The Impressive Power of Periphyton

It is usually green. It is almost always slimy. It is
seldom attractive. It is universally cursed and derided
by many pond keepers. It is, however, the most
important grouping of organisms in any aquatic ecosystem.
It is generally called periphyton.

Although the dictionary defines periphyton as “aquatic
organisms, such as certain algae, that live attached to rocks
or other surfaces,” there are a bevy of terms that refer to
the particulate organic matter (POM) attached to rocks
and other submerged surfaces:

  • Aufwuchs
  • Biofilm
  • Benthic algae
  • The “epis”:

  • Epilithon (rock)
  • Epipelon (mud)
  • Epissamon (sand)
  • Epixylon and epidendric (wood)
  • Epiphyton (plants)
  • Epizoic (animals, such as snails and Caddis fly larvae)
  • And, of course, periphyton!
  • The use of the term periphyton by the scientific
    community usually encompasses two communities of
    microorganisms:

    Biofilm: Microbial communities, predominantly
    bacteria, encased in a layer of extracellular polymeric
    substances (EPS).

    Aufwuchs: Pronounced OWF-vooks, this word is
    German for “growth upon.” Aufwuchs is the fuzzy,
    sort of furry-looking, slimy green coating that attaches
    or clings to stems and leaves of rooted plants or other
    objects projecting above the bottom without penetrating
    the surface. Unlike periphyton, it includes not only algae
    like Chlorophyta, but also diatoms, nematodes, protozoans,
    bacteria, fungi and myriad other tiny creatures such
    as tardigrades.

    It is only through the examination of these two groups
    of organisms, both in internal structure and function and
    the interrelations within and among the two groups, that
    we can truly understand the importance of these groups
    to overall water quality. In this article, we will dive (metaphorically
    speaking) into the first: biofilm.

    Part 1: Biofilm

    Biofilm is the foundational structure of these combined
    communities and may vary in thickness from only a few
    micrometers to several hundred micrometers — from the
    thickness of a single cell to multiple layers and community
    groupings.

    Perhaps the best definition of biofilm can be found in
    The American Heritage Science Dictionary:

    Biofilm: A complex structure adhering to surfaces that
    are regularly in contact with water, consisting of colonies of
    bacteria and usually other microorganisms such as yeasts,
    fungi, and protozoa that secrete a mucilaginous protective
    coating in which they are encased. Biofilms can form on solid
    or liquid surfaces as well as on soft tissue in living organisms,
    and are typically resistant to conventional methods of
    disinfection. Dental plaque, the slimy coating that fouls pipes
    and tanks, and algal mats on bodies of water are examples of
    biofilms. While biofilms are generally pathogenic in the body,
    causing such diseases as cystic fibrosis and otitis media, they
    can be used beneficially in treating sewage, industrial waste,
    and contaminated soil.

    Biofilms are a crucial part of an aquatic ecosystem. The microorganisms that make up biofilms form
    the basis for food webs that nourish larger
    organisms such as insect larvae, which are
    consumed by fish. Even plants benefit from
    naturally occurring biofilms.

    The instant that the first water contacts
    any surface of your pond — whether it be
    liner, rock, filter media, plants, et cetera —
    biofilm begins to form. Initially, the first
    surface deposits are transparent exopolymer
    particles, or TEPs: planktonic organic
    microgels that are ubiquitous in aqueous
    environments, which neutralize the electrical
    charge of the surface that would otherwise
    repel bacteria and other microorganisms.

    This initial layer of organics also serves
    as a nutrient source. Bacteria then begin
    to colonize the surface by secreting strands
    of sticky polymers (extracellular polymeric
    substances, or EPS), which holds the biofilm
    together in a structural matrix and secures
    it to the surface. These polymers also serve
    to trap nutrients and act as a very strong
    protective barrier against toxins.

    As nutrients accumulate, the original
    bacteria multiply. These offspring bacteria
    produce their own sticky polymer. Soon a
    colony of bacteria is established.

    According to Susan Borenstein in her
    1994 book, “Microbiologically Influenced
    Corrosion Handbook,” these “other bacteria
    and fungi become associated with
    the surface following colonization by the
    pioneering species over a matter of days.”

    The diagram above is based on Steinmand (1992, from Oecologia vol. 91) after Gregory (1980, PhD Oregon State University) and is a good summary of the various growth forms on stones.
    The diagram above is based on Steinmand (1992, from Oecologia vol. 91) after Gregory (1980, PhD Oregon State University) and is a good summary of the various growth forms on stones.

    Martin Wahl discussed the settling pattern of biofilm in four phases:

    1. Surface conditioning or adsorption of
    dissolved organic compounds where macromolecules
    attach to submerged surfaces
    following a spontaneous physical-chemical
    process;

    2. Primary colonization or bacterial settling
    following surface conditioning, and, after their
    colonization, bacteria start to produce EPS;

    3. Secondary colonization to bacterial
    layer and EPS pool by eukaryotic unicellular
    microorganisms — mainly protozoan,
    microalgae and cyanobacteria;

    4. Settling of eukaryotic multicellular
    organisms as a function of nutrient sharing,
    grazing and predation.

    According to Robert G. Wetzel, associated
    organization from secondary colonization
    onwards can be designated as
    “periphyton.” In that way, it could be defined
    as an advanced successional stage of biofilm.
    However, there could be a fifth phase:

    5. The tertiary colonization, where bacterioplankton
    colonized on the surfaces of
    unicellular and filamentous secondary colonizers
    (e.g. diatom, Oedogonium, et cetera). Once a certain bacterial population level
    is reached, a process called “quorum sensing”
    occurs. Quorum sensing is a cell-to-cell
    communication through the use of chemical
    autoinducers that allows populations of
    bacteria to simultaneously
    regulate gene expression in
    response to changes in cell
    density.

    Biofilm is made up of
    microorganisms and a polymeric
    web. Interestingly,
    in a well-established
    biofilm, most of the volume
    (between 75 and 95 percent)
    is the sticky polymer matrix.
    This matrix holds quite a
    bit of water and makes the
    biofilm-covered surface
    slippery. This is why, especially in bare liner
    ponds, it is difficult to maintain traction
    while you are wading in your pond.

    A fully developed biofilm is a complex,
    mutually beneficial community of various
    microorganisms living in a customized
    micro-niche. According to Andy Coghlan,
    author of “Slime City”:

    Different species live cheek-by-jowl in slime
    cities, helping each other to exploit food supplies
    and to resist antibiotics through neighborly
    interactions. Toxic waste produced by one species
    might be hungrily devoured by its neighbor. And
    by pooling their biochemical resources to build
    a communal slime city, several species of bacteria,
    each armed with different enzymes, can
    break down food supplies that no single species
    could digest alone. The biofilms are permeated
    at all levels by a network of channels through
    which water, bacterial garbage, nutrients,
    enzymes, metabolites and oxygen travel to and
    fro. Gradients of chemicals and ions between
    microzones provide the power
    to shunt the substances around
    the biofilm.”

    A mature biofilm may
    take anywhere from several
    hours to several weeks to
    develop. A fully developed
    biofilm is able to move water
    through the entire matrix,
    supplying nutrients and
    transporting wastes. Biofilms
    may be very thin to several
    inches thick. The biofilms
    that are usually encountered
    in an aquatic ecosystem are measured in
    microinches. A microinch is equal to one
    millionth of an inch. The congregation of
    multiple species into biofilm microcosms
    increases the range of organic and inorganic
    substances that can be biodegraded.

    In aquatic systems, the biofilm bacterial
    count per square centimeter of surface has
    been estimated to be approximately 1,000-
    fold higher than the corresponding planktonic
    count per cubic centimeter.

    meyer_drawing

    It’s Everywhere!

    Biofilm covers every submerged and
    constantly wet surface associated with a pond.
    It is on the rock, liner, plants, skimmer, biofilter
    and media — it’s even inside of the pump
    and related piping. The biofilm in one location
    will be different in makeup than that in
    another location. Factors such as light, water
    movement, temperature and availability of
    nutrients will determine the member microorganisms
    of each community. The very same
    parameters that we test for to ensure healthy
    fish also influence the membership of the
    biofilm community.

    It is within this biofilm that nitrification
    and denitrification take place along with other
    chemical and organic conversion processes.
    Biofilm is the primary source of production
    in an aquatic system. It is what sustains all
    higher levels of aquatic life.

    This is part one of a two-part article.
    Part two will be available on our website, in the coming months!

    One Response to The Impressive Power of Periphyton

    1. Steve Daniels January 31, 2017 at 1:00 AM #

      Very informative, thanks.

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