Diatoms
Diatoms are microscopic, golden algae that are among the most abundant and diverse marine microorganisms on Earth. Characterized by their rigid, glassy cell walls made of silica, diatoms have a distinct two-part structure that resembles a shoebox fitting into its lid. After they die, these cell walls can accumulate in sediments, contributing to large deposits known as diatomaceous earth, which are commercially valuable for various applications. Diatoms play a crucial role in aquatic ecosystems as primary producers, using photosynthesis to fix carbon dioxide into organic compounds that support the food chain.
These organisms are especially significant in the context of climate change, as they are responsible for an estimated 20 to 25 percent of global carbon fixation. Environmental factors such as light, temperature, and nutrient availability influence their growth, making their response to climate change complex and unpredictable. Changes in diatom populations can dramatically affect carbon cycling, potentially influencing global temperatures. Research, particularly in regions like Antarctica, highlights the interconnection between diatom health, atmospheric conditions, and ecosystem stability, underscoring their importance in understanding and addressing climate change.
Subject Terms
Diatoms
Definition
Diatoms are ubiquitous, microscopic, golden-colored algae. They are classified as the most abundant and diverse marine microorganism. The most distinctive characteristic of a diatom is its rigid cell wall. The cell wall is formed in two halves, with one half fitting inside the other as a shoebox fits its lid. This rigid, glassy wall containing silica is patterned with pores, variable thicknesses, and spine-like extensions projecting from its surface.
Once a diatom dies, its cell walls either dissolve into the water or fall to the bottom of the sea or lake, where they become part of its sediments. When large numbers of diatoms are present in a body of water, their cell walls tend to accumulate in the sediments. Large deposits of diatom cell walls have been found on land in areas that were once covered by seas. This material is mined as diatomaceous earth and is used commercially as a fine abrasive material or filtering agent.
The classification system for diatoms is based on two key features, the pattern of the cell wall and the shape of the cell. Two major groups are often distinguished: pennate diatoms are typically bilaterally symmetrical, while radially symmetrical diatoms are known as centric diatoms.
Although diatoms are found in most environments, these organisms are very important members of marine ecosystems and freshwater ecosystems. As photosynthesizers, these tiny organisms harvest energy from the sun, fixing carbon dioxide (CO2) into organic compounds that are used by the diatom and by organisms that consume the diatom. In this capacity, diatoms are at the first trophic level of the food chain, providing energy and organic compounds for the heterotrophic organisms in that ecosystem. Their ubiquity and longevity lean toward promising solutions which could be exploited in ecology, aquaculture, renewable fuels, value-added foods, feeds, nutraceuticals and pharmaceuticals.
Significance for Climate Change
Researchers have estimated that 20 to 25 percent of all carbon fixed on Earth via photosynthesis is fixed by planktonic marine organisms. In some oceans, diatoms are the most numerous members of the phytoplankton; in other areas, they are significant but not dominant. Through photosynthesis, diatoms also play a significant role in the carbon cycle, removing carbon from the atmosphere. The carbon may be made available to other organisms within the ecosystem, or it could be removed from the carbon cycle for millions of years as the dead diatoms become a part of the sediments.
Numerous environmental factors influence the growth of diatoms, including light, wind, currents, temperature, and available nutrients. The nutrients that most commonly limit growth of diatoms in aquatic environments are nitrogen, phosphorus, silicon, and iron. Since numerous factors affect the metabolism, growth, and reproduction of these organisms, the impact of climate change is likely to be complex and difficult to predict. It is unlikely that all environment factors would work in either a negative or a positive manner, so the end result is likely to be cumulative. Given the vital importance of diatoms to the local ecosystem and to the carbon cycle, it will be critical to monitor these organisms as changes occur.
Climate changes affecting diatoms could through them have a profound effect on the carbon cycle. Any factor that affects the rate of photosynthesis, the health, or the reproduction of these organisms would affect the role they play in the cycle. If there were a drastic decrease in the number of diatoms or a drop in their photosynthetic rates, less CO2 would be removed from the air, ultimately resulting in warmer atmospheric conditions and contributing to global warming. On the other hand, massive blooms of these organisms could lead to more CO2 being removed via both photosynthesis and greater sedimentation rates, resulting eventually in a cooler atmosphere. Using historical evidence, researchers have suggested that such changes in atmospheric CO2 levels during the glacial periods were correlated with changes in diatom abundance and carbon fixation.
Some of the best evidence linking diatoms and climate change has come from research in Antarctica. Diatoms are the dominant photosynthetic organisms in the cool southern ocean and on the ice shelves at the edges of the continent. Researchers have found that these organisms produce a compound known as dimethyl sulfide (DMS). Once airborne, DMS can serve as a nucleus for water condensation to form clouds or be converted to sulfuric acid and return to the ground as acid rain. Changes in numbers of diatoms in these areas could result in changes in cloud and moisture patterns in the Antarctic.
At least one aspect of climate change has generated concern regarding diatoms in Antarctica. These organisms are being exposed to increasing amounts of ultraviolet (UV) radiation, as the ozone layer in this part of the world continues to thin. Laboratory studies have shown that diatoms suffer damage to their photosynthetic pigments and deoxyribonucleic acid (DNA) when exposed to UV radiation.
Changes in the population or photosynthetic activity of diatoms due to climate change could have significant effects on ecosystems where diatoms contribute significantly to the first trophic level of the food chain. Areas most likely to be affected are those where diatoms tend to dominate, such as in the open ocean. The impact of climate change to shoreline marine ecosystems or to freshwater ecosystems could be less significant, as these ecosystems tend to contain a greater diversity of photosynthetic organisms that contribute to the primary productivity of the system.
Bibliography
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Hassan, Gabriela S., et al. “Differential Preservation of Freshwater Diatoms and Mollusks in Late Holocene Sediments; Paleoenvironmental Implications.” Palaios 29.12 (2014): 612–623. GeoRef. Web. 19 Mar. 2015.
Miller, G. Tyler, Jr., and Scott Spoolman. Environmental Science: Problems, Concepts, and Solutions. 12th ed. Belmont: Brooks, 2008.
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