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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