Download Trophic levels and the microbial loop in aquatic ecosystems

MMG301 Dr. Frank Dazzo Aquatic & Wastewater Microbiology
• Natural aquatic habitats for microorganisms include lakes, ponds,
rivers, springs, oceans estuaries, marshes.
• The concentration, mixing and movement of nutrients, O2, and
waste products are the dominant factors controlling the abundance,
distribution, and diversity of aquatic microbial communities
Trophic levels and the microbial loop in aquatic ecosystems:
In contrast to soil, phytoplankton (algae and cyanobacteria) are the
predominant photosynthetic organisms in aquatic habitats
•
Much of the organic
matter synthesized by
phytoplankton during
photosynthesis is
released as dissolved
organic matter (DOM).
• This DOM is
consumed by
bacterioplankton
which become part of
the suspended
particulate organic
matter (POM) pool.
• A portion of these
bacteria are then
consumed as food by
protozoa predators.
•
Some of the nutrients immobilized in bacteria and protozoa are
mineralized and then assimilated directly by phytoplankton without
transfer to higher trophic levels (e.g. fish) in the aquatic
ecosystem. This is called the "microbial loop" (arrows in red).
O2 plays a key role in microbial activity in aquatic habitats:
Photic zone, oxic zone, anoxic zone in lakes:
Eutrophic lake
Oligotrophic lake
O2 and organic nutrients are inversely interrelated in aquatic
habitats. Nutrient-poor (oligotrophic, A) lakes recycle
nutrients only within the water whereas nutrient-rich
(eutrophic, B) lakes have major nutrient inputs from outside.
Oligotrophic lakes are typically O2-saturated and have low
microbial populations. Eutrophic lakes develop high
microbial populations that deplete the dissolved O2 by
aerobic respiration during decomposition of abundant
organic matter, producing deep anoxic zones.
Temperature also impacts on the status of O2 and nutrients in
temperate lakes.
• Anoxic conditions develop in the depths of the lake as a result of
thermal stratification.
• The cool bottom waters (hypolimnion) are more dense and contain
H2S from anaerobic bacterial sulfate reduction.
• The depth zone of rapid temperature change is the “thermocline.”
• Typically as lake surface waters (epilimnion) cool in the fall and
early winter, they reach the temperature and density of the
hypolimnionic waters and then they sink, displacing bottom waters
and the sediments, affecting “lake turnover” and the redistribution
of nutrients for the aquatic microorganisms.
Marine microorganisms (eubacteria, archaea, and eukaryotes)
• True marine microorganisms are moderate osmiophiles, requiring
the salinity and ions (esp. Na+) of seawater for cultivation
• Most are psychrophiles (5°°C for most of ocean volume)
• Barophiles in deep sea (hydrostatic pressures ≤ 1,100 atmospheres)
• Most are oligocarbophiles adapted to the extremely low
concentration of organic C in ocean seawater (∼
∼ 1-2 mg C / liter)
• Recent exciting find of endosymbiotic bacteria inside Riftia tube
worms that develop on sides of black smoker vents at deep sea
ocean floor → life sustained by geothermal energy rather than sun.
Concept of Biochemical Oxygen Demand:
• The amount of biologically usable organic carbon in water is
indirectly measured by its Biochemical Oxygen Demand (BOD).
• It represents the portion of total carbon that can be oxidized by
microorganisms in a 5-day period under standard conditions.
• It equals the amount of dissolved O2 needed for microbial
oxidation of biodegradable organic matter in a water sample.
The effect of a point source discharge of an organic pollutant (e.g.,
untreated sewage) into a clean flowing river system is profound:
• Heterotrophic bacteria, organic carbon, and BOD immediately
increase at the pollutant input, and correspondingly, dissolved O2
levels decline due to the burst in microbial respiration. Kills all
aquatic life (fish, etc) dependent on dissolved O2.
• Microbes mineralize and oxidize the organic N and P into inorganic
nutrients (NO3-, NH4+, and PO4-3), resulting in eutrophication, with
development of noxious algal / cyanobacterial blooms.
• Further downstream, self-purification processes result in a decline
of BOD, the oligotrophic conditions and phototrophic microbial
communities regain dominance, and dissolved O2 levels replenish.
Microbiology of Domestic Sewage Wastewater Treatment
• The treatment of human fecal wastes (→
→ organic matter plus many
bacterial, protozoan & viral pathogens) is one of the most
important factors in maintaining an advanced healthy society.
• Effects of discharging organic wastes into aquatic ecosystems
can be drastic as described earlier
• Fecal pathogens are shed from patients with disease and from
carriers (maintain infection without expression of symptoms).
• Conventional sewage treatment is a controlled intensification of
natural self-purification processes involving 1°°, 2°°, and 3°° treatment.
Primary treatment: removal of
insoluble particulate materials
from raw sewage by screening
gravitational settling in tanks.
The resultant solid material is
called sludge.
Secondary treatment:
microbial conversion of
organic matter into microbial
biomass and final
decomposition products (9095% reduction in BOD), plus
removal of many bacterial
pathogens.
Tertiary treatment: biological
and chemical removal of inorganic nutrients (e.g., N and P)
to reduce eutrophication of
receiving ecosystem, virus
removal or inactivation, trace
chemical removal.
Role of microorganisms in secondary treatment of domestic sewage:
Activated sludge process:
Microbes (e.g., Zooglea ramigera)
in a forced aeration tank form
zooglea of activated sludge (active
biomass of suspended flocs) that
aerobically decomposes organic
matter. A portion of the activated
sludge is recycled as inoculum to
maintain the process.
Trickling filter system: a
rotating arm of an aeration
basin trickles wastewater
over a bed of rocks. Each
rock develops a large
microbial biofilm that
absorbs and aerobically
decomposes the dissolved
organic matter in the
wastewater as it trickles
over the rocks.
Anaerobic (anoxic) sludge digestion process:
Anaerobic
sludge digestor
The bioreactor has a lid cover to
maintain anoxic conditions. A very
complex community of anaerobes
actively decompose polymers by
the processes indicated to final
anaerobic metabolites dominated
by methane (CH4) and CO2 as the
major products of anaerobic biodegradation. The CH4 is burned or
used to power the treatment plant.
Use of Indicator Microorganisms to Detect Fecal Pollution
A wide range of bacterial, protozoan, and viral diseases result from
consuming water and food contaminated with human fecal wastes:
• Bacterial diseases; diarrhea caused by Salmonella, Shigella,
enteropathogenic E. coli; cholera (Vibrio cholerae)
• Protozoan diseases: amoebic dysentery (Entamoeba histolitica),
amoebic meningocephalitis (Naegleria fowleri)
• Viral diseases: hepatitis A, poliomyelitis, diarrhea (enteroviruses)
• Many bacteria from the intestinal tract become physiologically
stressed when introduced into the aquatic environment, and they
gradually lose their ability to form colonies on differential &
selective media. Also, enumeration of many intestinal pathogens in
wastewater is difficult. Therefore, various indicator microbes are
used to detect fecal wastes in food and water (used for both
potable and recreational uses).
Criteria for use of indicator microbes as an index of fecal pollution:
1.
2.
3.
4.
5.
Its normal habitat should be the gut of warm-blooded animals.
It should be suitable for the analysis of all types of water.
It should be present whenever enteric pathogens are present.
It should survive longer than the hardiest enteric pathogens.
It should not reproduce in the contaminated water so its population
level can indicate the degree of fecal pollution.
6. The methods to detect and enumerate them should be specific, very
sensitive, standardized, and easy to perform, yield results quickly.
Present status: The “ideal” indicator organism hasn’t been found.
The search continues….. Ones currently used as indicators include:
1. Fecal coliforms: (e.g., Escherichia coli ) Gram (-) non-sporulating
facultative anaerobic rods from the intestine of warm-blooded
animals that ferment lactose with the production of gas at 44.5 °C
2. Fecal streptococci: (Streptococcus faecalis), useful indicator of
fecal contamination in estaurine and marine waters, where it
survives better than E. coli.
3. Lytic bacteriophages of E. coli (viruses survive chlorination better
than bacteria