CHAPTER SIX

ECOSYSTEM AND DISTRIBUTION

6.1Ecology and Distribution of diatoms

Diatoms (Bacillariophyta) constitute one of the most diverse and ecologically important groups of phytoplankton algae. Diatoms are one of the most prolific phototrophic organisms on the planet. One of the primary reasons for this is the fact that they can live virtually everywhere, occurring in substantial numbers.

Many diatom species are cosmopolitan (found everywhere), though many species, genera and groups occur in certain climates and regions for both marine and freshwater forms. Diatoms live any and everywhere. They are found in rivers, oceans, lakes, terrestrial, soil, bogs, damp rock surfaces as epiphytes along with algae, on the leaf of forest trees, mostly in tropical rain forests, even on the surface of a whale’s skin - in short anywhere there is water. Diatoms are found in all possible habitats. It is instructive to know that they are one of the few organisms that can live in the open ocean, polar waters, tropical waters, all freshwater areas, soil, snow and even glacial ice. The different species of diatoms develop different adaptations to survive within each environment.

6.2Examples of Diatoms found in Different Habitats

Fresh Water (Synedra acus, Diploneis smithii, Nitzschia vermicularis, Meridion circulare, Cymbella ventricosa, Melosira variens, Amphora ovalis, etc.).

Sea Water (Chaetoceros, Ceratium, Odontella, Skeletonema, Fragilaria, Pleurosigma, etc.).

Soil (Pinnularia microstauron, Navicula muralis, Cyclotella kutzingiana, Surirella ovata, etc.).

Terrestrial Species (Frustulia vulgaris, Navicula frugalis, Pinnularia borealis, Hantzschia abundans, etc.) can withstand desiccation for a long period.

Epiphytic Diatoms (on other algae, for example, Gomphonima, Cymbella, etc. live on higher plants such as Enteromorpha, Cladophora, etc.)

Endozoic diatoms (growing in animals) Licmophora abbreviata.

6.3Diatoms found in Open Ocean

The species that live in the open ocean have lesser pressure from predation than those in other environments. However, they have a greater pressure from lack of some major elements needed for growth. The largest is often iron and diatoms develop special mechanism to obtain and retain iron. But it still causes a large limitation to their growth. Another key element that diatoms need is nitrogen in a usable form. This may not be a serious problem especially in areas of the open ocean where populations of Cyanobacteria thrive, since the later will produce nitrate that the diatoms will assimilate. The last major limiting element that diatoms need, and which may be a limiting agent in growth is silicon concentration in the water. If silicon concentration is in short supply, then diatoms are unable to reproduce at a satisfactory rate as this will affect frustule formation. Dead or discarded diatom cell walls undergo little or no dissociation and hence do not contribute to the concentration of silicon in the environment, although they are the most dominate primary producers of these ecosystems.

Diatoms in Polar Waters

The species that live in polar waters often have ample nutrients due to ocean currents and the abundance of organisms adding iron and nitrogen to the water through various means. However, they must adapt to the fact that a good portion of the year is enveloped in darkness when no photosynthesis can take place. They either develop into vegetative cells, or they resort to heterotrophic methods. This allows them to be the primary producers in polar ecosystems.

6.5Diatoms found in Tropical Waters

The species that live in tropical waters have a large amount of competition and probably suffer the greatest pressure from predation than any other habitat. They typically have slightly thick frustules and have protecting adaptations within their frustules to prevent predation. They don’t have to worry about nutrients shortage since tropical waters have typically high concentrations of just about all the elements that the organisms need to live. The dominance of diatoms will vary in tropical waters. The dominant primary producers in these waters will often be some other species of algae, or if it’s a reef system it will often be the dinoflagellates that are in association with the coral. Even so diatoms typically have a strong foot hold in these waters and can be very prevalent and end up still producing a large degree of the net primary production.

6.6Diatoms found in Fresh Waters

The freshwater habitats of diatoms include lakes, rivers, streams, marshes, swamps, etc. where they grow in abundance. There are almost no special adaptations for obtaining the scarce elements that they need for growth. Although, if they are in rivers or streams, they often have to develop mechanisms to allow them to stay relatively attached and prevent them from getting washed down the river. These often come in forms of the diatoms attaching themselves to various surfaces that will allow them to be steady or they will grow in the upper part of the sediment which allows light to still permeate down to them but prevents them from being washing away.

In lakes, marshes and swamps they don’t typically have any issues with growth because there is relatively little flow to the water, so their basic buoyancy is enough to allow them stay in the areas with relatively no problem. In these habitats, however, they do have a fair competition, which comes from various macrophytes, such as water lilies, cat tails, duck weeds and others. The competition isn’t extremely intense because light is still available to them as diatoms are able to use some wave lengths of light that these other plants are unable to use. Also, diatoms can photosynthesis all the year around if the light is available thus making them very efficient in these habitats.

6.7Diatoms found in the Soil

Some species of diatoms can grow in moist soils. They must develop new mechanisms that allow them to withstand a more dynamic temperature range as well as mechanisms that allow them to maintain water more effectively. They often grow in field soils rather than wooded soils because of light availability. They are usually pennate species that can move, usually by a slimy substance that they excrete. Even though there are very few of these species, it is impressive that they can establish themselves in large numbers in such soils.

6.8Glacial Ice Diatoms

There have been diatoms found in glacial ice. This is impressive. They must live in extremely limited environment. Having little silica, iron, nitrogen and sometimes even gasses they are inherently smaller and extremely slow growing. They can form vegetative cysts whenever there isn’t enough light present which would allow them to conserve energy and all their organic molecules.

6.9Diatoms Commonly found in different Microhabitats

These include:

Episammon - on and between sand particles. Epilithon - occurs on gravels, stones and bedrocks. Epipelon - inhabiting surface of sediments.

Epiphyton - attached to other algae, macrophytic, or higher plants plants.

Diatom occurrence is governed largely by the substratum stated above, though their composition can also be influenced by other factors such as chemical constituents in the water, water turbulence and disturbance, resource supply (mainly from inorganic nutrients), grazing by micro-organisms and microhabitat light regime (Taylor et al., 2007).

They can generally be found on surfaces of man-made objects like bricks, concrete, pillars etc,

6.10Adaptations of Diatoms

Diatoms are extremely unique organisms. They are extremely abundant photosynthetic algae, with a few exceptions which are heterotrophic or mixotrophic. They store energy in the form of oils, and they usually produce extra cellular polysaccharides. But above all, they have cell walls that are not made of cellulose or peptidoglycan or any other complex organic molecule. Instead, they are surrounded in a silica (glass) cell wall which is often called frustules.

Diatoms have a highly evolved system of photosynthesis which makes them one of the most productive photosynthetic groups of organisms on earth. Twenty (20) to twenty-five (25) percent of the total global net primary production is carried out by these organisms. Several of their adaptations allow them to do this. Three of these are:

They can store energy as oils which also help them with bouyancy in the water column.

They have the strong silica frustules that protect them from a large amount of predation.

They can grow virtually everywhere on the planet. There are species of diatoms that live in the soil, the snow, ice, arctic waters, Antarctic waters, tropical waters, rivers, etc. This allows them to be extremely prolific primary produces and a large portion of the base in many ecosystems.

Other specific adaptations will vary greatly among the over 200,000 estimated species. It is noteworthy, that very few species have developed adaptations that cause them to consistently form or need to participate in a symbiotic relationship. Different diatom species have specific adaptations to their specific habitats that will be covered under habitats rather than here.