Coral Reefs Flashcards
What are corals?
Most primary reef-building corals are colonial
calcifying Anthozoans
Anthozoa is a Class of ~6,000 species within Cnidaria that exist only as polyps, never as medusae
True stony corals are in the order Scleratinia
Coral Biology
Scleratinian reef corals form an aragonite corallum (colonial skeleton) composed of individual cuplike corallites around and under each polyp
Aragonite is one of two common naturally occurring crystal forms of calcium carbonate, CaCO3
Coral Growth
Growth occurs on top of previous calcification. Growth
in some Acropora corals can be >100mm annually
Large coral colonies can grow to several metres in height and weigh several tons, but the living tissue is typically <1mm thick on surface
Rates of calcification are controlled by both the function of a specific calicoblastic tissue (probably via direct roll in crystal formation), and photosynthetic contributions of symbiotic zooxanthellae
As a general rule, the large, reef-building (hermatypic) Scleratinian corals typical of shallow tropical seas (i.e. primary reef builders) are
zooxanthellate, i.e. contain algal symbionts
The Zooxanthellae
Symbiosis
Most scleratinian corals in shallow, tropical waters the have a symbiotic relationship with dinoflagellates of genus Symbiodinium
> 100 genotypes of Symbiodinium have been identified, some widely distributed across coral species, others restricted to particular hosts or environments
Zooxanthellae are packed within host gastrodermal cells (i.e. intracellular symbionts), 1-3 or more per cell, with typically c. 1,500,000 symbiont cells per cm2
The Three Roles of Zooxanthellae
Influence calcification process
Provide nutritional material to their hosts
Take up host waste products
Light-Enhanced Calcification
Light, photosynthesis, and calcification are
intimately linked
A cloudy day can reduce carbonate deposition
by 50%
Carbonate production 3x higher during day than
at night
Zooxanthellae and Coral
Nutrition
Carbon fixed by photosynthesis is translocated to the host in the form of animo and fatty acids, glucose, and glycerol
In some conditions photosynthesis respiration ratio can be >1 i.e. all metabolic requirements met by photosynthesis, and the symbiosis is autotrophic
This varies substantially with light, depth, season, habitat, etc.
Most corals do feed heterotrophically too, which significantly increases both tissue growth and calcification rates
Uptake of Waste Products
Zooxanthellae take up from their hosts significant quantities of nutrients including phosphorus and nitrogen
They transform these into organic compounds
and transfer them back to their hosts
Ammonium and nitrate are potentially toxic waste products to the coral, so the zooxanthellae act as metabolic kidneys
Coral Basics: Summary
The primary reef building organisms are simple animals that rely on symbiotic zooxanthellae in order to function efficiently
The symbiosis enables reef building corals to
exist in nutrient-poor shallow tropical waters
Some of the most biodiverse, complex, and productive ecosystems on earth are dependent on the success of this symbiosis
Corals and Stress
Reef corals are generally intolerant of environmental change, including:
- reduced salinity
- prolonged low tides
- disease
- pollution
- temperature extremes
- high light levels
Coral Stress Responses
At threshold levels:
- increased mucus secretion, polyp withdrawal
Beyond physiological thresholds:
- zooxanthellae symbionts are expelled = coral bleaching
Coral Bleaching
Complex reaction based on the physiology, biochemistry and genetics of both partners
The most obvious cases of bleaching result in the host tissue becoming transparent
This reveals the bleached white aragonite skeleton beneath
Not all bleaching is worrying
Sometimes it reflects natural seasonal variation
These cyclical changes are called physiological bleaching
Severe bleaching is accompanied by a loss of 60-90% of symbiont cells
There is usually an accompanying reduction in host tissue biomass
Bleaching is thus more than just a loss of photosynthetic capacity
Temperature and bleaching
Most reef corals are sensitive to small increases in temperature
Just 1-2°C above normal summer highs is sufficient to induce bleaching
El Niños generate wide swaths of such conditions, with sea temperatures >30-32°C widespread in the tropics
Previous mass bleachings
The super El Nino of 1998 caused coral mortality across the tropics
Coral coverage was reduced by >90% in many parts of the Indian Ocean (similar patterns in the pacific, carribean)
This bleaching event was unprecedented in global scope and severity
16% of the worlds corals were lost in shallow water reefs
Extreme el nino events are a major cause of mass bleaching
anomalously hot conditions not associated with el nino are also important
climate change interacts with natural processes to increase the severity and frequency of bleaching events
Interactions with other stressors make results more serious and make recovery less likely
Ecosystem effects of coral bleaching
corals provide habitat for a huge diversity of other organisms
many species entirely dependent on healthy corals for habitat and for food
coral cover and topographic complexity particularly important for fish
climate change now a major threat for reef fish communities
“In 2016, record ocean temperatures have led to record widespread coral bleaching on Australian coral reefs.
This bleaching is part of the ongoing third global bleaching event… Between February and May, the Great Barrier Reef experienced record warm sea surface temperatures.”
Surface temperature of the earth
will rise 4°C
Other stressors (e.g. acidifications, over-exploitation) are
reducing the capacity of corals to recover
Tropical reef systems are transitioning into a new era in which the interval between recurrent bouts of coral bleaching is too short for a full recovery of mature assemblages
The median return time between pairs of severe bleaching events has diminished steadily since 1980 and is now only 6 years
Annual bleaching may
become the norm in the coming decades
