Chemosynthetic Environments
- 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)
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)
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
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
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
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
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
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
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
Epibiont Relationships
Epibiotic bacteria - roles include nutrition and detoxification
Nutrition example - Lepetodrilus limpet have bacteria in gill lamellae in transverse section
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
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
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
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
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
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
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
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
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)
Whale Falls
Wood falls also common in deep seas with shorelines with forests
Scavenger stage -> Opportunist stage -> Sulforphilic stage
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
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
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
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
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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