Features of Microbial Mats and Stromatolites - Environmental Assessment Answer

Subject Name : Management

What do microbial mats and stromatolites tell us about the origin of life

Abstract
The paper discusses the characteristics and features of microbial mats and stromatolites that show hints of life. Origin of life has been associated with them due to the growth of microorganisms on the microbial mats and growing layers on limestone. Stromatolites are rock like structures having layers of limestone over them. These organisms use sunlight, water, oxygen, carbon dioxide, and other organisms from their surroundings for survival. Researchers are studying these microbial mats and stromatolites in order to relate the growth and survival of the organism found in them with the life and its origin on the Earth. The existence of microbial mats is believed to be much before when the earth was formed.

Introduction
Researchers are finding signs of primitive life. Ancient clusters of rock with some of the oldest microbes have a branch like formations. These branch-like formations are believed to have occurred due to microbial activity in the shallow lakes and seas. Scientists also believe that life first evolved in the lakes and seas. Researchers have found examples showing that life on Earth was dominated by microbial life thus it has become important to understand microbial evolution which can be done by studying sedimentary record. The aim of this essay is to analyse the relationship between microbial mats, stromatolites and life. In the following paragraphs, the evolution of microbial mats and stromatolites will be discussed. How they are related to the origin of life will also be analysed in the essay.
Microbial mats offer clues to life
According to Westall (2011), microbial mats are the most remarkable example of the origin of life on the Earth. They are valued due to their evolutionary and ecological standpoints in the history of life. Their occurrence is dated back to about 3.5 Ga BP in the geological record. This can also be dated as 1 Ga after the formation of Earth. The microbial mats are believed to be present extensively in coastal and terrestrial aquatic areas. These were the dominant sign of the existence of life on the earth and have shown a biological feature of life on the planet earth. They started dissolving due to eukaryotic organisms started feeding on them for their evolution. However, they are still present in the environment favourable for their growth such as thermal springs and hydrothermal vents (Foster and Green 2011). Archaea and bacteria also occur in such thermal settings. This finding has triggered further research in thermal environments. The interest of astrobiologists in the thermal environment has increased because they are believed to have influenced the early evolution of life not only on Earth but on other planets like Mars as well. Scientists are trying to identify the group which has become photosynthetic first. Some of them have identified anoxygenic photosynthetic group as a foremost group that has become photosynthetic. Mats are a place for microbes that are important for the study of bacterial phylogeny and bacterial physiology.

Fig. 1 Fossil record of Cynobacterial Mats Source www.mdpi.com/journal/life
Anoxygenic photosynthetic group further has Proteobacteria, Chloroflexi, Firmicutes and Chloroflexi. Contradicting this, Cynobacteria group is more complex and requires photosynthesis I and II were believed to be the first oxygenic photosynthesis (Abele et al. 1988). Genomes of various photosynthetic groups have also been compared to check which group was evolved but it is hard to determine its ancestral group. Moreover, these organisms are very close to each other which increase the chances of transmitting their DNA. Organisms have various transfer mechanisms to transmit DNA from one mat to another. These transfer mechanisms of microbial organisms community can select any gene from its environment and retain characteristics most suitable for their evolution and survival (Holdgate and Gallagher 2003). The microbial mat could answer the questions related to the first photosynthetic group and its ancestral group as well. Having so much of clues related to the existence of life still, these microbes are not studied in deep and researchers begin to take them seriously from the 1960s and began to study thermophile diversity and ecology. The microbial mats are the horizontally stratified microbial communities that display a structure that is defined by the physiochemical gradients that shows the diversity of microbes, their physiological activities and their dynamics as a whole system. Such ecosystems are generally associated with the aquatic habitats that include the hypersaline ponds, oligotrohic environments, hot springs, and the intertidal coastal zones. Microbial mats belong to the benthos community of organisms that are self sustaining communities and are vertically layered that forms in the liquid-solid interface of various environments. They comprise of millions of microorganisms that belong to distinguished species that interact5 and exchange signals that are fixed firmly in a matrix of exopolysaccharides, and to enable a huge flow of resources, they are embedded in nutrients (Preito-Barajas, 2018). They have restricted associations and some of them are symbiotic thus conferring a selective advantage. A set of metabolic reactions that are arranged in a loop that neither consumes nor produces the metabolites is a substrate cycle. In substrate cycle, a cofactor is transformed for example oxidizing a reducing equivalent.

Fig. 2 Microbial Mat Source https//phys.org/news/2016-03-microbial-mats-clues-life-early.htmlThese microbes are active at high temperatures and various other processes such as photosynthesis, chemosynthesis, and nitrogen fixation which support life also occur at high temperature. According to Stal (2012), mat setting provides a suitable place for the evolution of spirochetes. Researchers have found that there is a loss of microbial biodiversity which is a matter of concern. Due to mining practices, the decline in microbial species is observed. Loss of microbial activity will create constraints for researchers in determining their impact on the evolution of life on earth.
Hint of life in rocks
Like microbial mats, Stromatolites also have evidence of the evolution of life. These are the layered bio-chemical structures that are formed in water with the trapping and binding of sedimentary grains (Browne 2011). Stromatolites are internally laminated. They have been tracked for over 3.5 billion years. The stromatolites forms by the development of two separate microbial processes that is non-lithifying (type 1 mat that is a lose sediment used for microbial adherence) and lithifying (type 3 mat is a losse sediment that is hardened into rock). One of the key factors in explaining the role of microbes in the evaluation of Earths biosphere is Stromatolites. They are layered mounds, sheet-like sedimentary rocks and column that were originally formed by the growth of cyanobacterial layered growth, a single celled photosynthesizing microbe. Cyanobacteria are the DNA- packaging nucleus less prokaryotic cell.
Lichen Stromatolites is a process under which a layered rock structure is formed above water. This process takes place when rock and air meet and repeated colonisation of the rock by endolithic lichens take place. Stromatolites have been indicating ancient coastlines and water depth. Stromatolites are extensively found in Western Australia. In Western Australia, living and fossil forms of sediments have been found. These rocks age between 3.33 billion years to the modern age. Other than Western Australia, they are also found in the east to central Australia. These rocks are also popular for their mineralisation but they are mainly studied for the evidence of life on early Earth (Holdgate 2003). Since that time how the evolution has progressed and what changes have taken place have helped researchers for identifying the diversity with which animals and plants have been evolved. Stromatolites formed in lakes are different from those formed in the marine. Stromatolites formed in lakes do not trap layers of sediment however, marine Stromatolites trap them and allow growth of these layers in the presence of water.

Fig. 3 Stromatolites Source http//www.everythingselectric.com/stromatolites/
The composition of Stromatolites has also shown changes due to evolutionary development. In the primitive time, oxygen in the earths atmosphere has significantly increased due to photosynthesis. Cyanobacteria has shown significant sign of life which is growing and surviving using water, carbon dioxide and sunlight (Stal and Caumette 2013). With these components, Cyanobacteria creates its food. Along with this, a layer of mucus often forms over the microbial mats. The debris in the surroundings of the mucus gets trapped within it. Later on, the trapped debris gets cemented with calcium carbonate forming a thin layer of limestone. Over time, the banded pattern found on cyanobacterial cells forms biological stromatolites. The modern stromatolites were formed by the cyanobacteria that are photosynthesizing cyanobavcteria. They are the microbes that create their own energy by transformation into the chemical energy by the solar energy. The stromatolites were photosynthetically active from 1997 and they consist of mineral calcite and mud layers. Microbes still produce Stromatolites. The modern stromatolites are much similar to the ancient ones that give evidence of the life on earth from a long time. They both possess similar shapes and have similar layering which is fine. The microfossils of ancient cyanobacteria can be identified within these layers.
Summary
Evolution of microbial mats and stromatolites shows that the origin of life was much before researchers even started studying for it. Organisms on limestones and sediments are growing even before the earth was formed. To grow and survive, they use sunlight, water, oxygen, carbon dioxide, habitats in surroundings. While studying microbial mats, cyanobacteria have shown signs of life. Stromatolites, composed of calcite, are also growing in the shallow lakes. However, microbial mats are dissipating but stromatolites are extensively found in Australia which researchers are studying and relating their growth with the origin of life. The microbial mats and the stromatolites give evidence for the origin of life from past billions of years.

References
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