Potential Costs of Acclimatization to a Warmer Climate: Growth of a Reef Coral with Heat Tolerant vs. Sensitive Symbiont Types Alison Jones1*, Ray Berkelmans2 1Centre for Environmental Management, Central Queensland University, Rockhampton, Queensland, Australia, 2Australian Institute of Marine Science, Townsville,

Queensland, Australia

Environmental Science Graduate Program

CIAM 6117 Coastal Environment

Abimarie Otaño

Coral reefs are vulnerable to climatic change.

Coral survival rate depends on acclimatization to

warmer conditions by shuffling symbiotic

zooxanthellae algae, from thermal sensitive to a heat

resistant genotype.

To increase heat tolerance in a particular reef it must

occur zooxanthellae community shift of multiple coral

species.

Coral depends on the symbiont energy1 to carry

calcification process. External ion transport and

eventually CaCo3 precipitation.

1.Zooxanthellae function in the cora (rETRmax)l: Facilitates the nutrients needed for the secretion of calcium

carbonate skeleton. Produce 95% of the coral energy requirements through photosynthesis.

Introduction

Scleractinian are hard skeleton corals which polyps secrete

a high rate of carbonate, distinguishing as the primary reef

builder.

Benefits: coastal protection, carbon sink, provides habitat

for marine organism and touristic attractions.

Growth rate of this species determines reef resilience and

regeneration after extreme events (i.e. bleaching, hurricane

impacts, anthropogenic pressures).

Fast regeneration prevents phase shift directed to macro-

algae and soft corals dominance.

Study objectives: Determine physiological comparison

between type C2 (thermal sensitive) and type D (thermal

tolerant) symbiont function in Acropora millepora corals of

the Keppel island in the Great Barrier Reef, Australia.

Scleractinian order

4 genera, >160 species of Acropora in the Indo-Pacific

Acropora millepora: Near threatened species

1 genera, 2 species of Acropora in the Caribbean

Acropora palmata Acropora cervicornis

Acropora porifera

Scleractinian Acroporidae family

Listed as Endangered specie in the 2006. (ESA, 2006).

Field study, at reef slope of Miall Island,

NE Australia

43 pieces (15-20cm) of A. millepora colonies where cut and pruned to similar sizes.

Symbiodinum genotyped with Single Stranded Conformational Polymorphism(SSCP) analysis of the algal nuclear ribosomal DNA. Only colonies with intense bands where selected. March 2004 and May 2006.

Buoyant coral weight measurement, every 3 months from March to December 2005.

Growth measurement experiment after bleaching event in February 2006.

C2 and D colonies where placed in racks to allow recovery.

Methods

First experiment – before bleaching event in 2006

Growth rate of D colonies 38% lower that C2 (figure 3).

Growth varied with season, 71% higher in spring than in

winter (figure 4).

Results: Field study

Figure 3. Growth of A. millepora in the field

Figure 4: Seasonal growth

Second experiment- after bleaching event in February 2006.

Gained half of buoyant weight (figure 5).

Overall growth rate was 47% lower. Symbiodium was retained.

Highest growth rate rate in spring (76% lower than 2005).

Lowest growth rate in autumn and winter.

Results: Field study

Figure 5. Seasonal growth rate before and after bleaching.

16 colonies where transplanted from Keppel Islands to

Magnetic island to allow recovery and acclimatization.

6 explants (9 colonies type C2 and 7 type D) where cut and

distributed in three tanks.

Controlled temperature conditions 23oC (spring/autumn non-

stressful) and 29oC (summer stressful conditions).

Coral where fixed to plastic stand and rotated 180o daily to

allow enough light exposure.

Approximate natural diurnal light cycle: 3.5h shaded light, 5h

un-shaded, 3.5h shaded and 12h darkness.

Photosynthetically active radiation measurements F0 and Fm.

Fluorumeter (Fv/Fm): to monitor health of explant after dark-adapted max yield, assessed each morning after 8 hours of darkness.

Laboratory Australian Institute of Marine Science

Methods

Zooxanthellae densities and pigments:

Explants where frozen (-20oC) and tissues where

stripped with air gun. Volume was homogenized for 20s

Zooxanthellae count on 8 independent drops with a

compound light microscope.

Centrifugation for 5s at 4oC separated algal pellet.

Absorbance was measured with a spectrophotometer.

Total Chlorophyll a was calculated from the equation of

Jeffrey and Haxo.

Methods: Laboratory

Buoyant weight gained in explants was 29% less in

colonies with type D symbiont than in type C2 (figure

1).

Zooxanthellae density for type D colonies was 22%

lower.

Zooxanthellae density at 29oC density was 21% lower

than 23oC.

Chlorophyll a in type D was 16% lower.

Chlorophyll c2 in type D was 17% lower.

Concentration of chlorophyll a and chlorophyll c2 at

29C was 20% and 19%, respectively, higher than 23C.

Results: Laboratory study

Figure 1. Growth rate in the laboratory

Figure 2. Algal density and chlorophyll pigments

Points to discuss:

1. In your opinion, corals zooxanthellae shift to thermal resistant

genotype is beneficial or prejudicial to the reef community?

2. What are the main environmental factors that might influence

the laboratory and field studies? Which method provides the

most reliable results?

3. What can be done to ensure the coral reef diversity and

functions taking into consideration climate change

projections?

Growth rate is affected by:

1. Symbiont shuffling to thermally tolerant type after thermal

stress.

2. Bleaching stress.

✽Type D symbiodinium colonies had lower growth rate in

comparison with type C2, even in non-stressful conditions.

✽Shuffling to type D and C1 thermal tolerant symbiont

ocurred in A. millepora at Miall Island after bleaching event in

2006.

✽Coral growth reduced by 56% after bleaching.

✽Acclimation by shuffling to thermal resistant symbiont

reduce growth but improve heat tolerance and survival.

Discussion

✽Growth differed by a 50% in juvenile A. millepora between

type D and C1 symbiont (Mieog et. Al. ).

✽ rETRmax 87% higher in juvenile corals with type C1, correlates

with a double 14C fixation.

Lower rETRmax and growth in type D symbiont:

✽ Result of the retaining of photosynthetically fixed carbon for

metabolism and repair.

✽ Increased use of energy for respiration.

✽ Increased rate of photo-inhibition and reduces

photosynthesis.

Growth rate of A. millepora is affected by the shift to thermal-resistant zooxanthela.

In the long term, heat tolerance and resilience benefits are much greater due to the expected climatic change.

Further research is required to truly quantify the effect of symbiont genotypes on diverse coral growth as they acclimatize to climate change.

Evidence is needed to determine if there exist a correlation between thermally sensitive symbiont and a reduce photosynthesis carbon fixation in other scleractinian corals.

Conclusion