CLASS NOTES Nov. 25
(01/11:628:200) Fall 2003

Deep-Sea Biology

I. Definitions

The Deep Sea. Deep-sea areas are those below an ocean depth of about 200 m. The deep sea is found beyond the continental shelf and is divided into zones (see Sverdrup et al., Fig 3.7).

II. Characteristics of the deep sea.

a. cold, dark, high pressures (increasing by one atmosphere for every 10 m change in depth)

b. depths below 1,000 m occupy 62% of the earth's surface

b. little fluctuation in temperature or salinity but occasional increases in currents (storms) that resuspend sediment

c. sediments superficially appear homogeneous

d. topography is relatively smooth but variation is important

e. food supply is from the surface and variable in time and space.

f. the average size of animals, density (numbers per m2) and biomass (weight of animals per m2) declines with increasing depth and distance from land. With respect to density and/or biomass the deep sea is impoverished relative to shallow water.

g. the deep sea is not impoverished if we think about the diversity of life in the deep sea. Beyond the continental shelf the deep sea has a much higher diversity of species than most shallow water areas. Coral reefs are an exception; they have species diversities similar to deep-sea areas. The species diversity found in the deep sea may be as high as that found in rain forests.

III. What maintains the high diversity of species in the deep sea?

a. Intermediate levels of disturbance favor increased numbers of species.

b. The patchy input of food in both time and space (divided and ephemeral resources) allow many species to coexist.

c. Most deep-sea areas have a long undisturbed history.

d. Numbers of species increase with area and the deep-sea is a very large area with relatively few barriers to dispersal (e.g. it is an open system).

IV. Hydrothermal vents. Hydrothermal vents are found on the mid-ocean ridges where new sea floor is being formed at rates from 2 cm/yr to 16 cm/yr. Vents are found at depths ranging from 2000m to 3700m. Individual vent communities are generally less than one hectare in area and these areas usually last less than a couple decades. Vent communities may be clustered on a single segment of ridge crest to form vent fields which may be active for hundreds of years in some places and thousands of years in others. Cold seeps are found at the base of continental slopes where reduced compounds seep from the continental margin to support animals similar to but not the same as those at hydrothermal vents. Adding up all the known vent species, there are about 400 species living only at hydrothermal vents. At seeps there are about 100 species only about ten are known to occur in both seeps and vents.

How would you describe the diversity of species at hydrothermal vents?

A. Chemosynthesis

Vent organisms depend on chemosynthetic productivity by microorganisms for growth and survival instead of photosynthetic primary production. The source of this energy is the heat within the earth that results in hydrothermal fluid rich in reduced inorganic compounds. These compounds react spontaneously with compounds normally found in seawater. The most commonly observed reaction occurs in bacteria, when H2S is oxidized and CO2 is used as a source of carbon. The important enzymes involved in chemosynthesis are effectively the same structure as those in photosynthesis. Hydrogen, reduced iron, manganese, and nitrogen compounds can also be sources of energy for chemosynthesis. Some of the Archaea microorganisms at hydrothermal vents depend only on the reaction of H2 with CO2 at temperatures around 100o C to make organic compounds.

B. Symbiosis

Organisms that live closely together for mutual benefit have a symbiotic relationship. The largest vent animals depend on symbiotic bacteria living within the animal's body to produce organic compounds which are used as food. Clams and mussels can grow at a rate of up to 4 cm per year and the Riftia grow at rates up to 90 cm per year--the fastest growth known for a marine invertebrate.

C. Dispersal

Vents may be separated by hundreds of miles yet they are always colonized by dispersal stages--either planktonic larvae or swimming juveniles. These are carried by currents so it is advantageous for vent animals to grow rapidly and produce millions of offspring to increase the odds for colonizing newly-formed vents.

D. Biodiversity

Although vent animals are very unusual the species diversity of vent communities is low. Vent taxa are generally genetically quite different from relatives away from vents. Genetic diversity is high and there are considerable population differences from one vent field to another. The microorganisms are very diverse and this is a particularly exciting area of research.