Chapter 30: Microbes in Oceans, Lakes & Freshwater Ecosystems

A critical environmental concern addressed is the formation of ocean dead zones, regions of severe oxygen depletion caused when excess nutrients, often from agricultural runoff, fuel massive phytoplankton blooms that eventually sink, leading to widespread heterotrophic respiration and subsequent suffocation of animal and plant life. Marine environments are strongly influenced by factors like light penetration and the robust carbonate equilibrium system, which buffers seawater pH, though this system is currently threatened by ocean acidification resulting from atmospheric carbon dioxide absorption. Coastal regions, such as estuaries, present unique osmotic challenges, demanding that microbes be highly halotolerant; these areas are also prone to harmful algal blooms (HABs), which can introduce dangerous neurotoxins into the food web. In the nutrient-limited open ocean, microbial primary producers, primarily cyanobacteria, perform about half of all global carbon fixation, with the microbial loop defining efficient nutrient recycling. Microbes adapted to these oligotrophic conditions, such as the abundant SAR11 clade, supplement their energy stores using unique strategies like light-driven proteorhodopsin or lithoheterotrophy. Viruses are recognized as the most numerous biological entities on Earth, accelerating nutrient turnover by lysing their hosts and contributing significantly to particulate and dissolved organic matter. Exploration of the deep-sea benthos has revealed vast communities of piezophilic and psychrophilic organisms, including the economically important methane hydrates, and evidence of unusual subsurface microbial energetics that reverse typical thermodynamic stratification. Freshwater ecosystems, which are classified as lotic (flowing) or lentic (standing), are susceptible to pollution that can cause oxygen sag curves downstream of point sources; deep lakes exhibit seasonal thermal stratification, where the warm, oxygen-rich epilimnion is separated from the cold, nutrient-rich, anoxic hypolimnion, with seasonal mixing triggering temporary blooms.