Micromonas
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''Micromonas'' is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...
in the family '' Mamiellaceae''. ''Micromonas'' is a widespread prasinophyte alga that is very small in size, motile, and phototactic. Before characterization and naming of a second species, ''Micromonas commoda'' through genome analysis, ''Micromonas pusilla'' was considered to be the only species in the genus. This led to a disproportionate amount of research discussing a single species and the suggestion that it was the dominant photosynthetic picoeukaryote in some marine ecosystems. Unlike many marine algae, this single species was thought to be distributed widely in both warm and cold waters, but genome sequencing confirmed indications from single-gene studies that its global distribution really reflected presence of multiple species occupying different niches in the ocean.Genomes of Two Strains of Micromonas Algae Show Surprising Diversity
, ''Alternative Energy Newswire'', April 10, 2009
Some studies have divided ''Micromonas pusilla'' into 3 to 5 different
clades In biology, a clade (), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy ...
despite their similarity in morphologies and habitats. Varying ratios of clades contribute to the ''M. pusilla'' population throughout the marine ecosystem leading to the hypothesis of clades arising based on niche occupation and susceptibility to virus infection. Other studies have established the presence of at least seven phylogenetically distinct species for which global sequence analyses are beginning to delineate clear differences in the ocean regions they inhabit, with only some of the species actually co-occurring in the same environment.


Discovery

''Micromonas pusilla'' is considered the first
picoplankton Picoplankton is the fraction of plankton composed by cell (biology), cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs: * photosynthetic * heterotrophic They are prevalent amongst microbial p ...
studied, when it was discovered and named ''Chromulina pusilla'' in the 1950s by R. Butcher. Later, electron micrographs by the English scientists, Irene Manton and Mary Park, in the 1960s provided further details on ''M. pusilla''.


Cell morphology and structure

''Micromonas'' is a group of small
unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
pear-shaped micro-algae that do not have a visible
cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...
. Just like other members in the class, they have a single
mitochondrion A mitochondrion () is an organelle found in the cell (biology), cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosine tri ...
and a single
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
, which covers almost half of the cell. They are able to swim due to the presence of a scale-less
flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
. The axonemal structure of the
flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
for this genus is different in that the peripheral microtubules do not extend up to the termination of the central pair of
microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...
s, allowing a visible investigation of the motion of the central pair. In ''Micromonas'', the central pair constantly rotates in an anti-clockwise direction, despite the motion of other components of the
flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
. While the cell size, shape and the location of insertion of the
flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
into the cell are similar among strains and genetic clades, the variation in respective hair point length results in different lengths of the
flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
within the
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
.


Antibiotic

The
antibiotic susceptibility Antibiotic sensitivity testing or antibiotic susceptibility testing is the measurement of the wikt:susceptible#Adjective, susceptibility of bacteria to antibiotics. It is used because bacteria may have Antimicrobial resistance, resistance to some ...
was determined using a single strain of ''M. pusilla'' with the purpose to produce
axenic In biology, axenic (, ) describes the state of a culture in which only a single species, variety, or strain of organism is present and entirely free of all other contaminating organisms. The earliest axenic cultures were of bacteria or unicellul ...
cultures to be used in studies and experiments. The strain of ''M.pusilla'' was tested with a range of antibiotics to determine the possible effects of the particular antibiotic. Resistance:
benzylpenicillin Benzylpenicillin, also known as penicillin G (PenG) or BENPEN, is an antibiotic used to treat a number of bacterial infections. This includes pneumonia, strep throat, syphilis, necrotizing enterocolitis, diphtheria, gas gangrene, leptospiro ...
,
gentamicin Gentamicin is an aminoglycoside antibiotic used to treat several types of bacterial infections. This may include bone infections, endocarditis, pelvic inflammatory disease, meningitis, pneumonia, urinary tract infections, and sepsis amo ...
,
kanamycin Kanamycin A, often referred to simply as kanamycin, is an antibiotic used to treat severe bacterial infections and tuberculosis. It is not a first line treatment. It is used by mouth, injection into a vein, or injection into a muscle. Kanamy ...
,
neomycin Neomycin, also known as framycetin, is an aminoglycoside antibiotic that displays bactericidal activity against Gram-negative aerobic bacilli and some anaerobic bacilli where resistance has not yet arisen. It is generally not effective against ...
,
streptomycin Streptomycin is an antibiotic medication used to treat a number of bacterial infections, including tuberculosis, Mycobacterium avium complex, ''Mycobacterium avium'' complex, endocarditis, brucellosis, Burkholderia infection, ''Burkholderia'' i ...
Sensitive:
chloramphenicol Chloramphenicol is an antibiotic useful for the treatment of a number of bacterial infections. This includes use as an eye ointment to treat conjunctivitis. By mouth or by intravenous, injection into a vein, it is used to treat meningitis, pl ...
,
polymyxin B Polymyxin B, sold under the brand name Poly-Rx among others, is an antibiotic used to treat meningitis, pneumonia, sepsis, and urinary tract infections. While it is useful for many Gram negative infections, it is not useful for Gram positive infe ...
For ''M. pusilla'', sensitivity towards an antibiotic is likely defined by the impairment of growth, rather than a lethal effect, when subjected to
bactericidal A bactericide or bacteriocide, sometimes abbreviated Bcidal, is a substance which kills bacteria. Bactericides are disinfectants, antiseptics, or antibiotics. However, material surfaces can also have bactericidal properties based solely on their p ...
levels of that particular antibiotic. The susceptibility of other strains of ''M. pusilla'' towards this set of antibiotics should be the same.


Genetics


Evolutionary history

''Micromonas'' diverged early on from the lineage that led to all modern terrestrial plants. Individual species have very similar
18S ribosomal RNA 18S ribosomal RNA (abbreviated 18S rRNA) is a part of the ribosomal RNA in eukaryotes. It is a component of the Eukaryotic small ribosomal subunit (40S) and the cytosolic homologue of both the 12S ribosomal RNA, 12S rRNA in mitochondria and the 1 ...
gene sequences, a comparison often used to determine microscopic speciation; however, <90% of different genes are shared between the two genome sequenced ''Micromonas'' species. They have more notable differences in the V1-V2 region of the
16S ribosomal RNA 16S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome ( SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as 16S ...
genes (located in the chloroplast genome). More recent analyses show just how divergent they are in relation to other green lineage members, specifically land plants and chlorophyte green algae. Although ''Micromonas pusilla'' was thought to represent a single species, genetic studies have shown that ''Micromonas'' lineages diverged from each other as early as 65 million years ago, accumulating a large amount of genetic differences. The lack of morphological differentiation means that ''Micromonas pusilla'' may be considered a
cryptic species In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...
complex.


Strain isolation

The original ''Micromonas'' reference
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
(s) were created from strain CCMP1545 isolated from the North Atlantic and deposited in a culture collection in the 1980s, and strain CCMP2709 (RCC299 prior to being rendered axenic and clonal), isolated in 1998 from an Equatorial Pacific sample. These strains had been cultured for decades and are available from the National Center for Marine Algae and Microbiota (NCMA, US) and the Roscoff Culture Collection (RCC, FR).


Cellular mechanisms


Cell growth and division

''Micromonas'' reproduces asexually through fission. It has been observed that ''M. pusilla'' shows variability in optical characteristics, for example cell size and light scattering, throughout the day. There is an increase in these measurements during the period with light, followed by a decrease during period without light. This coincides with the findings that
proteomic Proteomics is the large-scale study of proteins. Proteins are vital macromolecules of all living organisms, with many functions such as the formation of structural fibers of muscle tissue, enzymatic digestion of food, or synthesis and replicatio ...
profiles change over the diel cycle, with an increase in expression of proteins related to
cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential increase in cell number and is therefore a rapid mechanism of tissue growth. Cell proliferation ...
,
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
and
cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
restructuring in the dark when cells start dividing and become smaller. However, the expression levels of genes and proteins can still vary within the same
metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell (biology), cell. The reactants, products, and Metabolic intermediate, intermediates of an enzymatic reaction are known as metabolites, which are ...
. It has also been suggested that the structure of 3’ UTR may play a role in the regulatory system.


Light-harvesting system

''Micromonas'' species still share the same collection of
photosynthetic pigment A photosynthetic pigment (accessory pigment; chloroplast pigment; antenna pigment) is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis. List of photosynthetic pigmen ...
s as the members of the class ''
Mamiellophyceae Mamiellophyceae is a class of green algae in the division Chlorophyta. The class contains three orders containing 25 species. * Order Dolichomastigales Marin & Melkonian (7 species) * Order Mamiellales Moestrup (13 species) * Order Monomas ...
,'' which includes the common pigments
chlorophyll a } Chlorophyll ''a'' is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light, and it is a poor absorber of green and near-green portions of the spectrum. Chlorop ...
and
chlorophyll b } Chlorophyll ''b'' is a form of chlorophyll. Chlorophyll ''b'' helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll ''a'' in polar solvents because of its carbonyl group. Its color is green, and it primarily ...
, as well as prasinoxanthin (xanthophyll K), the first algal
carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...
being assigned with a structure that has a γ-end group. It has been discovered that most of its
xanthophyll Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group; the other division is formed by the carotenes. The name is from Greek: (), meaning "yellow", an ...
s are in the oxidized state and show similarities to ones possessed by other important marine
plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
s like diatoms, golden and
brown algae Brown algae (: alga) are a large group of multicellular algae comprising the class (biology), class Phaeophyceae. They include many seaweeds located in colder waters of the Northern Hemisphere. Brown algae are the major seaweeds of the temperate ...
, and
dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...
s. In addition, there is another pigment called Chl cCS-170 can be found in some strains of ''Micromonas'' and '' Ostreococcus'' living in deeper part of the ocean, which may indicate a potential adaptation for organisms that reside under low light intensity; however, at least for '' Ostreococcus'', these strains are found throughout the water column in open ocean gyres, including in surface waters. The light-harvesting complexes of ''Micromonas'' are distinguishable from other
green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...
in terms of pigment composition and stability under unfavorable conditions. It has been shown that these proteins use three different pigments for light harvesting, and they are resistant to high temperature and the presence of detergent.


Peptidoglycan biosynthesis

Even though the
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s, which are suggested to be originated from
Cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
via
endosymbiosis An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...
, from ''Micromonas'' do not have a surrounding
peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
layer, the
peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
biosynthesis pathway is found to be complete in ''M.'' ''pusilla'' and partial in ''M. commoda'', with the presence of some relevant enzymes only. While the role of this pathway for ''Micromonas'' is still under investigation, this observation shows a lineage for different species of ''Micromonas'' along with
glaucophyte algae The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments, less common today than they were during the Proterozoic. The stated number of speci ...
which still have their
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s covered with
peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
.


Ecological significance

''Micromonas'' make up a significant amount of picoplanktonic biomass and productivity in both oceanic and coastal regions. The abundance of ''Micromonas'' has increased over the past decade. Evidence shows these spikes in numbers are induced through climate change, which has been felt more drastically in the Arctic. Many green algal species have been considered solely photosynthetic, and this appears to be the case for ''Micromonas''. Some years ago a study indicated that ''Micromonas'' had a predatory mixotrophic lifestyle that might have large impacts on prokaryotic populations within the Arctic. Due to the large consumption of prokaryotes by ''Micromonas'', this study and others building on it, suggested it might underlie why photosynthetic picoeukaryotes appear to be increasing in the arctic. However, the authors of that study lost the strain used, and two subsequent studies by other laboratories were unable to replicate the results, concluding that ''Micromonas'', including ''M. polaris'', is not a predatory mixotroph.


Viral infection

Virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...
es are important in the balance of marine ecosystem by regulating the composition of microbial communities, but their behaviors can be affected by several factors including temperature, mode of infection and host conditions. There is an increasing number of ''Micromonas''-infecting virus being discovered and studied, including studies of transcriptional responses to infection under differing nutrient conditions.


''Micromonas pusilla'' virus

There are currently 45 viral strains identified that coexist with ''M. pusilla'' populations. Virus infectivity is dependent on the host strain, light availability and virus adsorption. Per day average of death due to virus lysis is estimated to be about 2 to 10% of the ''M. pusilla'' population. * ''Micromonas pusilla'' reovirus (MpRV): The first isolation of a
reovirus ''Sedoreoviridae'' (''sedo'' = smooth) is a family of the ''Reovirales'' order of viruses. Viruses in this family are distinguished by the absence of a turreted protein on the inner capsid to produce a smooth surface. Taxonomy The family contain ...
that infects protist. This virus is found to be bigger than other members of the family.


''Micromonas polaris'' virus

It is the first phycodnavirus being isolated from polar ocean waters. It can infect ''M. polaris'', which is the polar ecotype of ''Micromonas'' that has adapted to waters with low temperatures. Evidence suggests that the increase in temperature due to climate change may shift the clonal composition of both the virus and host.


Metabolic engineering

With the growing population in the world, there is an increased demand for wild fishes and algae for their source of
polyunsaturated fatty acids In biochemistry and nutrition, a polyunsaturated fat is a fat that contains a polyunsaturated fatty acid (abbreviated PUFA), which is a subclass of fatty acid characterized by a backbone with two or more carbon–carbon double bonds. Some polyunsa ...
(PUFA), which is required for growth and development, as well as the maintenance of health in humans. Recent research is investigating an alternative mechanism for production of PUFA by using acyl-CoA Δ6-desaturase, an enzyme present in ''M. pusilla'', with plants. The ''M. pusilla'' strain of acyl-CoA Δ6-desaturase is highly effective in the polyunsaturated fatty acid synthesis pathway due to its strong binding preference for omega-3 substrates in land plants.


References


External links


Genes from tiny algae shed light on big role managing carbon in world's oceans
{{Taxonbar, from=Q6839686 Chlorophyta genera Mamiellophyceae Marine microorganisms