The word diatom is derived from a Greek word ‘diatomos’ meaning ‘cut in to two’. Diatoms are unicellular algae belonging to a phytoplankton group. It has its cell encapsulated in siliceous box known as frustules. Diatom frustule is composed of two intricate valves namely epivalve and hypovalve. It belongs to class Bacillariophyceae. Based on the shape of the frustules two orders are defined: Centrales (round diatoms) and Pennales (elongated diatoms). Order Centrales is divided into three sub-orders. The order Pennales is divided into two sub-order raphid (having raphe/slit on the valve) and araphid (without raphe). The frustules of diatoms are decorated with processes (labiates and strutted), areolaes, costae, spines and other distinguishing features. It is the siliceous composition of the diatom frustules which aids to its preservation. Thus diatom records preserved in the sediment is indicative of surface water condition. Hence, due to the specificity of diatom morphology and ecology it can be used in various ways to decipher the past environmental and oceanographic conditions including methods based on micropaleontology (diatom taxonomic identification), geochemistry (biogenic content, organic biomarker) and isotope geochemistry (of diatom-intrinsic organic matter or siliceous matrix). It is therefore essential to better understand the ecology, taxonomic position and morphology of diatom species to study the paleoceanograhic and paleo-environmental conditions.
Diatoms are non-motile; though motility is usually limited to a gliding motion. Reproduction among these organisms is primarily asexual by binary fission, with each daughter cell receiving one of the parent cell's two frustules. This is used by each daughter cell as the larger frustule (or epitheca) into which a second, small frustule (or hypotheca) is constructed.
This form of division results in a size reduction of the offspring and therefore the average cell size of a diatom population decreases, until the cells are about one-third their maximum size. Sexual reproduction and auxospore formation must occur. Vegetative cells of diatoms are diploid (2N) and so meiosis can take place, producing male and female gametes which then fuse to form the zygote. The zygote sheds its silica theca and grows into a large sphere covered by an organic membrane, the exospores. A new diatom cell of maximum size, the initial cell, forms within the exospores thus beginning a new generation.
Diagram taken from Grethe et al 1996
Diatoms live in a variety of environments, from salt to fresh water (they are even found in moist soil and mosses), and a wide range of pH levels, temperatures and organic pollution. This variety of living conditions can help to tell pollution or other ecological levels of the water. They also vary in their lifestyle, living singly or in a colony. Diatom does not always float freely in the water; they will attach themselves to a rock or another animal in the water. Diatoms live throughout the lighted zone of every ocean, as well as in freshwater streams and lakes. Some species live everywhere, but most have temperature, salinity, or other environmental preferences that restrict their ranges. Planktonic diatom shows boom and bust condition in both fresh and marine water throughout its life.
First recorded occurrences of diatoms are from the Jurassic, however, these are uncertain and the earliest recorded well preserved diatoms are centric forms from the Aptian-Albian stages of the Cretaceous. The earliest araphid (lacking a raphe) pennate diatoms appear in the Late Cretaceous, and raphid pennates in the Middle Eocene. The earliest freshwater diatoms appear in the Palaeocene in Russia and the Late Eocene in North America. In a similar manner to Radiolaria, it has been noticed that there has been a gradual progression towards less heavily silicified frustules, probably as a result of increasing competition for a limited resource (silica).
The evolutionary history of diatoms has been punctuated by several floristic turn overs, these have been utilized to allow basin wide biostratigraphic correlations. Diatoms are also used extensively in palaeo-environmental studies particularly in palaeoceanography. Dissolution of diatom frustules during descent through the water column, on the sediment surface and during diagenesis may seriously alter the preserved assemblage by preferentialy dissolving more lightly silicified forms. High alkalinity of pore waters and burial temperatures in excess of 50 degrees centigrade are also known to increase dissolution of silica. Incorporation into faecal pellets or muciligenous aggregations, rapid burial and the formation of heavily silicified resting spores tend to counteract these problems, however, in marine samples it is thought that only 1% to 5% of the living assemblage in surface plankton is represented in the death assemblage found on the sediment surface. Despite these problems diatoms are still a useful and to a certain extent under-utilized group in terms of biostratigraphy.
Living diatoms often have specific salinity, temperature and other environmental tolerences, this together with the fact that a high proportion of fossil genera and species are still extant, makes it possible to use transfer functions to produce accurate palaeonvironmental reconstructions.
Pennate Diatoms (A: Fragilariopsis kerguelensis, B: Fragilariopsis rhombic)
Centric Diatoms (C: Eucampia antarctica, D: Trigonium arcticum)