Disaster and Earth Science

What are acritarchs?


Among the oldest micro-organisms, acritarchs are one of them. Acritarchs are the organic-walled microfossils ranges in size from 20- 80 micrometers. They are widely distributed from the Archean age of 3000 million years to the recent. Acritarchs are the most important microfossils among the geologist because of its long geologic history of the existence. Geologist tries to find the correlation of the first life on the earth with this type of unicellular micro-organism to convey the link between the human being micro-organisms from the past to the present. Through the study of acritarchs, a scientist can explore the paleoenvironment of the earth. 

What are acritarchs?

Acritarchs are the unicellular, organic-walled micro-organism of unknown origin. They usually fall in the group of dinoflagellates. Because they usually found in the rock along with the dinoflagellates type of microorganisms. They have a spike or ornamentation on their wall. Based on the spike and the shape of the acritarchs scientist can classify the acritarchs. Acritarchs are usually in the 20- 80 micron in size. So they can not be seen with the naked eyes. Acritarchs are the most rigid and undegradable unicellular micro-organisms. It is a non-carbonate, non-silicious and non-acid soluble organism. So, in the rock sample, it is a very nicely preserved organism. It is very important for the geologist to explore the paleo-environments of the earth. The acritarchs are first appeared in the Proterozoic and reached the maximum in the Early to Middle Paleozoic. In the Carboniferous and Permian, they are nearly absent and the sudden absence of the acritarchs is called the “Phytoplankton Block Out”. Some scientist they try to reveal the similarity between the acritarch and the phytoplankton. Based on the value of the Carbon 13 of acritarchs and the Phytoplankton they found a similar value. Hence they emphasized that it may be in the phytoplankton type of microorganism. But still, the exact origin of acritarchs are unraveled!

How to Classify the Acritarchs?

Based on the ornamentation and the geometrical shape of the vesicle of acritarchs, they can be classified into five different types. The study from South China, Shangsi, Meishan, and some other section, geologist and paleontologist found a wide variety of acritarchs preserves in the samples. The Micrhystridium, Veryhachium type of acritarchs are very small in size usually the 20 micrometers. 

The classifications are;

  1. The Veryhachium cylindricum group: This group represents all the ellipsoidal shape of the Acritarchs.
  2. The Veryhachium trispinosum group: This group includes all triangular-shaped vesicles.
  3. The Veryhachium lairdii group: This Group includes all with rectangular forms of the acritarchs.
  4. The Micrhystridium pentagonale group: This group covers all the pentagonal shapes fo the acritarchs.
  5. The Micrhystridium breve group: This group includes all the spherical forms of acritarchs.

What is Known So far about  Unknow-Origin of Acritarchs?

Acritarchs are sometimes called the phytoplankton origin of organisms. So, Phytoplankton and acritarchs may have relationships. But before the final discovery of the exact origin of acritarchs still, there is a huge debate among the geologist, paleontologist, and other scientists.

  1. The acritarchs are mainly the marine phytoplankton or the algae or the unknown affinities of origin. Grice et al. (2005) state that similar value of stable carbon isotopic composition of the acritarchs derived biomarker (Geochemical fossils) and phytane sourced from chlorophyll a in phytoplankton
  2.  Another potential source reported in cold-seep carbonate crusts (Pancost et al., 2001). 
  3. A series of linear diphenyl alkanes have been reported in sulfur-rich coals containing biomarkers also derived from Chlorobiaceae (Gorchs et al., 2003). 
  4. Some are suggested that it might be derived from an organism that inhabited in a sulfide-rich environment (Grice et al., 2005; Fig. 15).

Though it is still a huge scope for the geologist to discover the exact sources of the acritarch in the Permian rocks or other older rocks to explore the paleo-history of the earth and can be linked up with the great mass extinction of the earth. During the last five mass extinction events, (Ordovician-Silurian, Devonian-carboniferous, Permian Triassic, Triassic Jurassic, and the last Cretaceous Paleogene mass extinction)  this micro-organism had been survived and experienced and until today we can found acritarchs in the recent sample. So, Acritarchs can live in stress environments and able to survive the event in the extinction span of time. Hence, we need more work on acritarchs to explore the past based on the micro-fossils.

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