15.4 What communities exist on the deep-ocean floor?

vast majority of ocean floor = submerged below several km of H2O

The Physical Environment

deep ocean floor includes: bathyal, abyssal, and hadal zones

• physical environment is very stable and uniform

• light only present in lowest concs down to max depth of 1000m and absent below that

• temp remains very constant

• salinity remains constant

• oxygen content constant and pretty high

• pressure = same

• bottom currents = slow but more variable than once believed

  • abyssal storms: warm and cold core eddies of surface currents affect certain areas lasting several weeks and causing bottom currents to reverse and/or increase in speed

thin layer of sediment covers most of deep-ocean floor

• abyssal plains and deep trenches - sediment = muddy abyssal clay deposits

• oceanic ridges and rises = oozes (dead plankton)

• cont rise = coarse sediment from land sources

• cont slope = sediment may be absent on steep areas

• crest of mid-ocean ridge, slopes of seamounts, and oceanic islands = sediment may be absent bc hasn’t had time to form there

Food Sources and Species Diversity

lack of light at deep-ocean floor —> photosynthetic primary production can’t occur

• except for chemosynthetic productivity at hydrothermal vents, ALL benthic organisms receive food from surface waters

  • only about 1-3% of food produced in euphotic zone reaches deep=ocean floor —> limiting benthic biomass

• some variability in supply of food casued by seasonal phytoplankton blooms at surface

many organisms inhabiting deep sea have special adaptations to help them detect food using chemical clues. once food is found, they’re efficient at consuming it

• large diversity of species

• deep-sea species diversity rivals that of tropical rainforests

• distribution of deep-sea life = patchy and depends to a large degree on presence of certain microenvironments

RECAP

deep-ocean floor = stable environment of darkness, cold water, and high pressure but still supports life. food for most deep-sea organisms comes from sunlit surface waters

Deep-Sea Hydrothermal Vent Biocommunities: Organisms and Their Adaptations

Discovery of Hydrothermal Vent Communities

1977 - discovered first time in Alvin

• hot water observed spewing from cracks in sea floor and from tall chimneys

most prominent species:

• giant tubeworms

• clams

• mussels

• white crabs

• microbial mats

hydrothermal communities have up to 1000x more biomass than rest of deep-ocean floor

hydrothermal vents may emit hot and acidifc fluids that are very toxic w/ high concs of dissolved hydrogen sulfide and heavy metals i.e. cadmium, arsenic, and lead

Chemosynthesis

most important members of hydrothermal vent = micro archaea

archaea = single-celled organisms similar to bacteria but have chemical similarities to multicelled organisms, thrive on sea floor chemicals, mostly hydrogen sulfide, and perform chemosynthesis, manufactugin carbs from water, CO2, and dissolved oxygen —> produce sulfuric acid as a byproduct

• archaea = base of food web

• some feed directly on archaea, some have symbiotic relationship w. archaea

  • Ex. tubeworms and clams depend on sulfur-oxidizing archaea that live symbiotically w/in tissues. tubeworms provide archaea w/ habitat and hydrogen sulfide - tubeworms get steady supply of food

archaea are super versatile w/ 2 diff methods to metabolize CO2 and can switch back and forth to accommodate fast-changing environmental conditions = asset

Discovery of Other Hydrothermal Vent Fields?

vents often differ dramatically from each other in chemical and geological characteristics

• giant tubeworms only in Pacific vents

• N Atlantic - shrimp and mussels dominate

• Antarctica - yeti crab dominates

Life Span of Hydrothermal Vents

vent may remain active only for limited periods bc hot-water comes from sporadic volcanic activity associated w/ MOR spreading centers

• inactive sites identified by accumulation of large #s of dead hyfrothermal vent organisms

  • organisms die when hot water stops coming out and they can’t move elsewhere

hydrothermal vent organisms = well adapted to temporary nature of hydrothermal vents:

• most have high metabolic rates —> mature rapidly so they can reproduce while vent is active

how do these communities survive after hot water is gone?

• dead whale hypothesis: when large animals die, they sink to deep-ocean floor, decompose, and provide an energy source for larvae of hydrothermal vent organisms —> organisms settle and grow where large dead animal rests, then breed and release their own larvae, some of which make it to next hydrothermal vent field

• rift valleys of MOR act as passageqays for larvae to traverse to inhabit new vent fields

Hydrothermal Vents and Origin of Life

uniformity of conditions and abundant energy of vents have led some scientists to propose that hydrothermal vents would have provided an ideal habitat for the origin of life ALSO archaea have ancient genetic makeup

additional evidence:

• discovery of deep-sea microbes w/ identical genes to those of microbes found in human body

however, vents are unstable and short-lived —> many scientists questions whether they could have adequately spawned life

Low-Temp Seep Communities: Organisms and Their Adaptations

Hypersaline Seeps

similar to hydrothermal vent communities:

• seeping water flows from fractures at base of a limestone and move out across clay deposits of abyssal plain

• hydrogen sulfide rich waters support white microbial growths called mats which do chemosynthesis

  • these support:

    • sea stars

    • shrimp

    • snails

    • limpets

    • brittle stars

    • anemones

    • tubeworms

    • crabs

    • clams

    • mussels

    • some fish

Hydrocarbon Seeps

fauna similar to those at hydrothermal and hypersaline

• chemosynthetic communities

• based on chemosynthesis deriving energy from hydrogen sulfide and/or methane

• microbial oxidation of methane produces calcium carbonate slabs found here and at other hydrocarbon seeps

Subduction Zone Seeps

trench filled w/ sediments which are folded into a ridge at seaward edge of the slope

at crest of the ridge, water slowly flows from the 2-million year old folded sedimentary rocks into a thin overlying layer of soft sediment on the sea floor

• eventually, water is released from sediment through seeps on ocean floor

seeps produce water slightly warmer than seawater at depth

vent contains methane produced by decomp of organic material in sedimentary rocks

microbes oxideze methane —> produce chemosynthetic food for themsevles and rest of community

The Deep Biosphere: A New Frontier

deep biosphere = environment w/in sea floor itself

• active and diverse microbial communities living w/in circulating fluids passing through porous sea floor

• as much as 2/3 of earth’s bacterial biomass might exist in deep biosphere

• biosphere microbes fuel their metabolisms by taking adv antage of chemical energy in various minerals

RECAP

hydrothermal vent communities occur near black smokes and rely on chemosynthetic archaea for food. other deep-sea cold seep biocommunities that depend on chemosynthesis exist around hypersaline, hydrocarbonm, and subduction zone seeps

CONCEPT CHECK 15.4

specify characteristics of communities existing on deep-ocean floor

(1) Where does the food come from to supply organisms living on the deep-ocean floor? How does this affect benthic biomass?

(2) Describe the characteristics of hydrothermal vents. What evidence suggests that hydrothermal vents have short life spans?

(3) What is the “dead whale hypothesis”? What other ideas have been suggested to help explain how organisms from hydrothermal vent biocommunities populate new vent sites?

(4) What are the major differences between the conditions and biocommunities of hydrothermal vents and cold seeps? How are they similar?

(5) Discuss changes in the physical environment that occur as one moves from the shoreline to the deep-ocean floor.