John R. Dolan is a senior research scientist with the French national science organization, the Centre National de la Recherche Scientifique. His expertise is in plankton ecology.
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The term ‘Marine microbes’ encompasses all microscopic organisms generally found in saltwater. Most micro-organisms are acellular and fall into the major categories of viruses, prokaryotes (’bacteria’), and protists, groups which differ considerably in biological characteristics. While representatives of these groups are found in virtually everywhere in marine waters and they play nearly every ecological role imaginable, their most important function is that they form the base of the food chain in marine ecosystems.
Major taxonomic groups of Marine Microbes
Viruses
Well-known to us as disease-causing agents, viruses are deceivingly simple organisms, little more than some nucleic acid within a protein container. They are ‘parasitic particles’ most about 40 nanometers in size. Viruses attach themselves to a living cell and inject a bit of nucleic acid into the cell; the injected nucleic acid directs the living cell to produce viruses. Generally, viruses are ‘host-specific’ only attacking or pirating a single species. They are very abundant in the sea; a tablespoon of seawater, 5 ml, commonly contains about 50 million viruses. As bacteria (or prokaryotes) are the most common potential host organisms in the sea, most viruses are bacteriophages (bacteria-consuming).
Prokaryotes
Prokaryotes are organisms without a distinct nucleus (their DNA is not bound within a membrane sac inside the cell). Typically, they are from 0.5 - 2 micrometers in size. Until recently known simply as ‘bacteria’, the 2 main groups recognized today are archaeabacteria and eubacteria which differ in the composition of their cell membranes. There appear to be no fundamental differences in the physiology or ecological roles played by the two types of prokaryotes but archaeabacteria appear do often to inhabit relatively extreme habitats such as the deep sea.
Most bacteria obtain energy by either absorbing marine dissolved organic matter through their cell membranes- osmotrophy (literally feeding through ‘osmosis’ in fact the material taken up is simply not obviously particulate, osmosis has little to do with the mechanisms used). However, some rely on sunlight and photosynthesis or the energy contained in some inorganic compounds (autotrophy or self-feeding). They are found in every environment, from sea ice at the poles to deep-sea hydrothermal vents. In seawater typical concentrations are about a million per ml or 5 million in a tablespoon.
Protists
Protists are eukaryotic, possessing a membrane-bound nucleus, but are single-celled or acellular organisms. The group includes all eukaryotic organisms which are not multi-cellular. Thus, it is a group of organisms united more by what they are not- multicellular- then by ancestry or common ecological characteristics. Marine protists typically range in size from 2 to 200 micrometers. Whereas viruses are parasites, and prokaryotes are osmotrophs or autotrophs, marine protist provide examples of these distinct life-styles as well as certain combinations of strategies.
The different types are found in different concentrations. Protists which have chloroplasts, allowing them to perform photosynthesis thus act as autotrophs, are generally found in the highest concentrations. The larger forms (10 - 200 micrometers in size) include diatoms and many dinoflagellates. The most abundant are small (1 - 10 micrometers long) flagellates. Autotrophic protists are restricted to the upper sunlit portion of the seas and found in abundances of a thousand per ml for the small flagellates. While they have few morphological characteristics allowing us to distinguish species, recent genetic studies suggest that small marine flagellates may be a very diverse group of organisms. Larger autrophic protists such as diatoms and dinoflagellates typically occur in concentrations of about one cell per ml.
Protists which rely on aquiring pre-formed organic matter are heterotrophic. Usually in surface waters there are about a thousand per ml of small flagellates which feed on bacteria (both autotrophic and heterotrophic prokaryotes) and 1 or 2 ciliates, oligotrichs (Fig 2B) and tintinnids (Fig 2C) or heterotrophic dinoflagellates which feed on autotrophic protists. Besides these two large, common life-styles there are parasitic protists as well ‘mixotrophic’ protists. Mixotrophic protists use both photosynthesis from chloroplasts as well as feeding on pre-formed organic matter, often in the form of other protists. Some protist species retain and use the chloroplasts in the prey they eat while other protists harbor symbionts, entire autotrophic bacteria or protists.