Author : Marissa Brett Hirst
Publisher :
ISBN 13 : 9781303792007
Total Pages : pages
Book Rating : 4.7/5 (92 download)
Book Synopsis Symbiotic Interactions Among Protists, Archaea, and Bacteria in Low Oxygen Environments by : Marissa Brett Hirst
Download or read book Symbiotic Interactions Among Protists, Archaea, and Bacteria in Low Oxygen Environments written by Marissa Brett Hirst and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In the natural world, most bacteria, archaea, and microbial eukaryotes live in close association with other microbes, and are often key symbiotic residents in protists (single-celled microbial eukaryotes excluding fungi) as well as multicellular eukaryotic hosts. Symbiosis, or "the living together of unlike organisms," has been a major driving force in shaping the evolution of the eukaryotic cell. Partnerships between eukaryotes and microorganisms are important because they have a wide taxonomic distribution across the tree of life, suggesting that symbioses play an essential role in the evolution of the species involved. Symbiosis-specific genes, pathways, and structures have also been identified, which are a direct result of evolution favoring the maintenance of the partnership. Lastly, microorganisms make up the greatest biomass and are also responsible for the most complex biochemical reactions on Earth, which makes symbioses between microbes and eukaryotes crucial for driving the evolution of communities. One common misconception regarding microbial eukaryotes is that they are absent from anaerobic environments, but in fact, they are common in a variety of anaerobic habitats including tidal marshes, microbial mats, anoxic marine basins, and the guts of many animals. Although the eukaryotic lineage of the tree of life is primarily composed of single-celled microbial eukaryotes, little is known about free-living protists (with the exception of pathogens). The second chapter of this dissertation focuses on a successful, new method used to describe the diversity of protists in diverse environments by linking culture-independent small subunit ribosomal RNA (SSU rRNA) sequencing to the morphology of protists. Anaerobic environments are habitats that are strongly influenced by microbially-mediated, symbiosis driven biogeochemical cycling. Many microorganisms cannot perform anaerobic respiration, but instead, ferment organic acids and generate ATP in the process. In anaerobic habitats; however, a single fermenting microbe cannot completely catabolize carbon substrates to carbon dioxide without the concerted activity with other microbial anaerobes. In this regard, one microbe lives off of the byproducts of another microorganism and neither microbe could survive on its own. This type of mutualism is known as syntrophy and is a thermodynamically interdependent lifestyle. One environment in which anaerobic microbial eukaryotes are prevalent and have intimate partnerships with bacteria and archaea, is the cow rumen. Rumen ciliates ferment organic acids to acetate or other volatile fatty acids while producing ATP and generating carbon dioxide and dihydrogen. Fermentation by rumen ciliates is an endergonic reaction in the rumen, but becomes exergonic when it is coupled to methanogenesis. Methanogens utilize carbon dioxide as a carbon source and dihydrogen as an energy source; the coupling of fermentation and methanogenesis is known as "interspecies hydrogen transfer" (IHT). IHT is known to occur between free-living methanogens and rumen ciliates, but a syntrophic symbiosis between ciliates and methanogens has not been confirmed in the literature. The third chapter of this dissertation identifies and describes the underlying metabolism of the first two putative, obligate, endosymbiotic methanogenic archaea in ciliates using fluorescence activated cell sorting (FACS), metagenomic sequencing, assembly, and annotation, and rRNA-targeted fluorescent in situ hybridization (FISH). Lastly, symbioses between bacteria in anaerobic environments can drive cycling in anoxic marine environments, and in particular Oxygen Minimum Zones (OMZs). In suboxic waters and OMZs specifically, denitrification (conversion of nitrate to dinitrogen) is limited by the diffusive flux of nitrate from water into the overlying sediments; however, the production of dinitrogen occurs below these limits, suggesting that an alternative, microbiologically driven metabolic process may be responsible for the loss of nitrogen from OMZs. Anammox bacteria are present in OMZs and gain valuable free energy by reducing ammonium to nitrite while producing dinitrogen (NO3− [arrow right] NO2− [arrow right] NH4+). In addition, Thioploca (macroscopic bacteria) are found in OMZs, and are chemolithoautotrophic sulfur-oxiding proteobacteria that glide vertically through marine sediments. Thioploca couples sulfide oxidation with dissimilatory nitrate/nitrite reduction at depth (NO3− + H2S + H2O [arrow right] SO4−2 + NH4+) but also converts large stores of nitrate to nitrite and generates elemental sulfur that it stores in vacuoles within its cells (NO3− + H2S [arrow right] NO2− + S0 + H2O). Based on geochemical and isotopic observations, a symbiosis between anammox bacteria and Thioploca was hypothesized to be the driving force behind the loss of dinitrogen from marine sediments underlying OMZs. The last chapter of this dissertation provides molecular (SSU rRNA sequence data), microscopic data (rRNA-targeted FISH), and isotopic evidence supporting the hypothesis that there is a symbiosis between Thioploca and anammox bacteria, responsible for upwards of 20% nitrogen loss from OMZs.