Plankton and planktonic bacteria

Plankton and planktonic bacteria share two features. First, they are both single-celled creatures. Second, they live as floating organisms in the respective environments.

Plankton and planktonic bacteria are actually quite different from one another. Plankton is comprised of two main types, neither of which is bacterial. One type of plankton, the one of most relevance to this volume, is phytoplankton. Phytoplankton are plants. The second type of plankton is zoo-plankton . These are microscopic animals. Phytoplankton form the basis of the food chain in the ocean.

Phytoplankton are fundamentally important to life on Earth for several reasons. In the oceans, they are the beginning of the food chain. Existing in the oceans in huge quantities, phytoplankton are eaten by small fish and animals that are in turn consumed by larger species. Their numbers can be so huge that they are detectable using specialized satellite imaging, which is exploited by the commercial fishing industry to pinpoint likely areas in which to catch fish.

Phytoplankton, through their central role in the carbon cycle, are also a critical part of the ocean chemistry. The carbon dioxide content in the atmosphere is in balance with the content in the oceans. The photosynthetic activity of phytoplankton removes carbon dioxide from the water and releases oxygen as a by-product to the atmosphere. This allows the oceans to absorb more carbon dioxide from the air. Phytoplankton, therefore, act to keep the atmospheric level of carbon dioxide from increasing, which causes the atmosphere to heat up, and also replenish the oxygen level of the atmosphere.

When phytoplankton die and sink to the ocean floor, the carbon contained in them is lost from global circulation. This is beneficial because if the carbon from all dead matter was recycled into the atmosphere as carbon dioxide, the balance of carbon dioxide would be thrown off, and a massive atmospheric temperature increase would occur.

Phytoplankton are also being recognized as an indicator for the physical status of the oceans. They require a fairly limited range of water temperature for healthy growth. So, a downturn in phytoplankton survival can be an early indicator of changing conditions, both at a local level (such as the presence of pollutants) and at a global level (global warming).

Planktonic bacteria are free-living bacteria. They are the populations that grow in the familiar test tube and flask cultures in the microbiology laboratory. The opposite mode of growth is the adherent, or sessile, type of growth.

Planktonic bacteria have been recognized for centuries. They are some of the "animalcules" described by Antoni van Leeuwenhoek in 1673 using a microscope of his own design. Indeed, much of the knowledge of microbiology is based on work using these free-floating organisms. Research over the past two decades has shown that the planktonic mode of growth is secondary to the adherent type of growth. Additionally, the character of planktonic bacteria is very different from their adherent counterparts. Planktonic bacteria tend to have surfaces that are relatively hydrophilic (water loving), and the pattern of gene expression is markedly different from bacteria growing on a surface. Also, planktonic bacteria tend not to have a surrounding coat made of various sugars (this coat is also called a glycocalyx ), and so the bacteria tend to be more susceptible to antibacterial agent such as antibiotics . Paradoxically, most of the knowledge of antibiotic activity has been based on experiments with planktonic bacteria.

When grown in a culture where no new nutrients are added, planktonic bacteria typically exhibit the four stages of population development that are known as lag phase, logarithmic (or exponential) phase, stationary phase, and death (or decline) phase. It is also possible to grow planktonic bacteria under conditions where fresh food is added at the same rate as culture is removed. Then, the bacteria will grow as fast as the rate of addition of the new food source and can remain in this state for as long as the conditions are maintained. Thus, planktonic bacteria display a great range in the speeds at which they can grow. These abilities, as well as other changes the bacteria are capable of, is possible because the bacteria are phenotypically "plastic;" that is, they are very adaptable. Their adherent counterparts tend to be less responsive to environmental change.

Planktonic bacteria are susceptible to eradication by the immune system of man and other animals. Examination of many infectious bacteria has demonstrated that once in a host, planktonic bacteria tend to adopt several strategies to evade the host reaction. These strategies include formation of the adherent, glycocalyx enclosed populations, the elaboration of the glycocalyx around individual bacteria, and entry into the cells of the host.

It is becoming increasingly evident that the planktonic bacteria first observed by Leeuwenhoek and which is the staple of lab studies even today is rather atypical of the state of the bacteria in nature and in infections. Thus, in a sense, the planktonic bacteria in the test tube culture is an artifact.

See also Carbon cycle in microorganisms