page_title: Marine microorganisms
section_title: Identifying microorganisms
hierarchical_section_title: Marine microorganisms / Identifying microorganisms
caption_attribution_description: English: DNA barcode alignment and comparison between two species of marine bacteria: Pelagibacter ubique, the most common bacterium in the ocean and Marinomonas arctica, which grows inside Arctic sea ice at subzero temperatures
context_page_description: Marine microorganisms are defined by their habitat as the microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism is any microscopic living organism, that is, any life form too small for the naked human eye to really see, needing a microscope. Microorganisms are very diverse. They can be single-celled or multicellular and include all bacteria and archaea and most protozoa, as well as some species of fungi, algae, and certain microscopic animals, such as rotifers and copepods. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify biologically active entities such as viruses and viroids as microorganisms, but others consider these as nonliving.
  Marine microorganisms make up about 70% of the biomass in the ocean. Taken together they form the marine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in the global cycling of almost all chemical elements. Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers.
context_section_description: Traditionally, the phylogeny of microorganisms was inferred and their taxonomy was established based on studies of morphology. However, developments in molecular phylogenetics have allowed evolutionary relationship of species to be established by analyzing deeper characteristics, such as their DNA and protein sequences, for example ribosomal DNA. The lack of easily accessible morphological features, such as those present in animals and plants, particularly hampered early efforts at classifying bacteria and archaea. This resulted in erroneous, distorted and confused classification, an example of which, noted Carl Woese, is Pseudomonas whose etymology ironically matched its taxonomy, namely "false unit". Many bacterial taxa have been reclassified or redefined using molecular phylogenetics.
  Recent developments in molecular sequencing have allowed for the recovery of genomes in situ, directly from environmental samples and avoiding the need for culturing. This has lead for example, to a rapid expansion in knowledge of the diversity of bacterial phyla. These techniques are genome-resolved metagenomics and single-cell genomics.
  The new sequencing technologies and the accumulation of sequence data have resulted in a paradigm shift, highlighted both the ubiquity of microbial communities in association within higher organisms and the critical roles of microbes in ecosystem health. These new possibilities have revolutionized microbial ecology, because the analysis of genomes and metagenomes in a high-throughput manner provides efficient methods for addressing the functional potential of individual microorganisms as well as of whole communities in their natural habitats.
caption: DNA barcode alignment and comparison between
  the two species of marine bacteria pictured above [266]
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