Coralline Algae and Ocean Acidification
Coralline algae are found in three orders (Corallinales, Hapalidialaes and Sporolithales) within the red algae (Rhodophyta) phylum. They contain calcium carbonate in their cell walls and come in two distinct forms – geniculate and non-geniculate. Geniculate corallines have upright branches that consist of alternating calcified and non-calcified segments, forming turfs on substrates. Non-geniculate corallines are completely calcified and form pink crusts over substrate surfaces, commonly known as ‘crustose’ coralline algae.
Non-geniculate coralline algae – Auckland Islands. Photography by Rebecca McLeod.
Coralline algal species can be
found in a broad range of marine habitats across the globe. They can be found
from the tropics to polar regions, from the intertidal zone to the euphotic
zone, from rocky shores to tropical reefs and seagrass meadows. Non-geniculate
coralline algae can also be found unattached to substrate, in a growth form
known as a rhodolith or maerl.
Educator at Sea Cherie Fenemor (Freyberg High School) and Young Blake Expedition voyagers Aidan Braid (Logan Park High School) and Guy McDonald (Timaru Boys High School) collect seaweed samples in Smith Harbour. Photography by Brendon O'Hagan.
Whilst corallines are known to inhabit a broad range of marine ecosystems, little is known about the diversity and taxonomy of coralline species around New Zealand’s Sub-Antarctic islands. In 2016 the Sir Peter Blake Trust Young Blake Expedition, under the supervision of Dr Rebecca McLeod (marine ecologist) collected coralline algae samples at a range of representative habitats in the intertidal zone around the east coast inlets of the Auckland Islands. Samples were taken at Smith Harbour, Musgrave Inlet, and an unnamed bay on the outer cost near Shag Rock. Student voyagers were responsible for collecting the samples, recording metadata, and then preserving the material so it could be analysed in a lab back on the mainland.
lab work is to be carried out by Brenton Twist, a University of Auckland PhD
student, under the guidance of Wendy Nelson at NIWA. Both genetic and
anatomical techniques will be used to determine the taxonomy of the samples
collected at the Auckland Islands. In terms of genetic analysis, DNA will be
extracted from the samples and then the segment of DNA that is needed for
examination will be copied millions of times using PCR (polymerase chain
reaction). One specific gene will be sequenced initially, as this is a good way
of sorting the samples and checking them against the existing dataset of
internationally as well as locally recorded DNA sequences, to determine differences.
Depending on the nucleotide differences between already recorded sequences and
those of the Auckland Island samples, an indication will be given as to whether
new species have been discovered. These analyses will help to improve the
knowledge on diversity and distribution of coralline algae in southern New
Brenton Twist pipetting coralline material for genetic analysis at NIWA in Wellington. Photography by Wendy Nelson.
Although thus far only baseline data has been collected, it is important to have an understanding of what is present in the marine ecosystem of the Auckland Islands. This is because if the impacts of climate change on ecological communities is going to be seen, the Sub-Antarctic will be one of the first places to experience change. So, with the baseline data collected, overtime, the rate and scale of change, and the effect that it is having can be determined. Coralline algae play an especially important role as an indicator of global climate change, as the calcium carbonate in their cell walls put them at risk of dissolution due to ocean acidification.
After remaining constant for millions of years, the pH of surface seawater has fallen from 8.2 to 8.1 in a few hundred years. This drop of 0.1 pH units translates to a 30% increase in acidity. Reactions that occur in ocean acidification are equilibrium reactions - they will remain at equilibrium until a change occurs that throws out the balance. In the case of ocean acidification, the change that has occurred is the increase in atmospheric carbon dioxide. This has subsequently increased the amount of carbon dioxide that is dissolved in the oceans. As carbon dioxide is more soluble in colder waters than warmer, this could result in greater impacts in the Southern Ocean which surround the Auckland Islands.
As carbon dioxide reacts with the
seawater, carbonic acid is formed (H2CO3). This disassociates
into hydrogen ions (H+) and bicarbonate ions (HCO3-).
The hydrogen ions produced from this reaction increase the acidity and lower
the pH of the oceans. This is intensified by the fact that the bicarbonate ions
also disassociate into more hydrogen ions (H+) and carbonate ions
circumstances carbonate ions help to resist changes in ocean acidity through
what is known as the carbonate buffer system. If an acid is added to seawater,
carbonate ions (CO32-) work to reduce the acidity by tying up excess hydrogen ions (H+), producing
bicarbonate ions. As ocean acidity is increasing, the concentration of
available carbonate ions for organisms, such as coralline algae, to use for the
secretion of calcium carbonate shells and skeletons is decreasing. This is
because the carbonate ions are becoming tied up, and biologically unavailable
as bicarbonate ions, leading to oceans being under-saturated in carbonate ions
and organisms requiring greater energy expenditure for calcification to occur.
Further to this, calcium carbonate (CaCO3) which is insoluble in seawater, reacts with carbonic acid (H2CO3) and dissolves. This results in the dissolution of calcium carbonate skeletons and shells of marine organisms. There are three different mineral forms of calcium carbonate (aragonite, calcite and the rare mineral vaterite) each of which have a different solubility. The structure of calcite means that calcium ions (Ca2+) can be substituted for magnesium ions (Mg2+), forming magnesium calcites, with varying magnesium ion concentrations. The three coralline algae orders have differing concentrations of magnesium – the Corallinales have the highest magnesium content, then the Sporolithales, and finally, the Hapalidialaes, having the lowest content. Those species with a high magnesium content (high Mg-calcite) are more soluble in seawater than aragonite and calcite. Coralline algae, particularly non-geniculate species, can also contain aragonite in their skeletons, predominantly in their attachment area. This form of calcium carbonate is less soluble than high-Mg calcite, but more soluble than pure calcite. Thus coralline algae are more susceptible to changes in ocean acidity than organisms that have other forms of calcium carbonate in their shells and skeletons.
As the consequences of global climate change begin to be seen in the oceans, the impacts of lowered calcification rates and higher dissolution rates in coralline algae will have possible impacts on their wider ecological community. Coralline algae have been shown to have an importance in the life history of other marine species. They are considered settlement inducers for marine invertebrates such as paua, corals and kina. Thus a change in a coralline algal population due to ocean acidification may also invoke a change in populations of these species. Furthermore, as carbon dioxide concentrations increase, and the energy needed for calcification increases, non-calcified fleshy algae will gain a competitive advantage over coralline algae, changing the composition of benthic communities. As all species are part of a dynamic ecological system, it is hard to predict the full range of consequences on coralline algae due to ocean acidification, but as sea water acidity increases it is highly likely that there will be an impact on calcifying species, which will in turn impact their ecological community.