiation
The two ecotypes for deep and surface water strains of Alteromonas macleodii provide different functions to their ecosystems. The “deep ecotype” is suited for lower oxygen conditions. These organisms degrade compounds that generally break down slowly, such as urea. The “surface ecotype” is better suited for the degradation of sugars and amino acids.

Interspecies interactions
Under nutrient stress, the cyanobacteria Prochlorococcus undergoes chlorosis, whereby photosynthetic pigments that are essential for primary production become non-viable. With the presence of Alteromonas macleodii, Prochlorococcus cells are able to survive without nutrients for an extended period of time, increasing its ecological function.

Trichodesmium is a filamentous cyanobacteria that contributes to nitrogen fixation into ocean ecosystems. Alteromonas macleodii is commonly found associated with Trichodesmium populations, although their role within the colonies is not fully understood. It is likely that the incorporation of A. macleodii into Trichodesmium populations modifies biochemical pathways, allowing the catabolization of methanol and the detoxification of radical oxygen species.

Iron limitation
Under iron replete conditions, the rate of respiration in Alteromonas macleodii is significantly reduced. Iron metal is associated with several key processes for bacterial metabolism, such as the citric acid cycle, glycolysis, and oxidative phosphorylation, all of which are functionally limited when iron availability is not sufficient. The growth rate of A. macleodii is therefore reduced when iron is limited, although the growth rate of strains from coastal populations is reduced more so than those from mid-oceanic populations.

Copper stress
Increased exposure of bacteria to copper may occur in several ways, such as nutrient leaching, metals from ship hulls, or natural mineral deposits. Under conditions of increased copper concentrations, biofilm production of Alteromonas macleodii significantly increases as a defensive response to copper induced stress.

Role in carbon cycle
As heterotrophic bacteria, Alteromonas macleodii consume dissolved organic carbon in seawater and are then consumed by higher trophic levels, acting as a gateway for carbon into ecosystems. While natural ecosystems consist of a variety of heterotrophs contributing to the carbon cycle, it has been found in laboratory settings that A. macleodii is capable of drawing down the complete pool of labile DOC present in coastal waters. This indicates that the relationship between A. macleodii and other bacteria in the microbial loop of coastal waters is one of functional redundancy: Alteromonas is capable of carbon cycling to the same extent as entire microbial communities.

Cell wall polysaccharides secreted by macroalga are degraded by microbes such as Alteromonas, and are a major source of carbon into marine ecosystems. A. macleodii exhibits two distinct strategies for carbon uptake, depending on the type of polysaccharide present in their habitat. When degrading the common polysaccharides laminarin, alginate, and pectin, A. macleodii releases different catabolites at different times to degrade the respective substrates. Laminarin is the first polysaccharide that is degraded, followed by alginate and pectin. This temporal variation in carbon utilization is a result of a shift in transcriptional activity of CAZymes and polysaccharide utilization gene fragments. The biphasic nature of these cellular adaptations indicates that Alteromonas macleodii’s role in the drawdown of polysaccharide DOC is adaptable to changing community structures of macroalgal communities.

Alginate is a gel textured polysaccharide that is a common component of macroalgal cell walls, and is a nutrient and carbon source for many organisms. Alteromonas macleodii is a strong component of the carbon cycle, in that they degrade alginate, increasing DOC drawdown in marine environments. Further, A. macleodii has been found to outcompete other species of bacteria in the degradation of alginate, indicating that A. macleodii plays a particularly relevant role in ecological carbon cycling.

Genome (finish writing and review refs)
The surface and deep-dwelling ecotypes of A. macleodii have distinct genomic content associated with their different lifestyles. Surface strains such as the type strain ATTC 21726 have a single circular genome of about 4.6 million base pairs, and share about 81% sequences similarity to the AltDE deep-type strain, which has a slightly smaller genome of 4.4 million base pairs. The two ecotypes each have an estimated 3200 core genes, with an estimated 1200-1600 genes unique to each strain. The pangenome of A. macleodii is vast  ; there is significant variation between the two ecotypes especially, but also between closely related strains isolated from surface and deep waters in different areas of the globe. Specifically, genomic islands (GIs) differ greatly between strains, especially those coding for polysaccharides that present on the flagellum and the outer surface of the cell, with possible roles in phage avoidance. Some are lysogenic or defective phages; one of these widespread GIs encodes virus-derived functional mismatch repair and RNA chaperone genes.

Surface ecotype
Alteromonas macleodii is globally distributed in the surface ocean (0-50m depth), these strains are highly variable functionally despite sharing 97-99% nucelotide identity. Functional differences between surface strains are conferred by horizontally transferred genes acquired through viral infection and plasmid transfer, and are reflective of the variable conditions of surface waters.

- The variability of surface water conditions is reflected in the higher number of genes associated with utilising different sugar and amino acid substrates as well as transcriptional regulators.

- Closely related (96-99% genetic content similarity) surface strains can contain distinct sets of functional genes, such as those associated with siderophore production, and degradation of algal substrates. Plasmids carried by some A. macleodii strains that enhance heavy-metal toxicity are found in genomic islands in other members of the Alteromonadales.

Deep ecotype
The deep ecotype contains over 60 transposable elements, an order of magnitude more than other strains, a common feature of bacteria found in the deep ocean.

- Contains many genes such as phase integrases and the CRISPR cluster that are likely involved in phage interactions.

- Contains more dioxygenases -> for degrading more recalcitrant DOM such as urea.

- Contains molecular chaperones for protein folding at lower temperatures.

- Contains high levels of hydrogenases associated heavy-metal tolerance, which are located in a large cluster on a single genomic island.

- Great deal of variability in how adaptable deep-strains are to

Phage infecting A. macleodii
Alteromonas macleodii populations present in different ecotypes display different variations in the flexible genomic islands (fGIs). Specifically, many fGIs of A. macleodii in deep ecotypes encode specific surface receptors that can be recognized by phages, thus increasing the susceptibility to phage infections. For example, the O-chain lipopolysaccharides of the bacteria may associate with the receptor binding proteins present on phage R8W, allowing attachment and entry

Burst size of alterphages varies. R8W has a burst size of approximately 88 virions per cell. P24 has a burst size of 147 virions per cell.

Reduction of potassium tellurite to elemental tellurium
Elemental tellurium is an extremely rare metalloid contained in the Earth's crust that has been seeing an increase in demand over this past decade due to its special optic and electronic properties. However, current industrial production of tellurium requires the usage of substances harmful to both humans and the environment. As a result, extraction of metalloids by biotechnological applications involving bacterial biosynthesis of nanoparticles from various uncommon and rare metals are increasingly being studied.

Tellurium is seen to have toxic effects on bacteria, although the mechanism for which this occurs remains unknown. Alteromonas macleodii is known to contain a plasmid that houses genes allowing for resistance to multiple metals and was first found in a study conducted in 2019 that a surface ecotype of A. macleodii had the ability to reduce potassium tellurite into elemental tellurium. The nanoparticles of the reduced tellurium was seen to be diffused in the cytoplasm in the form of electron-dense globules, as well as in the extracellular space in the form of both globules and metalloid crystals. As limited research has been done on A. macleodii usage to extract the rare metalloid, this remains as a future possibility for safe and eco-friendly extraction of tellurium.

Extraction of biomolecules from red seaweeds
When isolating a strain of Alteromonas macleodii from India in 2019, it was found that membrane vesicles containing κ-carrageenase, were secreted when grown on κ-carrageenan, a major polysaccharide found in the cell walls of red seaweeds. As κ-carrageenase is a κ-carrageenan degrading enzyme, red seaweed biomass is decreased and A. macleodii, along with other bacteria, are able to grow due to an increase in nutrients resulting from the hydrolytic action on the cell wall from the vesicles being released. Additionally, the κ-carrageenase-containing vesicles can be exploited for bioethanol production due to the conversion of carbohydrate-rich biomass to sugars. Biomolecules present in the red seaweeds, such as vitamins and carotenoids, can also be extracted for commercial use.

EPS deepsane usage in cosmetics
Alteromonas macleodii is seen to secrete the exopolysaccharide (EPS) "deepsane" when isolated in a laboratory setting. "Deepsane" is found to be extremely viscous and contains glucose, pyruvate and acetate amongst other substituents. The properties studied of "deepsane", including the high viscosity possibly due to the interaction between acetate and pyruvate, classify the EPS to be a good alternative to other available viscous polymers currently present for commercial use in the food and cosmetics industry. As of 2012, "deepsane" is also commercially available in the cosmetics industry and is referred to as Abyssine®. Abyssine® is commonly used in skincare products to reduce irritation and soothe sensitive skin that may be irritated by UVB aggression and chemical or mechanical aggression.

Additional future prospects in water treatments
Although research is limited, a study in 1996 found that cultures of Alteromonas macleodii, when grown in glucose-supplemented media, secreted an unexpectedly high-molecular-weight polymer that changed carbohydrate composition. The polymer was found to be rich in uronic acids and therefore expected to have a heavy-metal-binding ability that could be used and applied in the biodetoxification and treatment of wastewater.