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What Eats Diatoms

February 4, 2012 by staff 

What Eats Diatoms, What do they look like?
There are two major types of diatoms: centric and pennate. The centrics are basically cylindrical, but many species have very long radiating spines. Some centrics are polygonal or elliptical when viewed from above. Some species form chains, with each cell connected at one end to the next cell, sometimes only touching with their spines. They are generally freely floating. Pennate diatoms are elongate and usually have bilateral symmetry. Most pennates glide smoothly along surfaces. There are a few diatoms that live in branching or tube colonies. Diatoms are almost always a rich golden-brown colour and have a rigid shape, due to the solid casing (called a frustule) that completely encloses them. The frustule is constructed like a box with matching overlapping lid. The frustule is often finely sculpted and can look very beautiful with the right type of microscope. Although they are almost all too small to see with the nak*d eye as individuals, dense aggregation of diatoms may form golden-brown patches on surfaces such as mudflats.

Where do they live?
Diatoms live almost everywhere on earth that is even slightly wet. In the ocean, centric diatoms tend to be planktonic, while pennates are more often associated with surfaces. Planktonic diatoms live in the upper reaches of the ocean when nutrients are abundant however many sink again when nutrients are scarce. It is thought that they lie dormant at depth with some surviving to migrate to sunlit regions the next time nutrients become available. On reefs, pennate diatoms can dominate the communities of protists on surfaces such as seagrass leaves and sandflats.

How do they get their energy?
Diatoms are almost all photosynthetic. Most of the biological energy production (and oxygen production!) on earth is due to photosynthesis, and a little under half of it occurs in the ocean. Diatoms are responsible for a large proportion of the total energy production of the oceans, possibly as much as a half. A smaller figure than this is more likely as the role of even smaller photosynthetic protists and bacteria has probably been underestimated in the past. However, it seems certain that diatoms are one of the worlds most important groups of producers. The few diatoms that don’t photosynthesise live instead on dissolved nutrients from rich organic matter, and it is thought that photosynthetic species might also be capable of this whilst in the dark.

What eats them?
Diatoms are important sources of food for the smallest planktonic animals, especially when concentrations of nutrients are fairly high. Many protists, such as ciliates and dinoflagellates, consume smaller diatoms whole. There are a number of small protists that can invade large diatoms and eat them out of their frustule. At certain times (for example, the well known ‘spring bloom’) they are probably the major source of new energy into the ocean ecosystem. It is possible that sinking diatoms are an important source of energy for deep ocean ecosystems. Attached diatoms are also eaten by animals that graze on the wide variety of surfaces on which diatoms grow, including the blades of seagrass and kelp, mudflats, sandflats, “bare” rock and on the shells of other animals such as molluscs.

How do they grow and reproduce?
Under the right conditions, diatoms can reproduce very rapidly and a population of a small species can double each day. Diatoms normally reproduce by binary fission, where one ‘mother cell’ splits into two daughter cells. The two halves of the mother frustule become the ‘lids’ for the daughter frustules- the ‘box’ parts are made inside, as the daughters separate. One daughter cell will therefore be the same size as the parent, and the other will be slightly smaller. Over time, a lineage of diatoms will get smaller and smaller, although a few species avoid this problem by having stretchable frustules. When cells get too small, they discard their old frustules and build new, much larger ones. This process can be connected to sex. In many centrics, some individuals will break up into many small sperm-like cells. These ‘fertilise’ other diatoms, which then produce new, large frustules. In pennates, ‘sex’ involves two adult cells lining up alongside each other, dividing and then swapping one daughter cell each. The new pairs of daughter cells fuse, giving two cells that are each half of each ‘parent’. They then produces new, large frustules and glide away.

Who do they live with?
The majority of diatoms are free-living and solitary, although millions of cells may be found in a small area if conditions are favourable. The ability to form colonies has evolved several times in diatoms with various species forming chains of individual cells, often held together by their spines. Diatoms can form symbiotic associations with some other organisms, most notably, with some foraminifera. They provide the foraminifera with energy from their photosynthesis.

Their connection with people.
Like many micro-organisms, diatoms can bloom when conditions of nutrients, light and temperature are ideal. While diatom blooms are a major source of energy for the ocean, which ultimately supports the large predators such as fish and marine mammals and birds, severe blooms can be harmful. One diatom can kill farmed fish because its barbed spines can wound gills. Chemicals present in diatoms can build up in the bodies of animals that eat them and if concentrated enough, can affect people that eat the animals. In Australia, diatom blooms have been responsible for making shellfish unpleasant to taste and unsaleable for long periods of time. One diatom is responsible for Amnesic Shellfish Poisoning, a potentially fatal illness whose symptoms include memory loss. Worldwide outbreaks to date have been very rare, however there is concern that human activities are increasing the frequency of many sorts of micro-organism blooms

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