hidden-ecologies-header-107

Seasonal Change in Heron’s Head Salt Marsh Micro-community

Wayne

In the post prior to this one [see below – Heron’s Head Salt Marsh Losing Salt], I observed that both San Francisco Bay and salt marsh ponds on Heron’s Head were becoming much less salty with the winter rains. In this post, I describe how this has impacted the Heron’s Head salt marsh micro-community.

Figure 1 - Heron's Head Salt Marsh Pond Sample Site
Figure 1 – Heron’s Head Salt Marsh Pond Sample Site.

The pond that is our micro-community sample site is shown in Figure 1 [above], as it appeared at the end of the summer and during the fall of last year. Washed regularly by tides, one well-defined micro-community remained intact throughout the summer and fall, and into December.

This community is a thin layer of microorganisms at the bottom of the pond, immediately on top black mud. In Figure 2 [below], you can see how this community presented during sampling in mid-Autumn. The water was clear and the surrounding pickle weed and salt grass were clean. Floating algae was not evident in the sample pond. Measurement with a refractometer showed a salt content of about 3.6-percent.

This pond is about 10-feet from the shoreline at mean tide and about 3-feet above the mean tide level. Very high tides cover the pond, gently washing it. This counters evaporation from the pond, and maintains the salt concentration about the same as India Basin.

Figure 2 - Sampling the Pond in November, 2005
Figure 2 – Sampling the Pond in November, 2005.

This photograph shows the red and blue-green layer is being sampled. With care, this layer can be lifted from the black mud as a mat. It is sufficiently coherent to sometimes clog the narrow plastic tubing used to collect the sample.

The blue-green color is indicative of photosynthetic cyanobacteria. The orange-red color probably results from golden diatoms and sulfur-oxidizing photosynthetic “gamma” bacteria.

Notice the bubbles on the mat. These bubbles smelled of “rotten egg”, and were composed of oxygen containing traces of hydrogen sulfide. The oxygen resulted from cyanobacterial and diatom photosynthesis taking place in the mat. The hydrogen sulfide originated below the mat, in the black mud, where it was produced by sulfur-reducing “delta” bacteria.

Before discussing the cyanobacterial mat community in detail, I want to compare the pond as it appeared in mid-autumn to the pond as it appeared last week in April. Go back and look at figures 1 and 2 [above], then look at Figure 3 [below].

Figure 3 - April Photograph of the Sample Site in the Sample Pond
Figure 3 – April Photograph of the Sample Site in the Sample Pond.

It is striking is how untidy the pond has become. The smooth bottom is almost wooly with creamy yellow growth. The clean stems of pickle week and salt grass now sport wooly coats. The bubbles are absent, and the water does not look as clear. The blue-green patches are missing.

Probing the bottom with the pipette shown revealed that the wooly yellow growth is fluffy, and not organized into a coherent mat. I could gently wipe it away to reveal the black mud underneath. When this “winter mat” was sampled and examined under the field microscope, it presented a very different micro-community from the “summer mat”. To appreciate those differences, I need to show the “summer mat” in the photomicrograph below. Unless otherwise noted, these photomicrographs were taken at 400X magnification and a camera mechanical “zoom” setting of 1.

The summer mat is composed of a dense layer of cyanobacterial filaments. This is shown in the photomicrograph labeled Figure 4.

Figure 4 - The Cyanobacterial Mat as it appears in summer and fall
Figure 4 – The Cyanobacterial Mat as it appears in summer and fall.

I called this photomicrograph, “Diatoms ride the Cyanobacteria Highway”. The long cyanobacterial filaments give the mat sufficient integrity that layers of it stay together when transferred to a slide. I was able to make many similar photomicrographs at different magnification. All show long blue-green cyanobacterial filaments with diatoms and some different filaments of a bacterium species called Beggiatoa mixed in.

The cyanobacteria are probably a species of Oscillatoria, a motile cyanobacterium. Each of the filaments consists of many disk-shaped individual bacteria stacked together. As the bacteria divide, a filament becomes longer and longer. In stable ponds and streams, these filaments can become several feet long.

Watching the mat through the microscope, at 400X magnification, one could see the filaments slowly gliding past one another, with individual “boat shaped”, or pennate diatoms gliding at an even faster speed along the filaments. Presumably, at the site, the cyanobacteria glide up and down in response to light. This keeps the mat only a few filaments thick, almost a membrane of bacterial filaments.

Most of the diatoms were the small pennate golden diatoms and longer golden diatoms with a slight bend at each end.

The filaments of Beggiatoa glide almost as fast as the diatoms. They use sunlight to break down the hydrogen sulfide gas bubbling past them, releasing energy and elemental sulfur. At higher magnifications, Beggiatoa often shows yellow elemental sulfur inclusions.

Figure 5 - Components of the summer Cyanobacterial Mat.
Figure 5 – Components of the summer Cyanobacterial Mat.

In a sample jar, the Beggiatoa rise above the Cyanobacteria in the early morning, creating a white film. They are following the H2S gas that bubbled out of the mud during the night, as well as seeking sunlight. As the hydrogen sulfide is used up, the Beggiatoa sink back down to higher concentrations of the gas, always keeping a a level balancing hydrogen sulfide and sunlight.

Figure 6 - The Dominant Bacterial Members of the summer Mat are cyanobacteria and Beggiatoa.
Figure 6 – The Dominant Bacterial Members of the summer Mat are cyanobacteria and Beggiatoa.

Colors are sometimes deceptive in photomicrographs, but the yellow blobs in the Beggiatoa were visually apparent, and are probably sequestered elemental sulfur. Unlike Oscillatoria, in which the filaments are stacks of individual disk-shaped bacteria, the Beggiatoa filaments are entire organisms.

In both Figure 5 and Figure 6, you may see small oval objects that look somewhat like the Beggiatoa filaments. These are probably Chromatium sp., another species of gamma sulfur-reducing bacterium. In some ponds in San Francisco Bay salt marshes, Beggiatoa is in the minority and Chromatium dominates. Chromatium contains a red pigment, and in large numbers gives the mat a distinctly red color.

Figure 7 - Minority members of the summer Cyanobacterial mat [800X magnification].
Figure 7 – Minority members of the summer Cyanobacterial mat [800X magnification].

Other diatoms were found in the summer mat, but the “typical” pennate Diatom shown here occurred in by far the largest numbers.

I cannot resist showing a photomicrograph of one summer mat component that has fascinated me. In Figure 8, below, appears a “jittery line” next to the cyanobacterial filament. I have found these “jittery critters” in samples from most San Francisco Bay salt marshes. They are few in number, but readily apparent because of their quirky jittery motion. Interestingly, I have also found them in hot spring streams and in Mono Lake. All are probably different species of the bacterial genus Spirochaeta.

Spirochaeta are motile bacteria that have no flagella. They are helical and move by the flexing action of “fibrils”, or axial filaments, entirely within the bacterium. For a bacterium that appears so interesting because of its motion, they are little described in the field literature.

Figure 8 - The

Figure 8 – The “jittery critter”, probably Spirochaeta sp. [Jittery Bug filmclip.]

Photomicrographs of the “winter mat” showed many of the same organisms, but in different frequencies and organized into an entirely different micro-community. In addition, other organisms not seen in the summer mat, predominated in the winter mat.

Whether this difference was the result of the massive change in salinity, or mechanical disruption by storms and heavy tides, or a response to different nutrients washed in from run off is not clear. Perhaps all three environmental changes played a role in reorganizing the micro-community.

Shown below are photomicrographs of the winter mat. In addition, other photomicrographs were presented in the first post on this subject, Heron’s Head Salt Marsh Losing Salt. Compare these photomicrographs with the summer mat photographs [above].
Figure 9 - Cyanobacteria are few, small, and far between in the winter mat


Figure 9 – Cyanobacteria are few, small, and far between in the winter mat.

Cyanobacteria neither define, nor even play much of a role in the winter mat at the Heron’s Head salt marsh sample pond. In fact, if you look at my Heron’s Head post that preceded this one, you can see a photomicrograph of a cyanobacterial filament that is breaking up…!

Diatoms of all kinds dominated the winter mat. The most common, however, was a species of the communal diatom Melosira. Chains, or filaments of Melosira occurred in the winter mat itself and as a coating on pickle weed and salt grass in the continuously submerged parts of the pond. Like most diatoms, Melosira is golden. Its fuzzy growth, however, often gives it a creamy yellow appearance. Looking back at the winter pond photograph, you see this creamy yellow cast everywhere.

Figure 10 - The Communal Diatom Melosira sp.Was Found Throughout the Winter Mat and Winter Pond
Figure 10 – The Communal Diatom Melosira sp.Was Found Throughout the Winter Mat and Winter Pond.

Views of Melosira at higher magnification were shown in the previous Heron’s Head post. I have seen colonies of Melosira so dense on boat docks that it looks like creamy golden “seaweed”. Indeed, since Diatoms are a form of algae, it is a “seaweed” – but not the filamentous algae we normally think of as “sea weed”. Handling a bundle of Melosira is very different from handling “slimy seaweed”. A clump of Melosira feels rough and one can, indeed, imagine that it is made of tiny bits of glass.

With Melosira in the winter mat are numbers of other species of Diatoms. Some of these components of the winter mat are shown in the figures that follow:

Figure 11 - Diatoms of the Winter Mat
Figure 11 – Diatoms of the Winter Mat.

Figure 12 - Diatoms of the Winter Mat
Figure 12 – Diatoms of the Winter Mat.

Figure 13 - Diatoms of the Winter Mat
Figure 13 – Diatoms in the Winter Mat.
Figure 14 - Diatoms of the Winter Mat
Figure 14 – Diatoms of the Winter Mat

Figure 15 - Diatoms of the Winter Mat


Figure 15 – Diatoms of the Winter Mat.

Figure 16 - Diatoms of the Winter Mat

Figure 16 – Diatoms of the Winter Mat.

It will be interesting to follow the development of these pond micro-communities in the Heron’s Head salt marsh over this spring and summer. The snapshots from last fall and this winter do not show transitional forms or intermediate stages that might enable us to infer the environmental events that brought about such startling community transformations.

As some of you may have seen from my web site. I have seen other winter/summer differences in micro-communities. I suspect this is a driving theme in San Francisco Bay salt marsh ecology, an ecology hidden because of its microscopic size.

One Response to “Seasonal Change in Heron’s Head Salt Marsh Micro-community”

  1. Cris Says:

    A most entertaining post Wayne. I also look forward to seeing what happens as the summer arrives. Will you try to capture the (presumed) transition of the microcommunity through sampling at regular intervals?