Marine microbes is a term that 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
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 characteristic size. Viruses attach themselves to a living cell and inject a dose 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, with five ml commonly containing 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 are organisms without a distinct nucleus (their DNA is not bound within a membrane sac inside the cell). Typically, they are from 0.5 - 2.0 micrometers in size. Until recently known simply as 'bacteria', the two 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 five million in a tablespoon.
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- than 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.
Ecological roles of marine microbes
Primary production is 'first production' - the creation of organic matter. Usually it refers to the transformation, or fixation, of inorganic carbon into simple sugars using solar energy through photosynthesis. On land, the primary producers take the form of grasses, bushes, and trees and these plants are usually the most visible of all organisms within a given locality. However, in the sea (with the exception of coastal areas with seagrasses & seaweeds) the primary producers appear invisible because they microscopic and the organisms we see are high up in the food chain. The primary producers are microbes. This is not only in open water areas where the plants are in the form of the plankton (the plant plankton or phytoplankton) but it also true of many shallow areas.
Among the prokaryotes, the most abundant and important primary producers are species of the genera Synechococcus and Prochlorococcus. They are capable of reproducing once per day and form most of the 'plant' biomass in many areas of the open sea, especially in the tropics. As these cells are small (about a micrometer across) the most likely consumers of Synechococcus or Prochlorococcus are flagellate and ciliate protists.
In most coastal zones, protists form the bulk of the phytoplankton. Diatoms (Fig 2A), dinoflagellates (Fig 2D), and many different types of flagellates are responsible for most of the primary production. Many are large enough (> 100 micrometer) to be consumed by filter-feeding fish such as anchovies and sardines. However, throughout most of the seas, protist primary producers are small flagellates (2-10 micrometers) and fed upon by other protists, typically ciliates (Fig B,C). The ciliate herbivores are then fed upon by larger organisms such as copepods (small crustaceans).
Herbivory (or secondary) and tertiary production
Consumers of plants or primary producers, are herbivores and the production (increases in numbers or individual mass) of herbivores is called secondary production. Among marine microbes, consumers of primary producers are those which feed on autotrophic prokaryotes or autotrophic protists. Thus, the viruses which attack the autotrophic prokaryotes Synechococcus, the bacteria which absorb dissolved organic excreted by autotrophic protists such as diatoms and dinoflagellates, and the protists such as ciliates, radiolarians which feed on autotrophic protists are all consumers of primary production. However, these simple relationships exist alongside many others because among marine microbes there is not a food chain but rather a web.
Thinking in terms of food chains, more often than not marine microbes are not exactly akin to plant (phytoplankton) nor animal (zooplankton). Furthermore, relationships between different microbes are usually neither direct nor exclusive. For example, the dissolved organic matter absorbed by a bacterium is likely a mixture of that excreted by a primary producer and some from the viral lysis of another bacterium as well the excreta of yet another organism that fed on a herbivore or a primary producer. Similarly, among protists, a radiolarian may capture and ingest, more or less indifferently, a bacterium, an autotrophic flagellate, a herbivorous oligotrich ciliate, or another radiolarian (Fig 2E). While the ecological roles are not often clear cut among marine microbes, the rest of the marine food web ultimately depends on the microbial community.