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

• Funafuti Atoll (Ellice Islands - Tuvalu)

• 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 (20-40m 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

• 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 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

• 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