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Microbial metabolism is the engine that drives nutrient cycling and energy flow in aquatic ecosystems. From cellular respiration to photosynthesis, microbes employ diverse strategies to obtain energy and nutrients, shaping water quality and ecosystem dynamics.

Understanding microbial metabolism is crucial for managing aquatic environments. By breaking down organic matter, fixing nitrogen, and transforming inorganic compounds, microbes play a vital role in nutrient cycling and energy transfer throughout aquatic food webs.

Microbial metabolism overview

  • Microbial metabolism plays a crucial role in the functioning of aquatic ecosystems by driving nutrient cycling and energy flow
  • Microorganisms exhibit diverse metabolic strategies to obtain energy and nutrients from their environment
  • Understanding microbial metabolism is essential for managing water quality and predicting ecosystem responses to environmental changes

Cellular respiration in microbes

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  • Cellular respiration is the process by which microbes break down organic compounds to generate energy in the form of ATP
  • Aerobic respiration requires oxygen as the terminal electron acceptor and yields the highest amount of energy (glucose, pyruvate)
  • Anaerobic respiration utilizes alternative electron acceptors such as nitrate, sulfate, or iron when oxygen is limited
  • Microbes in aquatic sediments often rely on anaerobic respiration due to the low oxygen availability in these environments

Fermentation processes

  • Fermentation is an anaerobic metabolic process that generates energy through the breakdown of organic compounds without the use of an external electron acceptor
  • Lactic acid fermentation is common in bacteria and produces lactic acid as the end product (Lactobacillus)
  • Alcohol fermentation is carried out by yeasts and results in the production of ethanol and carbon dioxide (Saccharomyces)
  • Fermentation processes are important in the decomposition of organic matter in anoxic zones of aquatic ecosystems

Photosynthesis in aquatic microorganisms

  • Photosynthesis is the process by which microorganisms convert light energy into chemical energy stored in organic compounds
  • Cyanobacteria and algae are the primary photosynthetic microorganisms in aquatic environments
  • Oxygenic photosynthesis releases oxygen as a byproduct and is responsible for the oxygenation of Earth's atmosphere (cyanobacteria)
  • Anoxygenic photosynthesis utilizes alternative electron donors such as hydrogen sulfide or hydrogen and does not produce oxygen (purple and green sulfur bacteria)

Chemosynthesis in extreme environments

  • Chemosynthesis is the process by which microorganisms obtain energy from the oxidation of inorganic compounds
  • Chemosynthetic microbes are found in extreme environments such as hydrothermal vents, cold seeps, and sulfidic springs
  • Sulfur-oxidizing bacteria derive energy from the oxidation of reduced sulfur compounds (Beggiatoa, Thiobacillus)
  • Methanogenic archaea produce methane using hydrogen and carbon dioxide in anaerobic environments (deep-sea sediments, wetlands)

Energy sources for microbes

  • Microorganisms have evolved to utilize a wide range of energy sources to support their metabolic processes
  • The availability and type of energy sources in an aquatic environment shape the microbial community composition and function

Organic compounds as energy sources

  • Organic compounds such as carbohydrates, proteins, and lipids serve as energy sources for heterotrophic microbes
  • Microbes secrete extracellular enzymes to break down complex organic matter into simpler compounds that can be transported into the cell
  • The degradation of organic matter by microbes releases nutrients and carbon dioxide, contributing to nutrient cycling in aquatic ecosystems
  • Dissolved organic carbon (DOC) is a major energy source for aquatic microbes, especially in oligotrophic environments

Inorganic compounds as energy sources

  • Inorganic compounds can be used as energy sources by chemolithotrophic microbes through oxidation-reduction reactions
  • Hydrogen gas is an important energy source for methanogens and sulfate-reducing bacteria in anaerobic environments (deep-sea vents, sediments)
  • Reduced sulfur compounds (hydrogen sulfide, thiosulfate) are utilized by sulfur-oxidizing bacteria (Thiobacillus, Beggiatoa)
  • Ammonia and nitrite serve as energy sources for nitrifying bacteria (Nitrosomonas, Nitrobacter)

Light energy utilization

  • Light energy is harnessed by photosynthetic microorganisms to drive the synthesis of organic compounds
  • Phytoplankton (cyanobacteria and microalgae) are the primary producers in aquatic ecosystems, converting light energy into chemical energy
  • The availability and quality of light influence the distribution and productivity of photosynthetic microbes in the water column
  • Photoheterotrophic bacteria can utilize light energy to supplement their organic carbon requirements (aerobic anoxygenic phototrophic bacteria)

Nutrient cycling by microbes

  • Microorganisms play a vital role in the cycling of essential nutrients in aquatic ecosystems, including carbon, nitrogen, sulfur, and phosphorus
  • Microbial metabolic processes regulate the transformations and availability of these nutrients

Carbon cycle in aquatic ecosystems

  • Microbes are key drivers of the carbon cycle in aquatic environments, mediating the fixation, degradation, and mineralization of organic carbon
  • Photosynthetic microorganisms (phytoplankton) fix atmospheric carbon dioxide into organic compounds, forming the base of aquatic food webs
  • Heterotrophic bacteria and archaea decompose organic matter, releasing carbon dioxide and nutrients back into the water column
  • Methanogenic archaea produce methane as a byproduct of anaerobic carbon degradation, contributing to greenhouse gas emissions from aquatic ecosystems

Nitrogen cycle and microbial transformations

  • Microorganisms mediate the major transformations in the nitrogen cycle, including nitrogen fixation, nitrification, denitrification, and ammonification
  • Nitrogen-fixing bacteria (Rhizobium, Azotobacter) convert atmospheric nitrogen gas into ammonia, making it available for biological uptake
  • Nitrifying bacteria (Nitrosomonas, Nitrobacter) oxidize ammonia to nitrite and nitrate, important nutrients for primary producers
  • Denitrifying bacteria (Pseudomonas, Thiobacillus) reduce nitrate to nitrogen gas under anaerobic conditions, removing bioavailable nitrogen from the ecosystem

Sulfur cycle and microbial metabolism

  • Microbes play a central role in the sulfur cycle, participating in the oxidation and reduction of sulfur compounds
  • Sulfate-reducing bacteria (Desulfovibrio, Desulfobacter) reduce sulfate to hydrogen sulfide in anaerobic environments, contributing to the formation of sulfidic sediments
  • Sulfur-oxidizing bacteria (Beggiatoa, Thiobacillus) oxidize reduced sulfur compounds to sulfate, coupling this process with carbon fixation or heterotrophic growth
  • The microbial sulfur cycle is closely linked to the carbon and iron cycles in aquatic ecosystems

Phosphorus cycle and microbial interactions

  • Microorganisms influence the phosphorus cycle through the solubilization, immobilization, and mineralization of phosphorus compounds
  • Phosphate-solubilizing bacteria (Pseudomonas, Bacillus) release bound phosphorus from inorganic and organic sources, making it bioavailable for primary producers
  • Polyphosphate-accumulating organisms (PAOs) store excess phosphorus as intracellular polyphosphate granules, regulating phosphorus availability in the environment
  • The microbial phosphorus cycle is tightly coupled with the carbon and nitrogen cycles, as phosphorus is a limiting nutrient in many aquatic ecosystems

Microbial adaptations to aquatic environments

  • Aquatic microorganisms have evolved diverse adaptations to thrive in the unique conditions of their habitats
  • These adaptations enable microbes to optimize their metabolic processes and survive in challenging environments

Metabolic adaptations to low nutrient conditions

  • Oligotrophic environments (open oceans, deep lakes) have low concentrations of essential nutrients, requiring microbes to develop efficient nutrient acquisition strategies
  • Microbes in nutrient-poor environments often have high-affinity nutrient transport systems to scavenge trace amounts of nutrients from the water column
  • Some microorganisms form symbiotic associations with other organisms (coral-algae symbiosis) to optimize nutrient acquisition and recycling
  • Oligotrophic bacteria (SAR11 clade) have streamlined genomes and minimized cell sizes to reduce nutrient requirements

Strategies for survival in extreme pH

  • Acidophilic microorganisms (Acidithiobacillus, Sulfolobus) thrive in low pH environments (acid mine drainage, volcanic springs) by maintaining a near-neutral intracellular pH
  • Alkaliphilic microbes (Bacillus, Clostridium) adapt to high pH environments (soda lakes) by regulating intracellular pH and utilizing sodium ion gradients for energy production
  • Extremophiles have specialized cell membranes and enzymes that function optimally under extreme pH conditions
  • Some microorganisms can tolerate wide pH ranges (Euryarchaeota) by adjusting their metabolic processes and gene expression patterns

Temperature effects on microbial metabolism

  • Temperature influences the rate of microbial metabolic processes, with higher temperatures generally increasing reaction rates up to an optimal point
  • Psychrophilic microbes (Polaromonas, Psychrobacter) have adapted to cold environments (polar regions, deep oceans) with enzymes that function efficiently at low temperatures
  • Thermophilic microorganisms (Thermus, Pyrococcus) thrive in high-temperature environments (hot springs, hydrothermal vents) with heat-stable enzymes and proteins
  • Mesophilic microbes (Escherichia, Bacillus) grow optimally at moderate temperatures and are common in temperate aquatic environments

Pressure adaptations in deep-water microbes

  • High hydrostatic pressure in deep-water environments (deep-sea trenches, abyssal plains) affects microbial cell physiology and metabolic processes
  • Piezophilic (pressure-loving) microbes (Shewanella, Moritella) have adapted to high-pressure conditions with specialized membrane lipids and pressure-resistant proteins
  • Deep-sea microorganisms often have slower growth rates and lower metabolic activities compared to their shallow-water counterparts
  • Pressure-adapted microbes play important roles in the biogeochemical cycles and food webs of deep-water ecosystems

Microbial interactions in aquatic ecosystems

  • Microorganisms in aquatic environments engage in complex interactions with each other and with other organisms
  • These interactions shape the structure and function of microbial communities and influence ecosystem processes

Symbiotic relationships between microbes

  • Mutualistic symbioses involve the exchange of benefits between microbial partners, such as nutrient sharing or protection from environmental stressors (lichens, coral-algae symbiosis)
  • Commensalistic relationships occur when one microbial partner benefits while the other is unaffected (epibionts on aquatic plants)
  • Parasitic interactions involve one microbe exploiting another, often resulting in harm to the host (phytoplankton parasites, bacterial viruses)
  • Syntrophic associations involve the cooperative metabolism of substrates, where the byproducts of one microbe serve as the energy source for another (methanogenic consortia)

Competition for resources among microbes

  • Microorganisms compete for limited resources such as nutrients, space, and light in aquatic environments
  • Competitive interactions can lead to the dominance of certain microbial species or strains with superior resource acquisition strategies
  • Allelopathy, the production of inhibitory compounds, is a competitive strategy used by some microalgae to suppress the growth of rivals (Prymnesium parvum)
  • Resource partitioning and niche differentiation help to maintain microbial diversity by reducing direct competition

Microbial food webs and trophic interactions

  • Microorganisms form the foundation of aquatic food webs, channeling energy and nutrients to higher trophic levels
  • Phytoplankton (primary producers) are consumed by zooplankton (primary consumers), which in turn are eaten by larger organisms (secondary consumers)
  • The microbial loop describes the recycling of dissolved organic matter by bacteria and their subsequent consumption by protists, linking primary production to higher trophic levels
  • Viral lysis of microbial cells releases nutrients back into the environment, influencing biogeochemical cycles and food web dynamics

Microbe-invertebrate relationships

  • Microorganisms form symbiotic associations with aquatic invertebrates, contributing to their nutrition, defense, and development
  • Chemosynthetic bacteria provide energy and nutrients to invertebrates (tubeworms, mussels) in deep-sea hydrothermal vent communities
  • Microbial biofilms on the surfaces of aquatic invertebrates (crustaceans, corals) protect against pathogens and environmental stressors
  • Invertebrate larvae (sponges, corals) acquire specific microbial symbionts from the environment during their development, establishing long-term partnerships

Anthropogenic impacts on microbial metabolism

  • Human activities can significantly influence microbial metabolism in aquatic ecosystems, often with consequences for water quality and ecosystem health
  • Anthropogenic factors such as pollution, eutrophication, and climate change alter the physical and chemical conditions that shape microbial communities

Eutrophication effects on microbial processes

  • Eutrophication, the excessive enrichment of water bodies with nutrients (nitrogen, phosphorus), stimulates the growth of phytoplankton and cyanobacterial blooms
  • Increased primary production leads to the accumulation of organic matter, which is decomposed by heterotrophic bacteria, depleting dissolved oxygen levels (hypoxia, anoxia)
  • Anoxic conditions favor the growth of anaerobic microbes (sulfate reducers, methanogens), leading to the production of toxic hydrogen sulfide and methane
  • Eutrophication-induced changes in microbial metabolism can result in the loss of biodiversity, fish kills, and the degradation of water quality

Pollutant degradation by microorganisms

  • Microorganisms have the ability to degrade a wide range of pollutants, including hydrocarbons, pesticides, and heavy metals
  • Bioremediation technologies harness the metabolic capabilities of microbes to clean up contaminated aquatic environments (oil spills, industrial effluents)
  • Pollutant-degrading bacteria (Pseudomonas, Alcanivorax) use contaminants as carbon and energy sources, converting them into less harmful compounds
  • The efficiency of microbial pollutant degradation depends on factors such as pollutant bioavailability, microbial community composition, and environmental conditions

Climate change impacts on microbial metabolism

  • Climate change affects aquatic ecosystems through changes in temperature, precipitation patterns, and ocean acidification
  • Warming temperatures can accelerate microbial metabolic rates, leading to increased respiration and the release of greenhouse gases (carbon dioxide, methane)
  • Altered precipitation regimes influence the input of nutrients and organic matter into aquatic systems, modifying microbial community structure and function
  • Ocean acidification, caused by the absorption of atmospheric carbon dioxide, affects the calcification and metabolism of marine microorganisms (coccolithophores, foraminifera)

Microplastics as substrates for microbial growth

  • Microplastics, tiny plastic particles (<5 mm), have emerged as a pervasive pollutant in aquatic environments
  • Microorganisms can colonize the surfaces of microplastics, forming biofilms known as the "plastisphere"
  • The plastisphere provides a unique habitat for microbes, with distinct community compositions compared to the surrounding water column
  • Microbial degradation of microplastics is limited, as most plastics are resistant to biodegradation, leading to their accumulation in aquatic food webs


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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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