Diatoms – An Indicator of Marine Ecosystem-Erakina

Diatoms are microscopic silica-type algae found in marine and freshwater habitats, responsible for producing oxygen through photosynthesis. 

Green colored different shaped diatoms

Diatoms are a significant component in the phytoplankton (microalgae) community. They are abundantly found in the marine environment in freshwater and salt water. The diatoms are a substantial source of food for aquatic animals and are responsible for generating oxygen through photosynthesis. Diatoms have become an essential part of the early 20th century to study their significant role in the marine ecosystem, especially by Hustedt. From the fossilized diatoms, they have been recorded to be existence from the Jurassic era to further back to the Cretaceous period. In some countries, the fossilized diatoms have been estimated to have existed from the mid-Palaeogene period in freshwater. Based on the shape of diatoms, they are divided into two orders: the Centrales and the Pennales. Applications from studying the sediment diatoms are numerous since they play a vital role as biological indicators to determine the water quality and the marine health. 

Table of Contents

Overview of Diatoms

The life cycle of Diatoms

Characteristics of Diatoms

Applications

Overview of Diatoms 

Diatoms are unicellular alga having a cell wall made of silica. They are photosynthetic microorganisms that utilize light to produce energy and oxygen into sugars due to the presence of chlorophyll. Therefore, the diatoms transform into a primary food source for marine animals and additionally help in carbon fixation by removing the carbon dioxide and converting it into sugar and oxygen. Hence, the diatoms generate 20-30% of atmospheric oxygen. They are non-motile; instead, they are free-floating and sometimes attach themselves to other aquatic animals such as whales and marine plants. Some diatoms limited ability to move along a slit-like groove called raphe. Since they are photosynthetic, they are not found in the deepest part of the ocean where the light is less; instead, they are bound to the photic zone. However, once the diatoms die, they settle down at the bottom of the ocean, forming sediments. 

Different types of variety shaped structures of diatoms 

Diatoms belonging to the class Bacillariophyceae currently do not have an exact estimated total diatomic species. Around 200,000 species have been roughly estimated, and scientists keep discovering new species yearly. The size of diatoms varies from 20 to 200 microns, depending on the species. The outer cell wall of the diatoms is composed of frustule that is a particular glass-type silica and silica acid. Frustule functions as a porous membrane that helps move gases and organic matter in and out of the cell. Although the diatoms are a part of phytoplankton, however, they feed on various types of plankton. 

The diatoms are divided into two orders: Centrals and Pennales. The centric diatoms are typically circular, triangular, or rectangular and have to mark on the surface radiating from the central area in radial symmetry arrangement. In contrast, the pennate diatoms are generally elongated where the surface marking are at right angles towards the long axis having a groove called raphe in bilateral symmetry arrangement. Therefore, the centric diatoms belong to the family Coscinodisophyceae. In comparison, the family of pennate diatoms having raphe are Bacillariophyceae, while those without raphe belong to Fragilariophyceae.

The life cycle of Diatoms 

At first, the diatoms divide asexually by dividing the outer layer of the cell wall frustule into epitheca (larger half) and hypotheca (smaller half) that contain diploid cells on either half. Each of these two diploid cells undergoes mitotic cell division to produce two diploid daughter cells. Each of these two diploid cells will inherit one of their halves. One diploid cell will inherit the larger epitheca forming a hypotheca. In contrast, the diploid cell of the other half (from the first asexual division) will inherit the epitheca (hypotheca transformed into hypotheca) to develop into a new hypotheca. The hypotheca matches in size with their respective epitheca. During the first cell division, the first larger half epitheca is inherited by the second hypotheca, forming a larger cell (similar to the parent size). In contrast, the smaller half hypotheca that is later transformed into epitheca is inherited by the epitheca forming a smaller cell (smaller than the parent size). Therefore, the cell that inherits parental hypotheca becomes smaller in size. This asexual reproduction continues until the cells become too small to divide asexually. Hence, they undergo sexual reproduction to regain their normal size.

The cytoplasm of the smaller cell undergoes meiosis to produce haploid cells. Some species may have heterogametes, while others are homogametic. These haploid cells fertilize to produce a diploid cell that expands (from absorbing water) into an auxospore, which becomes the original (first parental) size. Here they develop into new epitheca and hypotheca similar to their original parental size, and the cycle continues. 

Characteristics of Diatoms

Diatoms belong to the domain Eukarya, consisting of unicellular and multicellular microorganisms, although diatoms are unicellular organisms. Some of the significant characteristics of diatoms are:

• They are photosynthetic as they convert carbon dioxide and water into oxygen and energy to form sugar with the help of sunlight. 
• They undergo both asexual and sexual reproduction during a certain point of their life cycle.
• The cell wall comprises specialized silica, and the outer surface includes the frustule.
• They have a two-layers shell, where the outer layer is called thecae, which fits together to form a frustule.
• They are free-floating, while some diatoms adhere to the various surfaces of marine plants and animals. 
• The diatoms are in various shapes and sizes, such as the oval, tubular, circular, and triangular.
• Sometimes these diatoms have different distinctive colors due to chlorophyll that gives them green, yellow, and brown shades.
• The food is reserved for producing energy while storing carbohydrates and lipids. 

Applications 

Various diatom applications are especially in biotechnology and nanotechnology after advancements in genetic engineering and molecular biology. Knowledge of diatoms is utilized for environmental reconstruction as they can remove carbon dioxide from the atmosphere, test water quality, and in forensic investigations. 

During a forensic investigation, in death by drowning cases, the presence of diatoms indicates the victim was still alive before drowning as the water entered the lungs. Therefore the presence of diatoms in the lungs is a piece of supporting evidence indicating death by drowning. Moreover, the diatomic species can be identified that will aid in determining the potential location of where the victim was drowned. 

Diatoms are important in determining water quality. Some species of diatoms can tolerate a certain level of water pollution. Collecting a water sample and choosing the species and population will help estimate the water quality and the pollution level. Furthermore, diatoms help fix carbon dioxide; they are utilized in various environmental studies such as palaeoenvironmental studies. 

When the diatoms die, the silica in their cell walls will fall into the ocean bottom, creating deposits of diatomaceous earth. These deposits are utilized for industrial purposes, such as fossil fuels. Moreover, usage of frustule for nanotechnological applications includes solar technology, bio-sensing, microfluidics, drug delivery, and catalyst production. 

Illustration of different colored diatoms

The diatoms are converted into sediments made up of diatomites (diatom frustules) when they are accumulated together and utilized for commercial purposes. These deposits contribute as an ingredient for manufacturing filters, toothpaste, paints, etc. Therefore, the importance of these microalgae is immense as they play a significant role in our lives. 

Asha Rachel Thomas

09/06/2022