Assessing ocean acidification impacts on the reef building properties of crustose coralline algae
Date
2015
Authors
Nash, Merinda Catherine
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Abstract
Crustose coralline algae (CCA), and in particular Porolithon
onkodes, play an important reef-building role in modern tropical
coral reefs. CCA form thick crusts of Mg-calcite and grow over
corals and loose substrate to bind these together. This binding
and cementing process is fundamental to the development of
structural reefs that are capable of withstanding the high-energy
waves in the shallow to inter-tidal areas of the reef. As
anthropogenic CO2 emissions continue to increase, the oceans
absorb part of this extra CO2 and become more acidic, a process
known as Ocean Acidification (OA). There are concerns that OA
will have a negative affect on the reef-building capacity of
coral reef organisms, in particular on CCA. This is because
Mg-calcite is meta-stable and more susceptible to dissolution
than aragonite, the mineral used by corals to build skeletons.
The goal of this thesis work was to firstly understand the
physical and mechanical properties that enable the CCA to cement
the reef and withstand damage from high-energy waves, bioerosion
and chemical dissolution. Secondly, to anticipate how OA may
interfere with these reef-building properties. These goals were
pursued by setting clear aims with associated specific objectives
designed to elucidate information relevant to these questions.
Methods were developed for X-ray diffraction to identify the
mineral composition of CCA. Nanoindentation was investigated as a
tool for determining the mechanical properties of CCA and the
measurement of fracture toughness was found to return physically
meaningful information relevant to structural reef development.
Study of CCA calcification showed that cell wall Mg-calcite
exhibited radial crystal morphology in agreement with published
studies on temperate species. However, high-resolution imaging
showed the radial crystals were made of banded stacked sub-micron
grains within an organic framework. Dolomite was found not only
as cell lining by submicron rhombs, but also as the primary
calcification of hypothallial cell walls. Dolomite is shown to be
resistant to bacterial erosion. A model is developed whereby it
is proposed that dolomite formation is dependent on
polysaccharide accumulation.
Using nanoindentation, P. onkodes are found to be extraordinarily
tough, on par with the measured fracture toughness for
metamorphic minerals quartz and corundum. The fracture toughness
is enabled by the presence of dolomite cell lining. Contrary to
the literature, bacterial erosion is found to be a constructive,
not destructive, process.
A survey of P. onkodes from Heron Island fore reef and reef flat
showed that dolomite was present in all the fore reef crusts but
none of the reef flat crusts. The reef flat crusts did not have
fracture resistance except where remineralised. The presence of
dolomite cell lining was shown to decrease skeletal dissolution
rates by an order of magnitude.
OA experiments showed that skeletal dissolution rates increased
with elevated pCO2, but dolomite continued to confer resistance
to dissolution. pCO2 levels did not affect the skeletal Mg
content or dolomite formation in living CCA. Of concern, and in
agreement with the literature, bacterial erosion is accelerated
under a combination of elevated pCO2 and temperatures, suggesting
this may be the main threat to CCA reef-building in the future.
The experimental findings were corroborated by results of a field
survey along a natural pCO2 gradient.
In summary, dolomite was found to be an essential component of
modern reef development via its contribution to enabling CCA P.
onkodes thick crust development and persistence. Reef building by
CCA P. onkodes is likely to continue as pCO2 rises up until a
tipping point is reached whereby bacterial erosion switches from
constructive to destructive.
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Crustose coralline algae, dolomite, ocean acidification, coral reefs, biomineralisation, biomineralization, nanoindentation, porolithon onkodes
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