Are Protists Heterotrophs? 9 Important Facts You Should Know

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There are protists heterotrophs, autotrophs and mixotrophs in nature, based on their mode of nutrition.

Protists are omnipresent, unicellular, eukaryotic organisms that are grouped together under the kingdom Protista because they do not identify with all the other kingdoms under the domain, Eukarya. As a result, they possess groups of unrelated organisms.

The autotrophic protists are the primary producers of aquatic habits, whereas, the heterotrophic protists act as the primary consumers of bacteria.

How are protists heterotrophic?

Majority of the extant eukaryotic species rely on heterotrophic mode of nutrition for survival. Phagocytosis is the most common among the different heterotrophic modes of nutrition. In fact, many phototrophic groups of protists possess the ability of phagocytosis as well, while some phototrophic groups have completely lost this ability.

Are all protists heterotrophic?

Not all protists are heterotrophs, as many extant species possess the ability to photosynthesize as well. The chloroplasts were engulfed by the primitive protistic cells through phagocytosis as endosymbiont. Some of such phototrophic species have even lost the ability for photosynthesis after obtaining chloroplast by endosymbiosis, to go back to heterotrophic mode of nutrition. Some groups show mixotrophic mode of nutrition as well, such as chrysomonads and dinoflagellates.

Heterotrophic protists examples

Heterotrophic protists are easily found in benthic habitat of both water bodies as these tend to have an accumulation of organic matter and detritus. Some like Bicosoeca sp. prefer to create a current to facilitate feeding while some possess specialised feeding appendages.

Amoeboids

Amoeboids are mostly present in aquatic ecosystems including marine, brackish water, freshwater and some are found in soil habitats as well. Their unique feature is the presence of pseudopodia which allows them to change their shape and also in phagocytosis. Amoeba is a common example and is pathogenic to humans.

Flagellates

Flagellates are both free living or parasitic unicellular protists that account for most of the abundance of zooplanktons in freshwater habitats. Some species of flagellates are phototrophic and some are mixotrophic as well. Their flagella are multipurpose as they help in feeding, attachment to substrate and in motility.

Ciliates

Ciliates are heterotrophic in nature, feeding on detritus, algae, bacteria and other protists. Ciliates can possess algal endosymbionts and take part in the primary production of the aquatic habitat they exist in. The only species of ciliates that are pathogenic humans is Balantidium coli.

Balantidium coli trophozoite
Image credit: Wikimedia Commons. Trophozoite of Balantidium coli.

Sporozoans

Sporozoans are unicellular, spore forming and parasitic protists which contain many species which are pathogenic to humans, such as malaria causing Plasmodium sp., trichomoniasis causing Trichomonas vaginalis and sleeping sickness causing Trypanosoma brucei.

1050px Trypanosoma brucei
Image credit: Wikimedia Commons. Trypanosoma brucei.

What types of protists are autotrophs?

When the Earth was in its primitive state where life started in the hot primordial soup, several extant species of protists obtained chloroplasts by the process of endosymbiosis. Acquisition of chloroplasts allowed the protists to photosynthesise and obtain nutrition by autotrophic mode. Some species of ciliates and foraminiferans can retain the chloroplasts from their prey cells and exploit them for a few days until they become non- functional.

Autotrophic protists examples

Most of the autotrophic protists usually live in freshwater bodies while some are found in marine habitats as well. The most common of all habitats are the small water bodies such as ditches, canals, ponds that undergo eutrophication very easily.

Euglenoids

They are easily found in habitats that have an abundance of decaying organic matters. During decomposition, the organic matters release sugars, vitamins B1 and B12, short chain fatty acids and the other organic compounds needed by euglenoids. Usually, chlorophyll a and b and carotenoid pigments are present in euglenoids.

Euglena, a common example, usually inhabit freshwater bodies, marine ecosystems, wetland habitats and the digestive tracts of small aquatic organisms.

are protists heterotrophs
Image credit: flickr. Euglena.

Dinoflagellates

Dinoflagellates mostly found in marine ecosystem but some are found in freshwater habitats as well. Marine dinoflagellates are generally present in the photic zone where light and nitrogen are easily available. The photosynthetic species of dinoflagellates contribute towards the primary production of the freshwater and marine habitats. They possess golden-brown plastids with chlorophylls a and c, along with some accessory pigments unique to them.

Hawaiian Dinoflagellates 1 edited
Image credit: Wikimedia Commons. Dinoflagellates.

Several autotrophic species act like mixotrophs as well and acquire nutrition by phagocytosis and osmosis as well, along with photosynthesis.

Diatoms

Diatoms are unicellular phytoplanktons which are mostly autotrophs but a few species are heterotrophs. They are believed to be a result of endosymbiosis of red algae which are also believed to have originated because of endosymbiosis. This is evident because the chloroplasts in diatoms have four membranes. Their cell wall is unique in being composed of silica dioxide and also in being formed of two distinct parts such that one fits into the other due to their size difference.

1200px Разнообразие форм диатомовых водорослей
Image credit: Wikimedia Commons. Diatoms.

Diatoms possess chlorophyll a, chlorophyll b and fucoxanthin, along with some carotenoids which are involved in photoprotection. When overexposed to sunlight, these carotenoids protect the photosynthetic systems from getting damaged by dissipating the excess solar energy in the form of heat.

Desmids

These are unicellular freshwater algae that exist only in clean stagnant freshwater bodies. Although most species fall under highly threatened group of organisms due to their very narrow highly specific range of niche, they act as good indicators of water pollution for the same reason.

Some examples of desmids are Cosmarium sp., Closterium sp., Euastrum sp., Straurastrum sp., Pleurotaenium sp. and Desmidium sp.

Algae Cosmarium edited
Image credit: Wikimedia Commons. Cosmarium sp.

Golden algae

They mostly found in freshwater habitats with some exceptional species which can thrive in marine and brackish ecosystems as well.  They are capable of producing various kinds of toxins which are fatal for the aquatic animals.

Some examples are Dinobryon sp., Chromulina sp., Chrysococcus sp., Kephyrion sp. and Pseudokaphyrion sp.

4878561929 50d09762ff b edited
Image credit: flickr. Dinobryon sp.

Are fungus like protists heterotrophs?

Slime molds and water molds are the major groups of protists that resemble fungus and are all usually heterotrophic. Some examples of slime molds are Dictyostelium sp., Lycogala sp., Polysphondylium, Fuligo septica and Physarum polycephalum.

1200px Dictyostelium discoideum 43
Image credit: Wikimedia Commons. Dictyostelium discoideum

Are animal like protists heterotrophs?

Protists that resemble animals are mostly grouped under amoebiods, flagellates, ciliates and sporozoans. Some examples are Amoeba proteus, Paramecium sp., Plasmodium berghei, Trichomonas and Trypanosoma.

All animal like protists are heterotrophs except for Chlorarchniophytes, marine amoeboids that show mixotrophic mode of nutrition. Many groups under foraminifera and radiolaria exist in a symbiotic relationship with some unicellular algae. Sporozoans are generally heterotrophs.

Many planktonic ciliates are autotrophs and contain plastids with plant like pigments due to endosymbiosis. An example is Myrionecta rubra which is an obligate phototroph.

Are plant like protists heterotrophs or autotrophs?

Algae, euglenoids, diatoms and chrysophytes or the golden algae represent the plant like autotrophs as most of the groups under them are photosynthetic.

Whereas, dinoflagellates show both autotrophic and heterotrophic modes of nutrition. Among them are some that show mixotrophic mode of nutrition and obtain food from both the ways. Some examples are Oxyrrhis marina, Protoperidinium sp. and Gyrodinium sp.

Some genera of algae have been reported to show growth in artificial environment, in the absence of sunlight, by heterotrophic mode too. Such as, Chlorella vulgaris, Chlorella regularis, Chlamydomonas reinhardtii, Spirulina sp. and Nostoc.

Certain species of diatoms, such as Navicula perpusilla and Nitzsckia clostredium can be cultured in dark and semi- dark conditions.

Conclusion

To conclude protists show autotrophic, heterotrophic and mixotrophic modes of nutrition. Autotrophic protists usually identify better with plants while heterotrophic protists have some similarity to either animals or fungi.

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