Terrestrial and Aquatic Environments as Microbial Habitats

• nearshore marine environments -> predominantly macrophages and benthic algae
• open ocean -> mainly phytoplankton
• What is the main difference between terrestrial and aquatic environments?

  who is responsible for the majority of primary production
• in terrestrial environments the environment that is most available to most microorganisms is either soil or plants
• terrestrial environments are divided into biome types based on vegetation characteristics
• soil may be organic (rarely) or inorganic -> mineral (most commonly)
• most soil -> combination of organic and inorganic material (minerals, gasses, microbes) -> support plants and other soil organisms
• soil formation -> a complex process; requires many, many years to reach mature soil profile
• microbial activities contribute to soil formation (decomposers ...etc.)
• plants contribute to soil formation by taking up water, and releasing nutrients into the soil
• soil animals (worms) contribute to soil structure and fertility by mixing and aerating upper soil layers
• physical and chemical processes contribute (freeze-thaw, wind and water erosion) to the formation of soil
• soil horizon -> a layer parallel to the soil surface, that has physical, chemical and biological characteristics that differ from the layers above and below it
• what is the microenvironment within a soil aggregate?

  surfaces of sand, silt, clay, organic matter, air-filled, and water-filled pores
• determinants of microbial activity in soil:

  • water availability -> affects oxygen availability
  • nutrient status -> microbial activity may be limited by C, N or P availability
• microbial cells tend to accumulate on the pores
• heterogeneous environments have immense microbial diversity
• very few microbes are common to all the soils sampled
• plants greatly influence soil habitat as well as serve as potential habitats
• rhizosphere -> soil that surrounds plant roots (an enviroment that is significantly different from the bulk soil environment)
• What is the R:S ratio?

  ratio of microbes in rhizosphere compared to microbes in bulk soil. Although they might be same bacterial group they may be different species within that group
• rhizoplane

  • is the actual root surface
  • is a source of root exudates (sugars, amino acids, hormones, vitamins)
  • can be very competitive environment (may contain a biofilm)
• phyllosphere

  • is the surface of a plant leaf
  • important microbial habitat in some rain forest trees
  • N2 fixation by phyllosphere microbes contribute nitrogen to plants in what can sometimes be a nutrient poor type of soil
• subsurface inhabitants are primarily prokaryotes, only a few microeukaryotes
• subsurface is typically low nutrient habitat with very low metabolic activity
• microbial species composition and their activity are affected by physical and chemical characteristics of their aquatic enviroment
• niches of aquatic environments are based generally on oxygen, sunlight, temperature, and salinity gradients
• in lakes oxygen produced near the surface and may be depleted at depth due to low solubility with depth and high consumption of sinking organic matter by heterotrophs with depth
• photic zone -> defined as the zone or depth to which light penetrates
• the photic zone is a crucial aspect in most aquatic systems
• the epilimnion is above the hypolimnion
• hypolimnion is anoxic
• epilimnion is oxic
• primary production occurs at the top of lakes
• aerobic heterotrophic activity occurs in the middle of the lake
• anaerobic heterotrophic activity occurs at the bottom of the lake
• flow and turbulence affects degree of re-oxygenation in rivers
• in rivers organic matter and nutrient inputs affect productivity, may lead to harmful algal growth and oxygen depletion
• Biochemical oxygen demand (BOD5) = change in dissolved oxygen after 5 days at 20 degrees
• Open Ocean

  • low primary productivity -> limited by N, P, Fe
  • photic zone of 0-200 m in depth