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 ##
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.”
#### 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.
## 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!
Very informative, thanks.