Transcript Polyp

Productivity and the Coral
Symbiosis II
• 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
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.
Alga stores CHO – starch
•
•
Broken down at night
Polyp stores lipid – fat bodies
•
•
Energy reserve
•
Algal PS: 90% fixed C to coral host
•
Used for metabolic functions
•
•
Growth, reproduction &
Calcium deposition