Download Tropical Marine Biology Productivity and the Coral Symbiosis

4701 Tropical Marine Biology
Productivity and the Coral
Symbiosis
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
• 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 blue 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
• 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
• Funafutii Atoll (Ellice Islands)
•
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 (Eniwetok)
• 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
• 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
• The septa radiate from the wall towards the
center of the corallite
• 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
• 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 10 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
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 (but the freeliving alga does get eaten !)
• 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 efficiency in precipitating
CaCO3
– without the alga, coral could not have such a high
rate of metabolism and 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 algal
– 1000x more conc. than in seawater
– Algae grow faster - helps polyp
Calcification - growth of the reef
•
Algal PS: 90% fixed C to coral host
•
Used for metabolic functions
•
•
Growth, reproduction &
Calcium deposition
•
What sort of calcium ?
•
CaCO3
• In ocean, mostly find 3 forms of CaC03
• Calcite
– Mostly of mineral origin
• Aragonite
– Fibrous, crystalline form, mostly from corals
• Magnesian calcite
– Smaller crystals, mostly plant origin
• Examples:
– Molluscs
– Corals
– Some green algae
– Red algae
– Sponges
– Some bryozoans
calcite & aragonite
just aragonite
just aragonite
magnesian calcite
aragonite (with
silica)
all 3
• Corals
–
–
–
–
–
–
–
remove Ca++ & CO3-- from seawater
Combines them to CaCO3
transports them to base of polyp
Calcicoblastic epidermis
minute crystals secreted from base of polyp
Energy expensive
Energy from metabolism of algal PS products
CO2 and seawater
• What forms of C are available to the coral ?
• Organic and inorganic forms
• DIC - dissolved inorganic carbon
– CO2 (aq)
– HCO3– CO3--
• DIC comes from:
– Weathering
– dissolution of oceanic rock
– Run-off from land
– Animal respiration
– Atmosphere
– etc.
• DIC in ocean constant over long periods
• Can change suddenly on local scale
– E.g. environmental change, pollution
• Average seawater DIC = 1800-2300 mol/Kg
• Average seawater pH = 8.0 - 8.2
• pH affects nature of DIC