Salt Marsh Life at the Bottom of the Food Chain

SPRING 2010 Volume 31, Number 1 Salt Marsh Life at the Bottom of the Food Chain By Wayne Lanier life that you can easily see: the birds; the Look in any shallow salt marsh pond animals; and, the marsh plants. At a stretch, on a sunny day and you will see the founda- this might number 100 species. The number tion of life at the bottom of the food chain. of species of microbial life in the microbial A blue-green mat may cover the mud of the mat of a salt marsh pond, as measured in pond; or perhaps a yellow fuzzy mat may DNA samples, exceeds 5,000 species. coat the stems of pickleweed growing in the Microbial mats are productive because water; or a thick green, yellow, red, or even they produce enormous quantities of oxygen multicolored mat may float on the surface and take up enormous quantities of carbon of the pond. dioxide. Almost all the green and red bacteria are photosynthetic as are the diatoms and the algae. This means they use the energy from the sun to fix carbon dioxide into the chemicals of Courtesy of Pelican Media life, while releasing oxygen. the salt marsh by 1,000-times. This makes If the microbial mat is sense, for salt marsh microbes are at the submerged, it is likely to be bottom of the food chain and must support covered with very many tiny all the other life. bubbles. These bubbles are Typically, the first step up the food almost pure oxygen, released chain is composed of the protozoa that during photosynthesis. live in and graze the microbial mat. Above Cyanobacterial Mat Surface tension keeps them attached to the them are the zooplankton, mostly the larvae Photo by Wayne Lanier mat until their size becomes great enough of insects and marine organisms, such as that their buoyancy detaches them and mollusks. These colorful mats support a multi- floats them to the surface. Counting the When you see an Avocet sweeping the tude of different microorganisms, collec- bubbles in a measured area and calculating bottom of the pond or mud flat with its tively called “plankton.” Each microbial mat their volume provides a minimum estimate curved beak, it is sweeping up microorgan- is characterized by one or more dominant of the amount of oxygen produced in isms from the mat, zooplankton, and the species of bacteria or diatoms. Such mats a day. A 10-ft x 10-ft area of salt marsh mollusks and worms that live in the mud. are often mistakenly called “algae,” although microbial mat minimally produces per day When you see a Clapper Rail pecking into algae is rarely the dominant species in a salt as much oxygen as a large hardwood tree marsh mat. with a trunk more than one foot in diameter the mud flat, it is feeding on mollusks that Microbial mats constitute one of the with a typical canopy more than 30 feet in filter-feed on plankton that depend on the most diverse and productive ecologies on diameter. microbial mat. All of these steps in the food earth. In addition to diversity and productiv- chain are vital for a healthy salt marsh, and Microbial mats are diverse because the ity, the biomass bound up in the microbial it all begins with the microbial life at the number of species in a typical mat is enor- mat and in the microbes in the salt marsh mous. Consider all the species of salt marsh mud exceeds that of all the other life in continued next page may be swimming around salt crystals from page 1 as the pond rapidly dries out. bottom of the food chain. As the ordinary salt marsh Not only does the microbial mat sup- pond varies in salinity, each species port the food chain, it also purifies the salt in the microbial mat gets a time at marsh water by sequestering and removing the salinity of its optimum growth. the pollutants so that the other dwellers of These microbes have dormant states the salt marsh can survive. The first stage in in which they can survive extremely recovery of a damaged salt marsh is recovery unfavorable conditions. A salt marsh of the microbial mat. pond that varies widely in salinity Microbial mats form in all salt marsh will continue to support communi- waters, from tidally-washed mud flats to ties of very many species of microbe, deep salt ponds. The richest microbial mat each getting a brief time at optimum communities form, however, in shallow growth. This is why tidally-washed ponds Colonial Diatom ponds with tidal inlets above the mean are so rich in numbers of species. It is also Photomicrograph by Wayne Lanier high tide. Such shallow ponds are the one of the reasons why they are so productive in producing oxygen and taking up Many members of a mat community most productive because the mat-forming appear to engage in mutually beneficial microbial community is photosynthetic and carbon dioxide. The dynamics of salt marsh pond relationships: One, perhaps, producing a less sunlight reaches the bottom of deep growth pheromone required, while the other microbial communities are very complex ponds. Ponds with tidal inlets at and above member provides shelter. These associations and poorly understood. Not all microbes the mean high tide are most rich in species are very important in the microbial commu- can form microbial mats. Among other because such ponds are not washed by every nity, but are also poorly understood. requirements for mat formation is ability tide, as is a mud flat, but are only tidally Although salt marsh microbial com- to either stick to a surface, or stick together washed at irregular intervals of days or even munities form the most important land while floating. Often microbial mat formers weeks. ecosystems for producing oxygen and are filamentous. Every one of the many species in the removing carbon dioxide from the atmo- Examples of filamentous mat form- sphere, they have been little studied until microbial mat has a salinity at which it ers are species of the Cyanobacterium recently. In part this is because, traditionally, grows best. Below and above that salinity, Oscillatoria, which form dark green mats. microbiologists have spent most of their growth diminishes, so the growth efforts studying the 1-in-a-million microbial curve is shaped like a hill. Most species that cause disease in humans. Only algae grow fast in fresh or brackish recently have microbiologists increasingly water; most large bacteria and dia- turned toward field microbiology. toms grow fast in water at or above Another reason for our limited knowl- the Bay salinity. Dinoflagellates edge is that microbes are much easier to grow at even higher salinities and study in the laboratory, but most ecologically Halobacteria only reach maximum important species do not grow well in the growth at about 5-times the salinity laboratory. Microorganisms are difficult to of seawater. identify in field observations, requiring either Salinity is measured by the culture or expensive DNA identification. Finally, without the use of field mi- number of grams of salt in each croscopes, the novelty and beauty of these liter of water. Since a liter of water organisms are not easily recognized, and they weighs a thousand grams, this Filament of Cyanobacteria are dismissed as “slime” or “smelly algae”. expressed as parts of salt per thousand water Photomicrograph by Wayne Lanier All of life is beautiful and interest- [PPT]. San Francisco Bay water averages a Another mat-former is the colonial ing, for it is part of the great web that has little below 35 PPT salinity. diatom Melosira, which forms yellow or uniquely shaped this planet for more than Every time a salt marsh pond is washed yellow-orange mats: three billion years. That is especially true in by the tide, its salinity is reset to the Bay the salt marsh, for many species in the mat salinity. If it is not washed again for several and almost all species down in the mud were days, evaporation causes the salinity in the not only around 3 billion years ago, but remaining water to rise. Depending upon To see these microbial they built our present oxygen atmosphere. its elevation above the mean tide, a salt communities up close, go We cannot conserve the salt marsh and we marsh pond is tidally washed from once cannot restore the salt marsh without its every day or so, to once or twice per month. on a walk with Dr. Lanier essential microbial communities. This causes the salinity to vary between 30 on April 10 or April 11. Wayne Lanier, PhD in microbial genetics, spends his to 35 PPT after washing, to as much as 50 retirement studying microbial ecology in the San Francisco PPT after weeks of evaporation. Salinity Bay salt marsh and in desert salt and alkaline lakes. He See the activity section has been professor in university and medical school. He in evaporative salt ponds exceeds 200 PPT, has also been Director of Research in several biotechnology where any Halobacteria or Dinoflagellates for details. companies; and a clinical studies consultant. Page 2 ItAs is we develop not plans for wetlandall resto about- not leave enough the of the open waterbirds habitats to modify our management procedures if ration around the bay, we are occasionally they prefer. impacts are found. We also monitor many reminded that a healthy San Francisco Bay We also recognize that some methods water quality parameters such as dissolved ecosystem is much more than healthy bird of restoration can have negative impacts on oxygen, salinity, temperature and biological populations.

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