Various
physical and chemical factors affect the microbial growth and distribution in
aquatic habitats. These conditions differ in various habitats. The main factors
include
·
Light
·
Nutrients
·
Turbidity
·
Water
currents
·
Temperature
·
Hydrogen
ion concentration
·
Hydrostatic
pressure
·
Salinity
Light
All forms of life in aquatic habitats are either
directly or indirectly influenced by light since primary production is mainly
through photosynthesis.
Algae and photosynthetic bacteria involved
in photosynthesis are the most
important ones with respect to light. Light is also important in the spatial distribution of microorganisms,
especially the photosynthetic forms. Photosynthetic microorganisms are usually
restricted to the upper layers of aquatic systems, the photic zone, where
effective light penetration occurs. Although photosynthesis is confined to the
upper 50-125 m of the waterbodies, the depth of the photic zone may vary
depending upon the latitude, season and turbidity. Apart from the decrease in
the quantity of light with depth, there is also a change in the colour of the
light. Blue light is transmitted most while red light is the least.
Light may also be bactericidal. The reason for the death of sewage bacteria in
seawater is suggested to be due to light. Sometimes, there may be a reduction in the bacterial activity
without killing. For example, reduction in the rate of oxidation occur in
Nitrosomonas and Nitrobacter due to the high light intensities.
Cell
division
may also be related to the day and night variation. The diatom Nitzschia
divides mostly in the light, while the dinoflagellate Ceratium divides, during
darkness.
Nutrients
Supply of nutrients is crucial for
microbial biomass and variety. Both organic and inorganic nutrients are
required. Nutrients may be available either in dissolved state or as
particulate matter and in varying amounts, as per season.
Aquatic environments with limited nutrient content are called oligotrophic and those with a high nutrient content are called eutrophic. Open sea has a stable and very low nutrient content while nearshore water shows variations in nutrient load due to additions from domestic and industrial waste waters.
Inorganic nutrients particularly,
nitrogen and phosphorous, may limit algal growth. However, the presence of
these in large amounts often lead to excessive algal growth, a condition known
as eutrophication.
Heavy
metals
like mercury from industrial waste waters can inhibit microbial growth. However,
some microorganisms have developed resistance towards these heavy metals.
Turbidity
Since turbidity of water in an
aquatic environment inhibits the effective penetration of light, it is also
considered an important factor affecting microbial life. Turbidity is mainly
caused by suspended materials, which include particles of inorganic mineral material originating from land, detritus, particulate organic material such as cellulose,
hemicellulose and chitin fragments and suspended microorganisms.
Particulate matter in a water body
may provide a substratum to which various microorganisms adhere. Many bacterial
species from marine ecosystems inhabit solid substrata. These attached
communities are often called epibacteria
or periphytes. Other organic nutrients and detritus matters act as
substrates for the metabolic activity.
Water Currents
Water currents are seen in lentic
habitats like rivers. Washing off by water current helps in dispersal, but it
can cause removal from suitable habitat also. The water current in rivers leads
to a phenomenon called nutrient
spiraling. In ocean, water currents or geothermal vents are responsible for
mixing up of nutrients. Usually in deep waters, the water currents may move in
opposite direction to those on the surface and usually they are slower. The
velocity of water in a waterbody, especially rivers, influence the nutrient
uptake and metabolism.
(contd)
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