Tag Archives: protists

Friday Fellow: Contractile Gentle-Scaled Centrohelid

by Piter Kehoma Boll

Unicellular eukaryotes, traditionally called protists, come in a variety of shapes and sizes and have a complex classification, especially because many lineages evolved similar features. The centrohelids, for example, have a round cell with several needle-like radially-distributed pseudopods, called actinopods, thus looking like radiolarians, but are only distantly related to them.

A single individual of the contractile gentle-scaled centrohelid. Credits to Wikimedia user NEON_ja.*

A centrohelid that has been considerably well studied recently is Raphidiophrys contractilis, to which I decided to coin the common name contractile gentle-scaled centrohelid. It was described in 1995 from specimens collected from brackish ponds in Hiroshima, Japan. As with other species of the genus Raphidiophrys, the contractile gentle-scaled centrohelid has many structures of silica, called scales, covering the cell and embedded in a gelatinous coat. These scales are more concentrated around the base of the actinopods and extend outward around part of them as well. In the contractile gentle-scaled centrohelid, the scales are oblong, flat and slightly curved, resembling a rubber boat. The convex side of the scale is always directed toward the cell.

The contractile gentle-scaled centrohelid is a predator of other protists, especially flagellates and ciliates. Small protists are captured by the actinopods and pulled quickly toward the cell body by a sudden contraction of the actinopod. This behavior is the reason for the species to be named contractilis. Once the prey is close to the cell body, it is surrounded by pseudopods and stored in a food vacuole, where it gets digested. The capture of prey using the actinopods is aided by a special organelle, called kinetocyst, found in large number below the cell membrane. When the centrohelid touches a prey, it discharges the kinetocysts, which immobilize the prey, working similarly to the cnidocytes of cnidarians.

Raphidiophrys contractilis with several flagellates of the species Chlorogonium elongatum trapped in its actinopods (A) and one of the being swallowed into a food vacuole (B). Extracted from Sakaguchi et al. (2002).

When a gentle-scale centrohelid finds a very large prey, much larger than itself sometimes, such as a ciliate Paramecium, it uses an extreme cooperative behavior. Several individual organisms fuse into a single large cell, pull chunks of the prey off and create a large common food vacuole. Isn’t that bizarre and amazing?

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References:

Kinoshita E, Suzaki T, Shigenaka Y, SugiyamaM (1995) Ultrastructure and rapid axopodial contraction of a Heliozoa, Raphidiophrys contractilis sp. nov. The Journal of Eukaryotic Microbiology 42(3): 283–288. doi: 10.1111/j.1550-7408.1995.tb01581.x

Sakaguchi M, Suzaki T, Khan SMMK, Hausmann K (2002) Food capture by kinetocysts in the heliozoon Raphidiophrys contractilis. European Journal of Protistology 37(4): 453–458. doi: 10.1078/0932-4739-00847

Siemensma FJ, Roijackers MM (1988) The genus Raphidiophrys (Actinopoda, Helozoea): scale morphology and species distinctions. Archiv für Protistenkunde 136(3): 237–248. doi: 10.1016/S0003-9365(88)80023-X

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Friday Fellow: Giant Gromia

by Piter Kehoma Boll

Some time ago I introduced a cool unicellular alga, the Sailor’s Eyeball, which can reach about 5 cm in diameter, being one of the largest unicellular organisms known to exist.

Today  we’ll know one more creature of this type, only it is not an alga, but a testate amoeba more closely related to foraminifers. Named Gromia sphaerica, I will here call it the giant gromia.

gromia fig2

Specimens of the giant gromia from the Bahamas. Image extracted from Matz et al. (2008).

The giant gromia was first found in the Arabian Sea at depths of more than 1100 m and was formally described in 2000. It lives lying on the substrate and is usually covered by a thin layer of sediment, appearing as small spheres scattered across the sea floor. The body is spherical or grape-shaped but hollow, with the interior filled with fecal material (called stercomata) or other fluids. This spherical cell is covered by a shell, or test, of organic material which shows several small perforations by which thin expansions of the cytoplasm, forming a kind of pseudopod, can be extended. The size of the test can reach up to 3 cm in diameter, being much larger than that of its best known relative, Gromia oviformis.

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Several specimens of Gromia sphaerica on the sea floor of the Bahamas with the tracks left by their movement. Extracted from Matz et al. (2008).

In 2008, another population of species was found in the waters around the Bahamas. Specimens there are not as spherical as in the population in the Arabican Sea and  were seen associated with tracks that indicate that these organisms slowly move across the sediment. The tracks clearly resemble some fossil tracks from the Pre-Cambrian period, which are usually considered an indication of the early evolution of multicellular animals. However, this discovery of unicellular organisms being able to produce tracks similar to those associated with animals raises doubt about the time of origin of multicellular animals.

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References:

Gooday AJ, Bowser SS, Bett BJ, Smith CR (2000) A large testate protist, Gromia sphaerica sp. nov. (Order Filosea), from the bathyal Arabian Sea. Deep-Sea Research II 47: 55–73.

Matz MV, Frank TM, Marshall NJ, Widder EA, Johnsen S (2008) Giant deep-sea protists produces bilaterian-like traces. Current Biology 18(23): 1849–1854. https://doi.org/10.1016/j.cub.2008.10.028

 

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Friday Fellow: Common Pelomyxa

by Piter Kehoma Boll

It’s time to dig deep into the mud again and bring up a peculiar protist, the third species of the clade Amoebozoa to be featured here. Its binomial name is Pelomyxa palustris, and I decided to call it the common pelomyxa.

Measuring up to 5 mm in length, although usually having less than 1 mm, the common pelomyxa is considered a “giant amoeba”. In fact, the Giant Amoeba Chaos carolinense, previously featured here, was once classified in the genus Pelomyxa, but currently we know that they are not closely related at all. While the true giant amoebas of the genus Chaos are closely related to the common amoebas of the genus Amoeba, the species in Pelomyxa belong to a completely different group of amoebas.

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A specimen of the common pelomyxa. The large pseudopod is to the right and the small uroid can be seen projecting from the cell at the upper left. Credits to Proyecto Agua.*

The cell of the common pelomyxa has a somewhat cylindrical shape with a single, large, semicircular pseudopod at the front, thus moving basically always in the same direction, forward, differently from the more classical amoebas with several pseudopods and the ability to move in any direction. At the opposite side of the cell, the common pelomyxa has a small, also semicircular appendix called the uroid that is covered by several small non-motile flagella. The flagella are surrounded by small cytoplasmic projections (villi) that are easily seen under the microscope.

The common pelomyxa lives buried in the sediments of freshwater lakes throughout the northern hemisphere, especially those rich in decaying organic matter. It slides through the mud while feeding on smaller microorganisms and organic debris. Such an environment is characterized by the complete absence or extremely low concentrations of oxygen. As a result, the common pelomyxa is anaerobic and even lacks mitochondria. For this reason, it was once considered part of a very primitive group of eukaryotes that diverged before the incorporation of the endosymbiotic bacteria that would evolve into mitochondria. Currently, however, it is known that their lack of mitochondria is actually due to a secondary loss and they seem to be related to true amoebas and slime molds.

More than only lacking mitochondria, the cell of the common pelomyxa has a lot of peculiar features. Depending on the size of the cell, it may contain a few to several hundred nuclei. The cytoplasm also appears to lack several typical eukaryotic organelles, such as the Golgi apparatus and the endoplasmic reticulum. There are, however, many, many small vacuoles, so many that the cell usually has a foamy appearance.

Endosymbiotic bacteria are also found in great numbers in the cytoplasm of the common pelomyxa. After several years of research, it seems that these bacteria are obligate symbionts. Perhaps they help this strange amoeba to perform some of the tasks that should be done by the several organelles that it lacks.

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References:

Goodkov, A. V.; Chistyakova, L. V.; Seravin, L. N.; Frolov, A. O. (2004) The concept of pelobionts (Class Peloflagellatea): Brief history and current stateEntomological Review 84(Suppl. 2): S10–S20.

Wikipedia. Pelomyxa. Available at < https://en.wikipedia.org/wiki/Pelomyxa >. Access on May 26, 2018.

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Friday Fellow: Handsome Asterisk-Diatom

by Piter Kehoma Boll

It’s time for the next diatom to be featured here. Differently from the previous ones, today’s diatom is a freshwater species commonly found in lakes of North America and Eurasia. It has also been reported for South America and Africa, but it is likely that these individuals actually belong to another, closely related species.

asterionella_formosa

Asterionella formosa from a lake of the Rocky Mountain National Park, United States

Named Asterionella formosa, this diatom has small rod-shaped cells that are 60 to 85 µm long and only 2 to 4 µm wide. The individuals usually organize themselves in colonies linked by one of the ends in a star fashion. Most colonies include eight organisms and look somewhat like an asterisk, hence I chose to give the common name asterisk-diatom to the genus, this species then being called the “handsome asterisk-diatom”, from the translation of the specific epithet formosa. However, some colonies may have up to 20 individuals and organize in a more spiral fashion.

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A spiral-shaped colony from a small lake in Spain. Credits to Proyecto Agua.*

Originally found and described from water supplies used in London, the handsome asterisk-diatom has a preference for cold waters, occurring commonly in temperate lakes under temperatures between 0 and 15 °C. During summer, when temperatures get too high and the light intensity also increases, its photosynthesis is inhibited by these two factors as well as by the increase in oxygen caused by the metabolism of the species itself as well of other algae from the phytoplanktonic community.

Sexual reproduction is not well known in the handsome asterisk-diatom, but must certainly occur, as asexual reproduction alone leads to a continuous decrease in cell size in all diatoms. Studies on genetic diversity show that this species is very genetically diverse, which proves that sexual reproduction indeed occurs and in a apparently high rate, contributing for the dominance of this species in many of the ecosystems of which it takes part.

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References:

AlgaeBase. Asterionella formosa Hassall. Available at < http://www.algaebase.org/search/species/detail/?species_id=31441 >. Access on May 25, 2018.

Belay, A.; Fogg, G. E. (1978) Photoinhibition of photosynthesis in Asterionella formosa (Bacillariophyceae). Journal of Phycology14(3): 341–347. https://doi.org/10.1111/j.1529-8817.1978.tb00310.x

De Bruin, A.; Ibelings, B. W.; Rijkeboer, M.; Brehm, M.; Van Donk, E. (2004) Genetic variation in Asterionella formosa (Bacillariophyceae): is it linked to frequent epidemics of host-specific parasitic fungi? Journal of Phycology40(5): 823–830. https://doi.org/10.1111/j.1529-8817.2004.04006.x

EOL – Enclyclopedia of Life. Asterionella formosa. Available at < http://eol.org/pages/917771/details >. Access on May 25, 2018.

Lund, J. W. G. (1950) Studies on Asterionella formosa Hass: II. Nutrient depletion and the spring maximum. Journal of Ecology38(1): 15–35.

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Friday Fellow: Pink Cellular Slime Mold

by Piter Kehoma Boll

Protists have always been problematic organisms, and today’s Friday Fellow is not different. In fact, it has probably been one of the most problematic ones. Known scientifically as Acrasis rosea, it has no common name, as you may have guessed already, so I will call him the pink cellular slime mold, as I saw him being called once.

acrasis_rosea_31330

Isolated (or not so much) cells of Acrasis rosea. Photo by Shirley Chio.*

The pink cellular slime mold is a single-celled organism with an amoeboid shape. It feeds on a variety of bacteria and yeasts and is commonly found in decaying plant matter. When the food supply is completely consumed and the cells start to starve, they gather and form a colony that act as a single organism that moves like a plasmodium similar to that of slime molds. For this reason they were originally called cellular slime molds and considered related to other organisms showing a similar behavior, such as those of the genus Dictyostelium.

This plasmodium moves through the formation of “pseudopods”. Eventually the cells start to form a pile reaching up into the air that produce fruiting bodies in the form of branched chains of spores. There is a slight division of labor between stalk and spore, but both groups of cells are viable to produce a new generation.

799px-acrasis_rosea_31095

The chains of spores are visible in this image of the pink cellular slime mold during its plasmodium phase. Photo by Shirley Chio.*

The whole process is similar to what is seen in species of Dictyostelium, but the division of labor and the morphology of the plasmodium and the fruiting bodies are a bit more complex. However, with the advancement of molecular phylogenetics, all the slime mold and cellular slime mold classification fell apart.

While Acrasis was revealed to be an excavate, being closely related to organisms such as the euglenas and parasitic flagellates, Dictyostelium is closely related to the true slime molds, such as the already presented here many-headed slime.

But the excavates are still a problematic group among the protists, and so the real position of the pink cellular slime mold may not be settled yet.

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References:

Bonner, J. T. (2003) Evolution of development in the cellular slime molds. Evolution and Development 5(3): 305–313. http://dx.doi.org/10.1046/j.1525-142X.2003.03037.x

Olive, L. S.; Dutta, S. K.; Stoianovitch, C. (1961) Variation in the cellular slime mold Acrasis rosea*. Journal of Protozoology 8(4): 467–472. https://dx.doi.org/10.1111/j.1550-7408.1961.tb01243.x

Page, F. C. (1978) Acrasis rosea and the possible relationship between Acrasida and Schizopyrenida. Archiv für Protistenkunde 120(1–2): 169–181. https://doi.org/10.1016/S0003-9365(78)80020-7

Weitzman, I. (1962) Studies on the nutrition of Acrasis roseaMycologia 54(1): 113–115.

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Friday Fellow: Bubble Globigerina

by Piter Kehoma Boll

A little more than a year ago I introduced the first foraminifer here, the tepid ammonia. Now it is time to bring the second one, this time a planctonic species that is rather famous and whose scientific name is Globigerina bulloides, or the bubble globigerina as I call it.

Globigerina_bulloides

A live specimen of Globigerina bulloides. Photo extracted from Words in mOcean.

This species can be found throughout the world, but it’s more common in cold subantarctic waters and a little less common in subarctic waters. The most common areas are the North and South Atlantic and the Indian Oceans, but the tropical records are most likely a misidentification of other closely related species.

The bubble globigerina usually lives in the upper 60 m of the water column, at least while reproducing, and feeds on other planktonic organisms, especially microscopic algae. In oder to maximize the ability of their gametes to meet in the vast extension of the ocean, the bubble globigerina synchronizes its sexual cycle with the moon cycle, reproducing during the first week after the new moon. It is, therefore, a kind of biological calendar.

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ResearchBlogging.orgReferences:

Bé, A. W. H.; Tolderlund, D. S. 1972. Distribution and ecology of living planktonic Foraminifera in surface waters of the Atlantic and Indian Oceans. In: Funnell, B. M.; Riedel, R. (Eds.) The Micropaleontology of Oceans, Cambridge University Press, pp. 105–150.

Schiebel, R., Bijma, J., & Hemleben, C. (1997). Population dynamics of the planktic foraminifer Globigerina bulloides from the eastern North Atlantic Deep Sea Research Part I: Oceanographic Research Papers, 44 (9-10), 1701-1713 DOI: 10.1016/S0967-0637(97)00036-8

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Friday Fellow: Giant Kelp

by Piter Kehoma Boll

This week we’ll stay in the sea and meet on of the most impressive algae, the giant kelp, Macrocystis pyrifera. It is called giant for a good reason, since it can grow up to 50 m in length and form real forests in the sea. Being able to grow 60 cm in a single day, it has the fastest linear growth of any organism on Earth.

The giant kelp is a brown algae, so it is not related (at least not closely) to green or red algae, but it is a relative of the tiny diatoms that cover the ocean. It grows in cold waters along the Pacific Coast of the Americas and close to the coast of the countries near Antarctica, such as Chile, Argentina, South Africa, Australia, and New Zealand.

macrocystis_pyrifera

It’s a really beautiful alga, isn’t it? Photo by California Academy of Sciences.*

This amazing organism is composed by a thallus that branches at the base and then continues as a single and very long stalk from which blades develop at regular intervals on only one side. At the base of each blade, there is a gas  bladder that helps the whole organism to stand in a more or less upright position.

The huge kelp forests in the oceans are an important ecosystem and many species depend on them to survive, including other algae. Humans also use the giant kelp either as a direct food source or as a source of dietary supplements, since the alga is rich in many minerals, especially iodine and potassium, as well as several vitamines.

macrocystis_pyrifera2

The kelp forests sustain a huge diversity of lifeforms in the oceans. Photo by Stef Maruch.**

In the last decades, the kelp populations are decreasing rapidly. This is most likely caused by climatic changes, as this alga cannot develop in temperatures above 21°C. The giant kelp is, thus, just one more victim of global warming. And if it goes extinct, a whole ecosystem will be gone with it.

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ResearchBlogging.orgReferences:

Foster, M. (1975). Algal succession in a Macrocystis pyrifera forest Marine Biology, 32 (4), 313-329 DOI: 10.1007/BF00388989

Wikipedia. Macrocystis pyrifera. Available at . Access on January 19, 2007.

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