Bacteria were the sole occupants of our planet for more than half of the entire history of life; yet these single-celled organisms are among the smallest living things. These diverse creatures are abundant in all marine habitats, from the sea surface to the greatest depths. The role bacteria plays in cycling organics through the marine food web is being intensely studied... a puzzle that Bigelow Laboratory scientists are helping to solve.

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Domains of Life, Bacteria & Viruses

For many years, living organisms were divided into two kingdoms: Animalia (animal) and Plantae (vegetable). In the 1800s, scientists realized these two kingdoms could not adequately express the diversity of life. Since the 1960s, the most widely used scheme for classifying life consists of five kingdoms. This "kingdom scheme" has co-existed with another classification of living organisms based on cell complexity:

  • Eukaryotes: Complex cells with organized structures inside called "organelles" (e.g., nuclei, mitochondria, chloroplasts). Protists, Fungi, Plants, and Animals are examples
  • Prokaryotes: Simple cells without organelles. Bacteria, for example
Recent studies have shown that bacteria are far more diverse than previously thought. Thus Prokaryotes are now divided into two domains: Bacteria and Archaea; these are as different from each other as either is from the Eukaryotes. Viruses are fourth group of biological entities, although not organisms in the same sense as Eukaryotes, Archaeans, and Bacteria. However, they are of considerable biological importance and, like bacteria, help cycle organic matter in the marine food web.
Ecosystems: Matter Cycling versus Energy Flow
At left (<<) is a land-based ecosystem. Arrows show how matter is cycled among organisms. To maintain itself, any ecosystem must have key gases (such as oxygen and carbon dioxide), nutrients, and organisms that can recycle these substances. Producers convert raw energy to organic molecules and nutrients useful to themselves and other organisms. Consumers capture these molecules by eating the producers; they use nutrients to build organic tissue. Decomposers liberate nutrients from dead consumers and producers; primary producers use these nutrients for growth.
Roll your cursor over the image to see a marine ecosystem. Marine organisms that help to recycle matter can be grouped into three key categories: 1) producers such as phytoplankton; 2) consumers such as zooplankton and fish; and 3) decomposers such as bacteria.
Click here for Phytoplankton "Phundamentals"
Below is a simple marine food chain. Arrows show how energy flows in one direction: from the Sun to producers to consumers. Note that primary consumers eat primary producers, whereas secondary consumers eat other consumers. Energy is lost at each trophic level as chemical energy is converted to heat.
Cycling of Carbon & Nitrogen

Life on earth is carbon-based, thus the cycling of carbon is vital. Shown below are the living (i.e., plankton, fish, bacteria) and non-living components in a simple carbon cycle. Place your cursor on the image to see processes (e.g., photosynthesis, decay, respiration, sinking) that move carbon among these components.
Nitrogen (N) is a component of proteins, chlorophyll, DNA, RNA, and many other biological molecules. Availability of N in "usable" form means life or death for organisms. For example, nitrate, nitrite and ammonium can be used by plants; but Earth's biggest source of N, atmospheric gas, cannot be used by most organisms.
To see various forms of N in our oceans, roll over the "Nitrogen components" button. (Note: "DON" and "DIN" are dissolved organic and inorganic nitrogen, respectively.)
Much of the N cycle is controlled by bacteria: some transform "unusable" N into "organism friendly" forms. A self-sustaining marine community consists of: bacteria converting ammonium to nitrite; bacteria converting nitrite to nitrate; plants taking up DIN; plants and animals releasing ammonium and organic N when they decay; and animals excreting ammonium and other nitrogenous wastes. To see some key bacterial-driven processes, roll your cursor over the "Nitrogen cycle" button and look for dashed arrows.
Microbial Loop
In the oceans, matter cycling is carried out by organisms often much smaller than 10 micrometers. In fact, biomass production by marine bacteria is 30-50% of the ocean´s primary production!
How the microbial loop is coupled with the more familiar ocean food chain is shown at left. In the microbial loop, bacteria consume dissolved organic material (DOM) that cannot be directly ingested by larger organisms. DOM includes liquid wastes of zooplankton and cytoplasm that leaks out of phytoplankton cells. Micro-flagellates and ciliates eat these marine bacteria, helping to recycle organic matter back into the marine food web. Bacteria also help to facilitate phytoplankton growth by releasing nutrients when they absorb DOM.
Viruses are the smallest and most abundant organisms in the sea, concentrations vary from millions to billions per milliliter. In the 1990s, scientists began to realize that marine viruses are important players in the microbial loop. Questions being investigated include: What are they doing? Whom do they infect? What is their role in the plankton ecosystem? Something that IS known: like all other members of the marine food web, viral activity produces DOM, thus helping to drive energy cycles for ocean life. (Click here to learn more.)
Did you know that many marine organisms are classified by how they move, their size, and source of energy? Click here for more.
There is much to discover about the microbial loop: we are working to directly measure the bacterial activity levels in the ocean. These findings will help us to understand the cycling of elements that are important to life.
Web site development was supported by the National Science Foundation.
If you'd like to learn more about Bigelow Laboratory for Ocean Sciences, please visit our web page.
Please direct comments about this web site to Annette deCharon.