Marine Biology: Reefs, Mars & Stromatolites


A reef is a bar of rock, sand , coral or similar material , lying beneath the surface of water. 

Many reefs result from abiotic processes (i.e. deposition of sand, wave erosion, and other natural processes), but the best known reefs are the coral reefs of tropical waters developed through  biotic processes dominated by corals and calcareous algae.

Coral reefs are complex and highly diverse ecosystems that exhibit a gross productivity 50 to 100 times greater than the surrounding tropical ocean waters (Sorokin 1990b, 1993).

Coral reefs are built by corals that can secrete calcium carbonate exoskeleton; not all corals build massive reefs, those do restricted to tropics, monthly mean sea temperature not less than 20 °C year round।

Source here.

Variety of corals matched by richness of other life dependent on reef. Yet all this massive ecological engineering  depends on symbiosis of coral polyps with single celled alga called Zooxanthellae living inside cells of polyp’s gut

Photosynthesis by alga provide most of corals carbon supply and polyp waste recycled in return algae receives N & P from coral

photosynthesis also creates an alkaline local environment allowing CaCO3 accretion; algae directly related to calcification

Corals without algae can build skeleton but at terrible energetic costs and massive reefs impossible.

Coral bleaching results from the loss of Zooxanthellae

Symbiotic algae & Coral reefs

The frequency of symbiotic associations between plants and animals in the tropics is probably  a result of lower nutrients, higher rates of predation, and the long period for evolution of coral reefs (Luning 1990). 

The most important endosymbionts are dinoflagellates, the most common species is Gymnodinium microadriaticum (Symbiodinium microadriaticum) , although unicellular green algae  and filamentous blue-green also occur.   

Corals may be viewed as living greenhouses; endosymbionts aid in calcification and supply photosynthates, while obtaining  nutrients from animal.

Infact, the survival and development of  coral reefs in oligotrophic tropical seas are largely due to the microalgal zooxanthellae. The unique and important mutualistic relationship has been well described (Sorokin, 1993).

In addition to the endosymbiotic  microalgae, which include  the zooxanthellae, other coral reef algae include phytoplankton, mat-forming and boring microfilamentous algae, and calcified, fleshy, and turf macroalgae (Berner 1990).



Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt. 

The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite, dolomite, and siderite may be present. 

Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions; specifically an earthy substance containing 35–65% clay and 65–35% carbonate. It also describes a habit of coralline red algae.

Fossil-rich marl. Source here.

The term is today often used to describe indurated marine deposits and lacustrine (lake) sediments which more accurately should be named ‘marlstone’. 

Marlstone is an indurated (resists crumbling or powdering) rock of about the same composition as marl, more correctly called an earthy or impure argillaceous limestone.

It has a blocky subconchoidal fracture, and is less fissile than shale.



Stromatolites  or stromatoliths  are layered mounds, columns, and sheet-like sedimentary rocks that were originally formed by the growth of layer upon layer of cyanobacteria, a single-celled photosynthesizing microbe.

Fossilized stromatolites, some of which may date from 3.7 billion years ago, provide records of ancient life on Earth.

Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens.

Source here.

Stromatolites are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. 

Source here.

They exhibit a variety of forms and structures, or morphologies, including conical, stratiform, branching, domal, and columnar types. Stromatolites occur widely in the fossil record of the Precambrian, but are rare today. 

Very few ancient stromatolites contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-biological) precipitation. 

Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an active area of research in geology.

(This article is completely based on the Lecture sheet provided by respected Dr. Mohammad Azmal Hossain Bhuiyan, Professor, Department of Botany, University of Dhaka.
Some pictures and info have been added by the author. Any mistake, error, misinformation and other related things found in this article is only author’s to blame)

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About Abulais Shomrat

Abulais Shomrat
Currently in 4th year (Hons) in the Department of Botany, University of Dhaka. Planning to have multiple careers one by one but promised to be with 'Plantlet' as long as it's primary stage remains unfinished.

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