The Biological Cycle

Copyright © 1997 Aqua biotechnology
[I have taken the liberty of correcting spelling, punctuation and formatting errors—Ed.]

Metabolism is the biochemical reactions by which living cells convert food to energy and growth.   Aerobic microbes in water feed on organic matter and dissolved oxygen to form additional cells. Carbon dioxide and water are the end products.  One third of the organic matter is oxidised. This yields energy to synthesise remaining organics into additional living cells.

Pollution occurs when excess wastes enter the water-stream system. the key factor is dissolved oxygen (DO).  Organic wastes stimulate bacterial growth which uses up oxygen. As the dissolved oxygen in the water is depleted, the fish are the first to die, then the lower forms of life including protozoa. When oxygen disappears the water becomes septic. Anaerobic bacteria then produce odours and dark colours in the water.  If no more waste enters the environment then eventually the bacteria will convert all waste to gases and trace elements and the dissolved oxygen in the water will begin to rise to the point where the water will be safe for fish again.  Almost all toxic waste can be removed from water using bacteria.  Some synthetic toxins are exceptions to this rule.

Controlling waste entering an aquatic environment from all sources and maximising the bacterial degradation of this waste will reduce the oxygen demand and make it available to other life forms whilst maintaining a safe environment for aerobic bacteria.  The rate at which organic waste is consumed depends upon the size and variety of bacterial population.  As metabolism proceeds the number of microbes increase and the food supply is consumed at a faster rate, (more efficient bacteria = faster conversion of waste).

Bacteria that have a plentiful supply of food will double their mass through cell division at set intervals.  This is called log growth or the exponential growth phase.  The interval varies according to different species and strains.  When food becomes scarce the growth rate slows until finally the food supply is depleted to the point where bacteria begin to absorb their own cellular mass.  This is called the endogenous phase.  In an aquatic environment algae use carbon dioxide and release oxygen, whilst aerobic bacteria need oxygen but release carbon dioxide and so assist each other to survive.

The Role of Bacteria

Micro organisms play a vital role in the normal biological cycle.  Bacteria are the most important biological converters of organic wastes (animal and plant life) to water, new cells and gases.  Bacteria generally thrive in a pH range from 6.5 to 8.5 and will not survive at a pH of less than 4.3 or above 10.5.  There are both pathogenic and non pathogenic form of bacteria.  Aerobic bacteria require oxygen and anaerobic bacteria require an absence of oxygen.  Since pathogenic (generally anaerobic) bacteria cause disease, it is preferable to introduce and promote growth of a larger number of non-pathogenic aerobic bacteria.

In the absence of oxygen, anaerobic bacteria produce end products such as organic acids, aldehydes, keytones and alcohols, and produce odours and dark colors in the water.  Bacteria are single cell organisms (classified as plants but non photosynthetic) and all their food must be hydrolysed.  To accomplish this the bacteria excretes enzymes which break down particulate matter. The resulting solution is then absorbed into the cellular membrane and diffused through the inner cell wall as cellular enzymes before they can then convert it to food. This process produces mainly CO₂ , water, and bacterial cells. Carbohydrates and proteins are readily hydrolysed.

Maintaining Water Quality In An Aquatic Environment

Water quality is important to aquatic organisms as air quality is to you and I.  Every molecule of waste product will affect the health of your aquarium.  In the natural environment all waste in the form of organic compounds will eventually be broken down into gases and elements.  The organisms most suited to the existing conditions break down waste and still maintain a healthy aquarium.  Algae are photosynthetic plants which convert energy from sunlight and use inorganic compounds to grow new cells (single and multi cell forms) and in turn become a food supply for protozoa, rotifers and crustaceans.  Bacteria and algae are the prime food source for protozoa.  Protozoa are single celled animals and can easily be observed under a magnification of 100X. Rotifers are multi-cellular animals with flexible bodies and cillia near their mouth.  Small fish feed on protozoa, rotifers, crustaceans, and larger bacteria and algae. Bigger fish feed on the smaller fish and they in turn are caught and eaten by man. Man and animals discharge wastes back into the stream where bacteria metabolise the organic matter and the cycle starts over again.