11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 25: Predicting Reef Futures in the Context of Climate Change
25-42
Factors Affecting The Evolution Of Bleaching Resistance in Corals
Julian CALEY* 1 , Troy DAY 2,3 , Laura NAGEL 4 , Madeleine VAN OPPEN 1
1 Australian Institute of Marine Science, Townsville, Australia, 2 Dept. of Mathematics and
Statistics, Queen’s University, Kingston, ON, Canada, 3 Dept. of Biology, Queen’s
University, Kingston, Canada, 4 Dept. of Biology, Queen’s University, Kingston, ON,
Canada
We present a mathematical model of coevolutionary interactions between partners in a
coral-algae mutualistic symbiosis. Our goal is to better understand factors affecting the
potential evolution of bleaching resistance in corals, in response to increased average sea
temperatures. We explore the evolutionary consequences of four factors: (i) tradeoffs
among fitness components, (ii) different proximate mechanisms of coral bleaching, (iii)
the genetic determination of bleaching resistance, and (iv) the mode of sexual
reproduction. We show that traits in mutualistic symbioses, such as thermal tolerance in
corals, are potentially subject to novel kinds of evolutionary constraints, and that these
constraints are mediated by ecological dynamics. We also show that some proximate
mechanisms of bleaching yield faster evolutionary responses to temperature stress, and
that the nature of interspecific control of bleaching resistance and the mode of sexual
reproduction interact to strongly influence the rate of spread of resistance alleles. These
qualitative theoretical results highlight important future directions for empirical research
in order to quantify the potential for coral reefs to evolve resistance to thermal stress.
25-43
Estimating Potential For Adaptation Of Corals To Climate Change
Nikolaus CSÁSZÁR* 1,2 , Peter RALPH 1 , Richard FRANKHAM 3,4 , Ray
BERKELMANS 2 , Madeleine VAN OPPEN 2
1 Department of Environmental Sciences, University of Technology, Sydney, Sydney,
Australia, 2 Australian Institute of Marine Science, Townsville, Australia, 3 Macquarie
University, Sydney, Sydney, Australia, 4 Australian Museum, Sydney, Sydney, Australia
Climate models predict rising global sea-surface temperatures accompanied by increasing
frequencies of coral mortalities and shifts in ecosystem structure throughout the 21st
century. Today, it is well known that geographically different coral populations display
substantial intra-specific variation in thermal tolerance. This suggests local adaptation to
different temperature regimes. The ability of the coral/algal symbiosis to adapt to further
increasing sea-surface temperatures in an evolutionary sense will depend on the extent of
the underlying genetic variation in thermal tolerance. We investigated the genetic- and
environmental variation in two phenotypic traits that are directly linked to thermal
tolerance in corals. A controlled temperature experiment was conducted on a
predominantly clade D zooxanthellae harbouring population of the widely abundant
scleractinian coral species Acropora millepora from the Great Barrier Reef, Australia.
Under sub-bleaching (31 ºC) and bleaching (32 ºC) conditions, the proportion of the
observed phenotypic variation that was due to genetic factors (i.e. the heritability in the
broad-sense; h 2 ) was high for both traits: the decrease in maximum capacity of
photosystem II (Fv/Fm) for in hospite zooxanthellae, and the increase in coral growth
rates. Under control conditions (27 ºC), however, the phenotypic performance of these
traits was determined to a lesser extent by genetic factors. This suggests that under high
temperatures, there is an underlying heritable component for both the maximum
efficiency of photosystem II in the zooxanthellae, and coral growth rates. Additional
thermal-stress related phenotypic traits for this particular population are currently under
investigation.
This is the first study to show that thermal tolerance in corals has an
underlying genetic component, and provides direct insight into the
evolutionary potential of this species to adapt to climate change.
25-44
Coral Mortality From Bleaching: Pre-Adaptation Of Host Corals To Increased Seawater
Temperatures With Dependence Upon Rapidly Adapting Zooxanthellae
Kevin STRYCHAR* 1 , Paul SAMMARCO 2
1 Texas A&M University - Corpus Christi, Corpus Christi, TX, 2 Louisiana Universities Marine
Consortium, Chauvin, LA
Bleaching in corals is the loss of obligate symbiotic zooxanthellae (required for growth and
survival) from their tissue. Increased sea-surface temperatures (SSTs) associated with climate
change have caused mass coral bleaching on a global basis. This in turn has caused widespread
coral mortality, increasing in frequency through time. Some studies suggest that the host
‘ejects’ the symbiont during these stressful periods, but the mechanism(s) by which this occurs
is still unknown. Recent evidence suggests that processes of cell death, apoptosis and necrosis,
associated with zooxanthellae cells increase within host tissues as temperatures increase and
may be the primary causes of zooxanthellar loss. The host response to increased temperatures
is not known, however, nor is the fact of “who releases whom” during the bleaching process.
Here we determine whether the host coral or the zooxanthellae symbionts are more temperaturesensitive
and which is being subjected to higher selection pressure due to temperature.
Assessment of two types of cell death, apoptosis and necrosis, of cell and tissue states was done
by flow cytometry, fluorescent microscopy, and transmission electron microscopy. We
examined the temperature resistance properties of three scleractinian coral hosts - Acropora
hyacinthus, Porites solida, and Favites complanata - and their zooxanthellae by exposing them
to experimental temperatures of 28, 30, 32, and 34C for 48 hrs. Coral host cells did not exhibit
any signs of apoptosis until exposed to temperatures of 34C. Zooxanthellae cells, however,
exhibited apoptosis at temperatures as low as 30C, and this effect increased markedly in the
symbionts as temperature increased. These findings suggest that the host corals are pre-adapted
to temperature increases and not suffering mortality from this stress but from the loss of their
zooxanthellar endosymbionts – a process driving natural selection in Symbiodinium and
necessitating their rapid adaptation to a changing environment.
25-45
Climate Change Can Supersensitise Corals To Natural Levels Of Ultra Violet Radiation
Ruth REEF* 1 , Sophie DOVE 1 , Maya CARMI 1 , Ann MOONEY 1 , Paulina KANIEWSKA 1 ,
Oren LEVY 1 , Ove HOEGH-GULDBERG 1
1 Centre for Marine Studies, The University of Queensland, St Lucia QLD, Australia
Corals are exceptional for their ability to tolerate ultra violet radiation (UVR) intensities that
can be lethal to many reef organisms. Their sessile nature combined with their obligatory
symbiosis with the photosynthetic dinoflagellate Symbiodinium means exposure to solar
radiation, five percent of which is harmful UVR, is high. While some of the UVR protection
associated with corals comes from the previously documented UVR screening compounds in
the tissue, we find that this is complemented by unique properties of the coral skeleton that
reduce the intensity of UVR in the overlying tissue. This property becomes even more
important during heat stress, when we demonstrate that a vast and rapid reduction in the UVR
screening ability of the tissue occurs and that this leads to a significant increase in the amount
of UVR induced DNA damage in the cells. However, when grown under high levels of carbon
dioxide (and hence low concentrations of carbonate ions) some of the ability of the skeleton to
reduce the levels of UVR in the overlying tissue is lost. Thus, the remarkable ability of corals to
thrive in the UVR levels of shallow tropical waters might be greatly lessened by the predicted
changes to sea surface temperature and carbon dioxide levels. This research therefore indicates
yet another subtlety associated with the change in environmental conditions surrounding coral
reefs associated with rapid anthropogenic climate change.
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