Coral Reefs

Question 1

What are corals?

Answer 1

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

Question 2

Coral Biology

Answer 2

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

Question 3

Coral Growth

Answer 3

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

Question 4

As a general rule, the large, reef-building (hermatypic) Scleratinian corals typical of shallow tropical seas (i.e. primary reef builders) are

Answer 4

zooxanthellate, i.e. contain algal symbionts

Question 5

The Zooxanthellae

Symbiosis

Answer 5

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

Question 6

The Three Roles of Zooxanthellae

Answer 6

Influence calcification process
Provide nutritional material to their hosts
Take up host waste products

Question 7

Light-Enhanced Calcification

Answer 7

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

Question 8

Zooxanthellae and Coral

Nutrition

Answer 8

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

Question 9

Uptake of Waste Products

Answer 9

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

Question 10

Coral Basics: Summary

Answer 10

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

Question 11

Corals and Stress

Answer 11

Reef corals are generally intolerant of environmental change, including:

• reduced salinity
• prolonged low tides
• disease
• pollution
• temperature extremes
• high light levels

Question 12

Coral Stress Responses

Answer 12

At threshold levels:
- increased mucus secretion, polyp withdrawal

Beyond physiological thresholds:
- zooxanthellae symbionts are expelled = coral bleaching

Question 13

Coral Bleaching

Answer 13

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

Question 14

Not all bleaching is worrying

Answer 14

Sometimes it reflects natural seasonal variation

These cyclical changes are called physiological bleaching

Question 15

Severe bleaching is accompanied by a loss of 60-90% of symbiont cells

Answer 15

There is usually an accompanying reduction in host tissue biomass

Bleaching is thus more than just a loss of photosynthetic capacity

Question 16

Temperature and bleaching

Answer 16

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

Question 17

Previous mass bleachings

Answer 17

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

Question 18

Extreme el nino events are a major cause of mass bleaching

Answer 18

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

Question 19

Ecosystem effects of coral bleaching

Answer 19

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

Question 20

“In 2016, record ocean temperatures have led to record widespread coral bleaching on Australian coral reefs.

Answer 20

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.”

Question 21

Surface temperature of the earth

Answer 21

will rise 4°C

Question 22

Other stressors (e.g. acidifications, over-exploitation) are

Answer 22

reducing the capacity of corals to recover

Question 23

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

Answer 23

The median return time between pairs of severe bleaching events has diminished steadily since 1980 and is now only 6 years

Question 24

Annual bleaching may

Answer 24

become the norm in the coming decades