Transcript Corals - Structure and Function I

Coral Structure and Function I
Maritime coastal
- greenish
- particulate
Caribbean
- blue
- clear
• BLUE
– water reflects blue of the sky
– water refracts sunlight (more blue light)
– no interference from green plants
• BLUE
– water reflects blue of the sky
– water refracts sunlight (more blue light)
– no interference from green plants
• CLEAR
– little particulate matter
– few phytoplankton in the water
• PHYTOPLANKTON
– microscopic algae - flourish in colder ocean waters
– live in upper 60m - the PHOTIC ZONE
– give local Maritime waters their colour
• as you descend through water column
– lose more and more light
– reds go first (lower energy)
– gives a bluish cast to everything
• as you descend through water column
– lose more and more light
– reds go first (lower energy)
– gives a bluish cast to everything
• much more pronounced locally than in
the Caribbean
– we have far more photosynthetic
organisms in the water
– absorb the light (red & blue ) for
photosynthesis - leaves green
• So- the blue colour & clear water of tropics
due to few photosynthetic organisms in
tropical waters
• Tropical waters are still very PRODUCTIVE
• bottom of food chain events
– primary production
– production of organic material from inorganic
• trophic pyramids - find plants at the bottom
– use SUNLIGHT energy to fix CO2 into
organic molecules
– Primary Production
• plants consumed by primary consumers etc.
• less total biomass as you go up the pyramid
• increase size of organism as you go up the
pyramid
• eximine coral reefs ecosytem:
• “how does this flourishing ecosystem survive with so
few producers - the plants ” ?
• clear water, few phytoplankton ???
• In the reef system primary production is
mostly BENTHIC (bottom)
• Open ocean (or local Maritime), primary
production is mostly PELAGIC (water
column)
• Much of the productivity from corals
• Cnidaria - from the Latin “nettle” – a plant
• have often been mistaken for plants
– attached to a substrate
– do not wander about
– same colour as many
marine plants
– same branched nature
and growth habit
• were originally classified as plants
• by the naturalist John Ray (1627-1705)
• In 1723, Jean Peyssonel
decided they were animals
• naturalist John Ellis 1776
• a microscope modified for aquatic work
• found the animal polyps on many reef organisms
• then considered to be animals for a while - with no plant
component
• improvements in microscopy confirmed their animal
nature, with polyps filtering out plankton with their
tentacles
• subsequent studies showed that the reef is composed of
many organisms, as well as the Cnidarians
• The Royal Society Coral Reef Expedition 1896-1898
• Funafuti Atoll
•
analysis of cores - mostly:
1. Calcareous red algae
2. Calcareous green algae (Halimeda)
3. Foraminifera (20-40m protists, porous CaCO3 shell)
4. Corals
• Top 18m of the core was 80-90% Halimeda
• Calcareous red algae
• Calcareous green algae (Halimeda)
Foraminifera
• Corals
• 20C - new understanding of trophic pyramids,
attention turned to reef productivity
–
–
–
–
very productive (produce lots of biomass)
lots of life
lots of diversity
productivity couldn’t be due just to the calcareous
green and red algae
• so where were the primary producers ??
• Extensive examination of atolls (Eniwetak –
Marshall Islands)
• lots of encrusting algae on the surface of
corals, but also ...
• examine corals in more detail
• true nature of the Cnidarians
• algae growing inside the cells of the coral
polyp
• These algae - ZOOXANTHELLAE
• enough algae inside the coral polyp to
account for massive primary production
• their presence explained the plant-like growth
habit of the Cnidarian – to increase surface area for light absorption
• Also explained the colours of the corals
• 1950s - Tom & Gene Odum
• suggested the coral polyp and the alga were
in some sort of mutualistic relationship
– the polyp itself is a miniature ecosytem
– the two organisms exchange nutrients and
other benefits
• Corals are predacious animals - suspension
feeders
• two main methods of prey capture
– nematocysts
– mucus
• extend tentacles - mostly at night
– zooplankton are most plentiful (move up from
deeper waters)
• whole surface of the coral becomes a trap for
plankton
• paralyze prey
– sting with NEMATOCYSTS
• trap prey
– sticky MUCUS on
tentacles
• tentacles produce WAVE-LIKE action
sweeping the mucus and prey into the mouth
• down the pharynx (gullet) to the
gastrovascular cavity for digestion
• tentacles produce WAVE-LIKE action
sweeping the mucus and prey into the mouth
• down the pharynx (gullet) to the
gastrovascular cavity for digestion
• prey digested, mucus recycled, solid,
undigestible material (eg silt) ejected
• Keep tentacles retracted during the day
– help corals avoid predation
– protect from UV
• Corals also get some nutrients from seawater
–
–
–
–
dissolved amino acids
glucose
inorganics
not usually much, except in locally polluted areas
• structure of the polyps and skeleton of the
coral is a simple combination
• Most hermatypic scleractinian corals
– colonies of polyps
– linked by common gastrovascular system
(coenosarc)
• polyp made up of two cell layers
– outer epidermis (or ectoderm)
– inner gastrodermis (endoderm)
• non-tissue layer between gastrodermis and
epidermis = mesoglea
– made of collagen & mucopolysaccharides
• "lower layer" of epidermis = calicoblastic
epidermis
– secretes the calcareous external skeleton
• "upper layer" of epidermis is in contact with
seawater
• The corallite is the part of the skeleton
deposited by one polyp
• The skeletal wall around each polyp is called
the theca
• The coral structure also includes calcareous
plate-like structure known as septa
• One of the epidermal cell types is the
cnidocyte
– contains organelles called nematocysts
– discharge toxic barbed threads
– capture zooplankton prey
• gastroderm cells line the body cavity
– capable of phagocytosis (food particles)
– contain the intracellular algae
– extend into tentacles
• zooxanthellae not in direct contact with the
cytoplasm of the coral gastroderm cell
• zooxanthellae reside inside a vacuole
– the symbiosome (animal origin)
• Much of the food needed by the polyp comes
from the SYMBIONT
• Many corals have different growth forms - can
vary with local environment - light, depth etc.
• Local environment affects distribution of the
zooxanthellae
• Zooxanthellae:
– ZOO - animal
– XANTHE - gold-coloured
• single-celled alga, with 2 flagellae
– a dinoflagellate
• spherical, 8 - 12um dia
• Most dinoflagellates are free-living
– unusual group of algae
– feeding modes ranging from photosynthetic
autotrophy to heterotroph
• Many dinoflagellate produce toxins
– e.g. ciguatoxin causes ciguatera "fish
poisoining”
• Other toxic dinoflagellates responsible for
algal blooms
– e.g. red tides (Gymnodinium)
– paralytic shellfish poisoining (Alexandrium)
• dinoflagellates
– chlorophylls a and c
– lack chlorophyll b
– characteristic dinoflagellate pigments
diadinoxanthin and peridinin
• ~ 3 x 106 cells/cm2
• coloured tinge to the coral
• brown to yellow brown
• Zooxanthellae can live outside their host
– essential in some species for finding a host
• Dinomastigotes stage
– motile free-living state, have two flagellae
• Coccoid stage
– living in animal cells, lack flagellae
• In culture, zooxanthellae alternate between
coccoid and dinomastigote stages
• Almost all zooxanthellae are in the
dinflagellate genus Symbiodinium (1959)
• taxonomy of Symbiodinium in a state of flux
• 1980 - Symbiodinium microadriaticum
assumed to be the one species found in
almost all corals
• Recent work
– great genetic diversity in zooxanthellae
– clearly more than one species
– at least 16 different algal taxa
– zooxanthellae found in closely related coral
species not necessarily closely related themselves
– zooxanthellae found in distantly related coral
species may, in fact, be closely related
– may have multiple species in same coral
Acquisition of Zooxanthellae by Corals
either
1. open (or indirect) transmission or acquisition
– from the environment
or
2. closed (or direct) transmission or acquisition
- via gametes or
- during asexual reproduction
• Indirect acquisition
– provides potential for host to establish a symbiosis
with a different strain or species of zooxanthellae
than was in symbiosis with the host’s parents
• Coral bleaching
– may also allow establishment of new symbiosis
with different zooxanthellae strain,
– has been proposed as a possible adaptive
mechanism to environmental change
• Shifting symbioses
– controversial topic
• In all hermatypic corals endosymbiotic algae
provide an important source of nutrients
• can demonstrate mutualistic relationship
• feed 14CO2 to the coral
– quickly taken up by alga and ends up in the polyp
• feed zooplankton raised on 15N to coral
– quickly taken up by polyp and ends up in the alga
• clear they exchange a lot of material
– benefit each other
• reef-shading experiments
– 3 months in the dark
• algae expelled from the polyps
• later the polyps died
• Most coral polyps have absolute requirement
for alga - but not vice-versa
• MUTUALISM - benefits for algae?
– shelter
– protection from nematocysts, & other predation
– receive waste products of polyp - CO2 & N
• N is v.limiting in marine environment
– the major limitation to plant growth
– algal blooms occur in response to
small changes in N
– pressure exists to optimize N scavenging
– favours such a mutualistic relationship
• Disadvantage
– algae restricted to shallow tropical waters
• MUTUALISM - benefits for polyp?
– food (CHO)
– O2
– greatly increased ability to precipitate CaCO3
– without the alga, coral could not have such a high
rate of metabolism
• could not build such extensive reef structures
• Polyp can survive extended periods with no external
food source
• Tight internal N-cycling and algal PS
• Polyp lays down extensive lipid reserves to be drawn
on in times of starvation
• High light and high food availability
– ejection of pellets containing viable algal cells
• Control of algal cell number ?
• Algae divide within host polyp
• Analyze algal cell
– C,H,O from PS
– N,P,S, from host (normally limiting)
• Symbiosis controlled by host
• Polyp controls permeability of algal membrane
• “signal molecules”
• Freshly isolated zooxanthellae
• Incubate in light with 14CO2
• Release very little organic C into medium
• Add some polyp extract - releases lots of
organic carbon into medium
• Other cnidarian extracts work
• Alga donates most of it’s fixed C to polyp
– used for resp, growth, etc.
• Polyp respires
– releases CO2 to alga
• Polyp excretes N waste - NH3
– used by alga
• Polyp also releases PO4-, SO4-, NO3- to alga
– 1000x more conc. than in seawater
– Algae grow faster - helps polyp