Diatomaceous Mats


Not all salt marsh pond mats are formed and dominated by Cyanobacteria [see following post]. In some San Francisco Bay salt marsh ponds, the mat on the bottom of the pond is formed by species of the colonial diatom Melosira. The pond site I have designated as New Site 1 [NS-1] has a Melosira mat.


Notice that the color of the mat is dark yellow. Most diatoms are pigmented either yellow or orange. Depending on location, species of Melosira may range from redish-orange to yellow. Clicking on the species name will bring up a Wikipedia link. The text is in German, but I have linked to an English translation by Google. Diatoms and their habits are also discussed in the Hidden Ecologies Diatom page.

The photomicrograph below, taken at 100x magnification, shows the structure of this colonial diatom. The shell, or Frustule, of each individual cell is composed of silicon, so diatoms “live in glass houses”. Each Frustule is composed of two valves make up an individual cell, fitting together like a Petri plate and its cover.


The individual cell structure is more clearly shown in the 1,000x photomicrograph below. The detail and shadowing is the result of Hoffman Modulation Contrast optics, a technique for enhancing resolution.


Differences in color are shown in this photomicrograph. The refraction of light differs in different media, so this is not “true color”, but an artifact of microscopy, as well as pigmentation of the cell.


And the detailed structure of an individual cell is also shown in the photomicrograph here:


Diatoms belong to the Algae, and are plants, capable of photosynthesis.

Diatoms reproduce by vegetative cell division and by sexual union producing an auxospore. An auxospore may also serve as an over-wintering device, or to preserve the diatom when the pond dries up. Vegetative reproduction results in decreasing cell size, limited by the rigid silicon valves.

Most diatoms are free living, but Melosira is a genus of colonial diatoms. Each frustule is attached in linear fashion to the next, forming a long, relatively flexible chain. The frustule at one end of a filament may attach to a structure such as a soil particle, the post of a pier piling, or the cell of a higher plant. Melosira is often found growing as red “seaweed” on the underside of boat docks. Such filaments are rough to the touch, rather than “slimy”. The filaments may be very long, up to 10-centimeters.

When a species of Melosira forms a mat at the bottom of a salt marsh pond, some cells may be attached to plants. Usually this happens when marsh grass grows close to, or even in the pond. Often the Melosira coats the stems of such plants, giving them a yellow fuzzy appearance. Melosira can also coat the muddy bottom of a pond with a thick mat, up to several millimeters.

Unlike the Cyanobacterium Oscillatora, the filaments of Melosira are not motile. Some filaments lie on the bottom horizontally, but most filaments are fixed to soil particles or plant stems at one end, with the body of the Melosira filament free to wave gently in the water currents.

It is not clear to me why pond site NS-1, which is only a few meters away and sometimes connected to pond side OS-1, has evolved a Diatomaceous Mat, while OS-1 has a Cyanobacterial Mat. NS-1 is slightly lower in elevation, is more shallow, and is washed more frequently by tides [see also, in the post linked above, the aerial photograph showing the pond study sites]. The difference in salinity between the two ponds is rarely great. On January 3rd of 2009, when the top pond photograph was taken, NS-1 was very low and its salinity was 40-PPT; while OS-1 was closer to its usual 5-cm depth and its salinity was 39-PPT. The salinity of India Basdin was 37.5-PPT and there have been several recent tides exceeding 2-meters, therefore sufficient to wash both salt marsh ponds.

I have not yet measured the rate of oxygen formation in the Diatomaceous mats, but I suspect it is roughly equivalent to that of the Cyanobacterial mat [see the post immediately previous to this one].

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