Microbes and the Environment F.2 Objectives

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F.2.1 List the roles of microbes in ecosystems, including producers, nitrogen fixers and decomposers.

• Producers
• Microscopic algae and some bacteria use chlorophyll to trap sunlight
• Chemosynthetic bacteria use chemical energy
• Change inorganic molecules into organic molecules that can be used by other organisms for food
• Nitrogen fixers
• Bacteria which remove nitrogen as from the atmosphere and fix it into nitrates which are usable by producers.
• Decomposers
• Breakdown detritus (organic molecules) and release inorganic nutrients back into the ecosystem

F.2.2 Draw and label a diagram of the nitrogen cycle

...

F.2.3 State the roles of Rhizobium, Azotobacter, Nitrosomonas, Nitrobacter and Pseudomonas denitrificans in the nitrogen cycle.

• Nitrogen Fixation
• Mutalistic: Rhizobium lives in symbiosis with legumes (its root nodules) and fixes nitrogen for them
• Free-living: Azotobacter fixes nitrogen and lives freely in the soil without a host
• Nitrification
• Nitrosomonas converts ammonia (NH3) into nitrite (NO2-)
• Nitrobacter changes nitrite into nitrate (NO3-) which is usable by plants
• Denitrification
• Conversion of nitrates to nitrogen gas
• Pseudomonas denitrificans removes nitrates and nitrites and puts nitrogen gas back in atmosphere

F.2.4 Outline the conditions that favour denitrification and nitrification.

• Conditions favouring nitrification
• available oxygen/aerated soils
• neutral pH
• warm temperature
• Conditions favouring denitrification
• No available oxygen/anaerobic soils (flooding or compacted soil)
• High nitrogen input

F.2.5 Explain the consequences of releasing raw sewage and nitrate fertilizer into rivers

• Raw Sewage
• Raw sewage consists of organic matter and may contain pathogens, which are dangerous if drunk/bathed in => amount of saprotrophs increase to break down organic matter => a biochemical oxygen demand (BOD) occurs due to high levels of oxygen used =>deoxygenation of water => oxygen-dependent organisms are forced to emigrate/die => death and decay => decomposition => ammonia, phosphorus and minerals released => nitrification => eutrophication occurs due to high nutrient levels => algae proliferate => provided no algal bloom occurs, the rivers recovers eventually
• Nitrate Fertilizer
• Rivers leech off nitrate from soil => if application of nitrate fertilizer is great enough, eutrophication occurs => algae proliferate (increasing oxygen levels) => if nitrate levels in excess, algal bloom occurs => due to large amount of algae, some are deprived of sunlight and die => saprotrophs are needed to break down the organic matter => this creates a biochemical oxygen demand (BOD) => deoxygenation occurs => oxygen-dependent organisms are forced to emigrate/die=> increase in ammonia and phosphorus levels => nitrification => eutrophication => algae proliferate => provided no new algal bloon occurs, river recovers eventually.
• Simpler Module:
• High and excess nitrates and phosphates fertilize the algae in water
• Increased growth of algae (algal bloom)
• Algae decomposed by aerobic bacteria which use up oxygen in water, resulting in deoxygenation
• The high use of oxygen is called biochemical oxygen demand (BOD)

F.2.6 Outline the role of saprotrophic bacteria in the treatment of sewage using trickling filter beds and reed bed systems.

• Trickling Filter System
• Bed of stones 1-2 meters wide
• A biofilm of aerobic saprotrophs are on the rocks, which feed on organic mater, cling to the stones and act on the sewage trickled over (this is done to aerate the sewage), until it is broken down.
• Cleaner water trickles out the bottom of the bed to another tank where the bacteria are removed and the water treated with chlorine to disinfect it
• Reed Bed System
• Artificial wetland used to treat waste water
• Waste water provides both the water and nutrients to the growing reeds
• The reeds are harvested for compost and the organic waste is broken down by saprotrophic bacteria
• Nitrification of ammonia to nitrite and nitrite to nitrate
• Nitrates and phosphates released are used as fertilizer by the reeds
• Remaining nitrates are denitrified

F.2.7 State that biomass can be used as raw material for the production of fuels such as methane and ethanol.

• Biomass (organic matter) can be used as raw material for the production of fuels such as methane and ethanol. Examples include manure and cellulose.

F.2.8 Explain the principles involved in the generation of methane from biomass, including the conditions needed, organisms involved and the basic chemical reactions that occur.

• One group of Eubacteria are needed to convert the organic mater into organic acids and alcohol
• A second group of Eubacteria convert these into acetate, carbon dioxide and hydrogen
• Methanogenic bacteria are needed to create the methane, by two chemical reactions:
• carbon dioxide + hydrogen -> methane + water
• acetate -> methane + carbon dioxide (breakdown of acetate)
• Conditions required:
• No free oxygen (anaerobic)
• Constant temperature of about 35°C
• pH not too acidic

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