Chemosynthetic Environments

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  • Created by: rosieevie
  • Created on: 26-05-17 11:09

Chemosynthetic Envrionments

Usually an expontential decline in biomass with depth - limits process and influences adaptations

Chemosynthetic environments are exceptions = in situ sites of primary production in the deep sea

Chemosynthesis - synthesis of organic compounds by bacteria using energy derived from reactions with inorganic chemicals (usually in absence of light)

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Chemosynthetic Primary Production

Fixation of inorganic carbon using chemical energy

Reduced chemical compounds provide source of electrons e.g. H2S

2 stage process:

  • Production of reducing power
  • Fixation of inorganic carbon

Process requires a terminal electron accpetor e.g. oxygen in aerobic)

= Although process occurs deep at sea, not completely independent of photosynthesis (requires oxygen)

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Chemoautotrophy

Chemoautotrophy - fix inorganic carbon into carbon-based molecules by oxidation of substances, usually inorganic minerals

Carried out by prokaryotic microbes

Variety of electron donors e.g. sulphur, iron, manganese, H2S, CH4, hydrogen

Variety of electron acceptors e.g. oxygen, nitrates, sulphates, iron, sulphur

= Variety of chemosynthetic pathways e.g. sulfide oxidation, methanotrophy,

Dominating pathway depends on the environment

  • Availability of electron donors
  • Availability of electron acceptors
  • Energy yield

Most able to use a range of electron donors/acceptors so can take advantage of the environment by using most energetically favourable pathways possible

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Diversity of Chemosynthetic Environments

High abundance and (genetic) diversity of prokaryotes 

In situ chemosynthetic primary production supports faunal assemblages with high abundance/biomass

Faunal assemblages occur where reduced chemical for chemosynthesis available:

  • Hydrothermal vents
  • Cold seeps
  • Whale falls
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Hydrothermal Vents

Steep envrionmental gradients and endemic flora and fauna

Driven by geological processes where cold seawater seeps into Earth's crust, heated and then emerges, chemically altered

Vents found in mid-ocean ridges (plates move apart and heated seawater rises out of vent) and volcanic arcs (chains) - both different fauna

Mg and SO4 from seawater removed and H2S, Mn, Fe, Zn, Pb, H2, Ch4 are added

Primary (undiluted) vent fluids hot (<400C), acidic (pH3-5), anoxic and clear until it mixes with cold, oxygenated water and precipitates

  • 'Black smoke' chimneys - >225C with direct plumbing - precipitating mineral particles
  • 'White smoke' chimneys - 100-225C with greater mixing and cooling
  • 'Shimmering water' diffuse flow - lower temperatues due to differences in density

Highest temperatures only occur in primary vent fluid, background temperatures -1.5-4.5C = sharp temperature gradients

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Adaptations for Hydrothermal Vents

Hottest known microbe cultured at 122C

Most animals live at temperatures no different to shallow tropical marine - <40C

= Diffuse flows where most fauana is (no death due to high heat)

Polychates have highly variable environments - high inside burrow (81C), low outside (22C) = large temperature range required for survival

  • Pompeii worm (14-80C) - molecular structure of collagen adapted for thermotolerance
  • Sulfide form - tolerate 50-55C but prefers 25-50C
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Chemosynthesis at Hydrothermal Vents

Sulfide oxidation = dominant as H2S readily available in vent fluids and most energetically favourable

Some geological settings CH4 and H2 used

Oxygen readily available in background deep seawater = anaerobic pathways less imporant (may dominant high-temperature fluids)

Ways animals exploit chemosynthetic primary production:

  • Endosymbiotic relationships
  • Microbial epibionts (parasites on organism surface)
  • Grazing/sepension feeding of free-living microbes
  • Predation/scavenging on primary consumer animals
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Animal-Microbe Endosymbiosis - Giant Vent Tubeworm

  • Siboglinid polychaete
  • Large and fast growing
  • Ecosystem-structuring organism
  • 15% body weight is bacteria in little lobules - lots of blood flow around it
  • Endosymbionts need inorganic carbon - heterotrophic CO2 from tissues is 1/2 symbiont demand
    • CO2 produce by respiration and absorbed by seawater
    • Tubeworm blood alkaline - favours HCO3- = maintains diffusion gradient
  • Endosymbionts need sulfide - highly toxic to animal tissues (replaces oxygen at haemoglobin binding sites and poisons e- transport chain)
    • Tubeworm takes up HS- = less toxic
    • HS- also carried in blood by highly adapted haem molecule - sep bidning site for HS- as well as O2 = reduces toxicity in tissues
  • Endosymbionts need O2
    • Tubeworm haem high affinity for O2 = maintain uptake gradient
    • Supply of O2 and HS- separated in time due to fluctuating zone of mxing vent fluids
    • Haem affinity for O2 reduced at elevated temp = environment temp gradient along body to aid unloading of O2 at trophosome
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Animal-Microbe Endosymbiosis - Bathymodiolus musse

  • Host bacterial symbionts inside gill cells
  • Host dual symbionts - both sulfide-oxidising and methanotrophic bacteria
  • Ancestor species have extracellular gill symbionts - species living at whale falls today
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Epibiont Relationships

Epibiotic bacteria - roles include nutrition and detoxification

Nutrition example - Lepetodrilus limpet have bacteria in gill lamellae in transverse section

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Grazers and Filter Feeders

Primary consumers graze on bacteria in biofilms or as mats of filamentous bacteria

Grazers include limpets and polychaetes

Some species filterfeed on organic matter at vents in water column e.g. Eolepadid stalked barnacles

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Predators and Scavengers

Less predators and scavengers but still prevalent

Include crabs, zoarcid fish, anemones and octopus

Opportunistic 'non-vent' species that come in when food sources are low

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Zonation at Vents

Steep physico-chemical gradients including temperature, sulfide and O2 concentration

Energetic costs limits species' liveable ranges

Animals with endosymbiotic bacteria tend to be closer to vents

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Insular (Islandic) Nature of Vents

Vents occur in 'fields' - clusters of chimneys

Occurrence of fields depends on underlying geology - availability of heat source, pathways for circulation

Fast-spreading mid-ocean ridge (East Pacific Rise) - vent fields 10s km apart

Slow-spreading mid-ocean ridge (Mid-Atlantic Ridge) - vent fields 100s km apart

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Ephemeral (Short-Lived) Nature of Vents

Venting does not last forever

Volcanic eruptions/techtonic activity - disrupt 'plumming' of vents

How long vent lasts depends on how frequently disturbances occur (rate of geological activity)

Fast-spreading mid-ocean ridge (e.g. East Pacific Rise) - 10s years

Slow-spreading mid-ocean ridge (e.g. Mid-Atlantic Ridge) - 1000s years

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Cold Seeps

Environments where non-volcanic geological processes generate reduced chemicals = chemosynthesis

Very diverse types - salt dispair systems, mud volcanoes, asphalt seeps, gas hydrate beds

Found on continental margins - organic matter compressed, also occur in ocean trenches

Generallt soft-sediment seafloor settings

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Chemosynthesis at Cold Seeps

Geological processes forces organic compounds from deep reservoir up to seafloor sediments 

Examples include salt diapirism, tectonic compression of sediments

Organic compounds involved of ancient origin - hydrocarbons - degredation produces methane

Anaerobic subsurface microbes in sediments oxidise methane using sulphate = H2S

H2S provides energy source for chemosynthesis plus escaped methane

Typically a flux of sulfide and methane at seafloor where chemosynthetic microbes use reduced compounds

Animals expoit chemosynthetic primary productivity - endosymbiosis, epibionts, grazing, predation

Bicarbonate (waste product in reaction) provides solid structure in otherwise soft (mud) systems = animals can colonise

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Cold Seep Example - Brine Pool, North Gulf of Mexi

Underlying geological process is salt diapirism - dense layer of salt water over layers of organic hydrocarbons

Brine Pool anoxic and hypersaline

  • Pool surrounded by bed of mussels that habour gill endosymbionts
  • Grazers - gastropods and polychaetes
  • Scavengers - shrimp and squat lobsters

Species zonation within and around Brine pool mussel bed - gradients in sulfide flux and oxygen conditions

Siboglinid tubeworms occur beyond bed

  • Genera and species differ from hydrothermal vents
  • Key difference - seep tubeworms acquire sulfide via roots, not plumes
  • Large roots embedded in sediment 
  • Oxygen uptake via plume
  • Long-lived and slow growing
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Ephemeral (Short-Lived) Nature of Cold Seeps

Ephemeral at timescales of centuries (longer than vents)

Bicarbonate by-product of methane oxidation in underlying sediments = formation of biogenic carbonate rock = blocks seeping at site

Drives succession pattern at seeps - dominance by mussels (depends on fluids emerging) -> tubeworms (reach sulfide by roots) -> scleractinian corals (exploit hard-rock substratum from capping)

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Whale Falls

Wood falls also common in deep seas with shorelines with forests

Scavenger stage -> Opportunist stage -> Sulforphilic stage

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Chemosynthesis at Whale Falls

Degradation of whale carcasses results in cheemosynthetic environment 

Last for many years

Can occur in areas with limited food - scavangers originally colonise and ***** carcass (w/in months)

Whale bones - rich in lipids - broken down by anaerobic bacteria in heterotrophic processes, reducing seawater sulphate to sulfide = sulfide flux

Sulfide flux from bones - supports chemosynthetic primary production by other microbes

How big and where whale is, lengths of stages may vary

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Whale Fall Fauna

Not many animal species shared with vents and seeps, but do exploit in similar ways

Endosymbiont-hosting species = Adipicola mussels - both extracellular and intracellular symbionts in gills = flexible relationship so number of sources used for survival

Grazing species = Bathykurila guaymasensis - 'snowboarding scale worm'

Siboglinid polychaetes - Osedax spp. = 'bone-eating/zombie' worms

  • Heterotrophic rather than chemosynthetic nutrition
  • Female osedax have heterotrophic y-proteobacteria living in roots = break down bone material directly
  • Live off all types of bones
  • Males are dwarf parasites
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Ephemeral and Insular Nature of Whale Falls

Whale skeleton lipids depleted after decades

Natural mortality of whales = ~1,600 new carcasses each year

Typical distances may only by 10s km

7 natural whale falls studied so far

Processes studied by emplacement experiments

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Evolutionary History of Chemosynthetic Fauna

Vent animals have recent evolutionary origins - young ancestors

Species at chemosynthetics share evolutionary history with other deep-sea fauna - affected by climate changes, ocean circulation and deep-water anoxic events

Mass extinctions occur - recolonisation from shallow-water species

Taxa adapted first to whale falls, then vents and seeps = STEPPING STONE HYPOTHESIS

Radiations can occur between different types of chemosynthetic environments in a taxon

Vicariance (geographic separation) caused by geological changes in ridge activity and/or changes in hydrography important in speciation

Chemosynthetics are insular/ephemeral = dispersal of fauna by larval stages

Interruption to gene flow = movements of tectonic plates, ridges, contients, changes in ocean currents = speciation = biogeographic patterns

Best understood at hydrothermal vents but still poorly understood

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Significance of Chemosynthetic Environments

Chemosynthetic primary production - supports faunal assemblages w/ high abundance and biomass in deepsea

Insular and ephemeral - provide model system to study dispersal and evolution

Exploration only just begun

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