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A growing problem


 
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Jennifer Clark, from the School of Life Sciences at the University of Technology Sydney, outlines her doctoral research studies on the heat tolerance and adaptation potential of the macroalga, known as Neptune’s Necklace, to global warming.    
Her research involves examining how macroalgal communities will respond to changing climates, particularly in relation to increasing temperatures and the extent to which they possess the capacity to adapt to these changes.
By analysing the physiological response of different life stages and underlying genetic variation of traits related to stress tolerance within different populations, a better understanding of how communities will persist with future warming can be achieved.
 

 
Neptune’s Necklace is an intertidal macroalga, (or seaweed) that is iconic in the wave-swept platforms of temperate Australia. In NSW it is found as far north as Yamba, and extends down the south coast of Australia and also in New Zealand. It is characterised by its long chains of vesicles, which look like a pearl necklace, hence its name.

 
Like rainforests and coral reefs, many seaweeds such as Neptune’s Necklace, are habitat formers or ecosystem engineers in which they provide a 3D habitat and resources for other organisms such as a source of food, a home, a shelter from predators and a nursery for juveniles for many different organisms. They are important in facilitating biodiversity within coastal ecosystems and are important to many fisheries. Being more robust than coral, which can only tolerate a few degrees difference in temperature, Neptune’s Necklace has a wider tolerance, dealing with a temperature span of around 12ºC in any one day. On hot days, for instance, Neptune’s Necklace might have to cope with 40°C temperatures when exposed to the air and 24°C temperatures when re-immersed in the ocean.

 
Many organisms that live in the intertidal are exposed to stressful environmental conditions which revolve around the rise and fall of tidal cycles. Although quite robust, previous studies have found that many intertidal organisms are living close to their temperature limits, which means they may not be able to cope with any more increases in temperature.
Furthermore, it may not be the gradual increase in temperature that will have the biggest impact on macroalgal communities, but instead the increase in the frequency and intensity of extreme climate events such as heatwaves which may have the biggest effect. Interestingly, Jennifer found that juveniles of Neptune’s Necklace, which inhabited warmer areas (those closer to the equator), were more susceptible to temperature rises and had a lower heat tolerance than juveniles growing in cooler temperatures. This means that in heatwaves, warmer populations may be the first affected, reflecting world shifts in species distribution towards cooler climates.
 

 
Given that seaweeds support a lot of biodiversity, other species would also be affected. Such events can be particularly detrimental for slow growing seaweed like Neptune’s Necklace as it can take years to recover. Intertidal organisms are living very close to their thermal limits so they are, in effect, an early warning system with regards to climate change because they cannot acclimate any further.
Neptune’s necklace may one of many species of seaweed to show that climate change is a growing problem.

 

A pair of Neptune’s Necklace juveniles that are 120 hours old. Embryos ‘stick’ beneath the adult canopy within hours of being fertilised. They then undergo germination in which the fertilised eggs divide and put energy into extending the rhizoid which attaches the embryo to the substrate. At this age, juveniles are about 400 – 600 microns (µm) in length.


 
An aggregate of Neptune’s Necklace juveniles that are 120 hours old. Neptune’s Necklace are free spawners which means that male and female seaweeds can put out thousands of gametes during a single low tide. Some can aggregate beneath the adult fronds. Once fertilised, rhizoids will grow away from light towards the substrate, where the basal cell will grow towards the light which is indicated in this image.

 

Jennifer Clark was interviewed by Ruby Vincent for A Question of Balance. All images provided by Jennifer Clark. Summary text by Victor Barry, May 2015.


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