Transcript Tropical Marine Biology Productivity and the Coral Symbiosis

Coral Structure and Function II
– Primary Production
– Benthic
– Cnidaria
– Atoll analysis
1. Calcareous red algae
2. Calcareous green algae (Halimeda)
3. Foraminifera (20-40m protists, porous CaCO3 shell)
4. Corals
– Coral polyp – predacious
• Endosymbiont – zooxanthellae
• 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
• 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
mucus
nematocysts
• 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
– now dozens of 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
Usually a single
Symbiodinium clone in a
coral colony.
Sometimes two.
Adjacent colonies have
same or similar
Symbiodinium
populations.
Symbiodinium Diversity Patterns
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
Davy et al: Microbiol. Mol. Biol. Rev. June 2012 vol. 76 no. 2 229-261
• 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
• “host release factor” HRF
• Various suggestions:
– 10kDa protein
– Free amino acid suite
– Mycosporine-like amino acids
– Taurine
• Tension between HRF and PIF
– photosynthesis-inhibiting factor
Davy et al: Microbiol. Mol. Biol. Rev. June 2012 vol. 76 no. 2 229-261
• 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
FOOD
Polyp
Protein
CHO
Lipid
AAs
Sugars
Fatty acids
Growth &
metabolism
ATP
NH3
CO2
NH3
CO2
O2
O2
glycerol
AAs
AAs
Sugars
Fatty acids
LIGHT
ATP
NADPH
Protein
PO4-
PO4-
SO4-
SO4-
CHO
Growth &
metabolism
H2O
Alga
H2O
Mar Drugs. 2010; 8(10): 2546–2568.
Calcification - growth of the reef
Alga stores CHO – starch
•
•
Polyp stores lipid – fat bodies
•
•
•
Broken down at night
Energy reserve
Algal Photosynthesis
• base of reef productivity
• energy source for reef building
• Huge ATP demand
• Overall productivity of the reef:
4.1 - 14.6 gC/m2/d
• this is organic carbon production
• must also consider carbonate production
(deposition of physical structure of the reef)
– Get about half of this from the coral
symbiosis
– the rest from the calcareous green & reds
algae
CALCAREOUS ALGAE (greens & reds) are major
contributors to reef calcification
– the more flexible magnesian calcite
• last 30 years - role of these algae receive more
attention
– play a much bigger role in calcium deposition than
previously thought
• 10% of all algae CALCIFY (about 100 genera)
• 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
Calcification
Calcite
Aragonite
Magnesian calcite
(Mg carbonate)
• Examples:
organism
Molluscs
Corals
Some green algae
Red algae
Sponges
Some bryozoans
CaCO3
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
Calcification