Periphyton
Introduction
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 eco-system. 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 epi-s: (epilithon [rock], epipelon [mud], epissamon [sand],
epixylon and epidendric [wood],
epiphyton [plants] and 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 (pronounce: OWF-vooks,
German, "growth upon") 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 (2) groups
of organisms both in internal structure and function and the interrelations
within and among these two (2) groups can we truly understand the importance of
these groups to overall water quality.
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.
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.” (The
American Heritage® Science Dictionary)
Biofilms are a crucial part of an aquatic eco-system. 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 etc., biofilm begins
to form. Initially the first surface deposits are TEPs (transparent exopolymer
particles) planktonic organic microgels that are ubiquitous in aqueous
environments, which neutralize the electrical charge of the surface which 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 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.
These “other bacteria and fungi become
associated with the surface following colonization by the
pioneering species over a matter of days.” Borenstein
(1994),
(Whal, 1989) discussed the settling pattern of biofilm in
four phases: (i) surface conditioning or adsorption of dissolved organic
compounds where macromolecules attach to submerged surfaces following a
spontaneous physical-chemical process; (ii) primary colonization or bacterial
settling following surface conditioning and after their colonization, bacteria
start to produce EPS, (iii) secondary colonization to bacterial layer and EPS
pool by eukaryotic unicellular microorganisms, mainly protozoan, microalgae and
cyanobacteria and (iv) settling of eukaryotic multicellular organisms as a
function of nutrient sharing, grazing and predation. According to (Wetzel,
1983), 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 (v) phase; the
tertiary colonization where bacterioplankton colonized on the surfaces of
unicellular and filamentous secondary colonizers (e.g. diatom, Oedogonium etc.).
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 is the sticky polymer matrix
(75%-95%). 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 microniche.
“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.”
Slime City (Coghlan 1996)
A mature biofilm may take 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 eco-system 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 approx 1000-fold higher than the corresponding planktonic count per cubic
centimeter
Biofilm covers every submerged and constantly wet surface
associated with a pond. It is on the rock, liner, plants, skimmer, biofilter
and media, even inside of the pump and related piping. The biofilm in one
location will be different in make up 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.
Updated- Feb. 23, 2014
