Dr Greg Torda has been awarded LIRRF’s John & Laurine Proud Fellowship to study population genomics of coral recovery at Lizard Island following the 2016 bleaching events. He is interested in understanding how quickly corals adapt (genetically) or acclimatise (non-genetically) to changes in their environment, and whether they will be able to keep pace with climate change. His research will examine how rapidly coral that survived the 2016 bleaching is able to genetically adapt to higher sea temperatures, and the extent to which any such enhanced resilience is diluted by settlement of coral larvae spawned in non-stressed areas. Greg kindly provided the information for this post.
Coral polyps are small animals. Coral is also usefully considered as a holobiont of inter-dependent species, comprising the polyps, their Symbiodinium (symbiotic unicellular algal partners, commonly called zooxanthellae, within the phylum Dinoflagellata) and associated bacteria, archaea, fungi, viruses and protists.
Symbiodinium live within the tissues of coral polyps and provide nutrients through photosynthesis. They also provide colour. Changes in water quality and condition, such as when sea temperatures exceed their normal range for an extended period, can cause an expulsion or lysis (breakdown) of Symbiodinium. The white coral polyp skeletons becomes visible through the polyp tissue and the polyps starve. This is called ‘bleaching’. Each of the constituents of the coral holobiont may be involved in this complex physiology.
Coral colonies can recover from bleaching, but a prolonged heat stress usually results in their starvation to death, or a serious decrease in their immune response which leads to diseases and death. See Death by bleaching. Because water temperatures are rising, (see NOAA, NASA, DoEE Climate Science and Climate Change in Australia) the frequency and severity of bleaching events is also predicted to rise.
The schematic graphs below come from Terry Hughes’ 2003 Science paper. The orange lines represent fluctuating but gradually increasing sea temperatures. Scenario A assumes all corals have the same bleaching threshold, represented by the black horizontal line. Scenario B assumes there are various bleaching thresholds, represented by the black, yellow and blue lines and determined by factors such as species, depth and location. Scenario C, considered to be the more realistic, shows these various thresholds increasing over time through acclimation and evolution,
If they are not able to adaptively respond to changes in their environment, reef-building corals could depopulate to a point of insignificance. Corals have been building reefs for over 400 million years, but geological core samples also indicate long periods of hiatus – see here and here.
It is often assumed that genetic adaptation requires many generations, and because corals have relatively long generation time (3–8 years), some researchers think they will not be able to keep up with ocean warming. Greg Torda and his fellow scientists are hoping to challenge this view. Still early days to be optimistic, but they have hypotheses that provide hope.
Some coral species are more tolerant to heat stress than others. There is also variability in resilience within single species. Some of the variability may be due to characteristics of the unicellular partners in the coral holobiont, such as the Symbiodinium algae (Jones et al 2008) or the bacteria; some variability is thought to be genetically coded (Bay and Palumbi 2014); and there is also a hypothesis that epigenetic processes may play a role in stress tolerance (Putnam et al. 2015).
Greg and his team are trying to tease apart is how much each of these mechanisms contribute to the overall fitness of the coral holobiont, and how rapidly are they able to respond to environmental changes. They have a range of aquarium experiments running at the AIMS National Sea Simulator (SeaSim). They are also undertaking field research to obtain data from the 2016 bleaching event.
To understand the contribution of hard wired, genetic adaptation in heat tolerance, they are comparing the genetic makeup (full genome-wide scan with shotgun sequencing) of corals that died in the 2016 heat stress to that of those that survived it. Their hypothesis is that the large standing genetic variation in corals may be sufficient to facilitate adaptation in a single selective sweep, e.g. the survivors of the 2016 bleaching event may represent genetically more stress tolerant populations, and their offspring will consequently be genetically pre-conditioned for stress tolerance. One of the main questions here is whether this stress tolerant gene pool will be diluted by immigrants from non-stressed populations. In this sense, a rapid reef recovery via exogenous larval import, while desirable ecologically, may be counter-adaptive genetically.
Greg and his team are also seeking to determine whether the trans-generational inheritance of non-genetic mechanisms plays a role in coral acclimatisation. Corals show a substantial amount of plasticity in their physiology within the lifetime of colonies. For example, when transplanted between different habitats, corals have the capacity to modify their colony form to optimise for the new environment (Hoogenboom et al 2008). Some huge colonies of massive Porites are estimated to be as old as 7-800 years (Brown et al. 2009), and hence may have had to tolerate many different temperature ranges, including through the Medieval Warm Period (~950 to 1250 AC) and the ‘Little Ice Age’ (~1300 to 1850 AC).
The researchers hypothesise that mechanisms which allow such plasticity involve the rapidly evolving microbiome (bacteria and Symbiodinium), as well as epigenetic modifications – molecular pathways that regulate the expression of the genetic code. At AIMS’ SeaSim, they are rearing corals and their offspring under control and elevated temperature conditions, comparing the microbiome and epigenome of the parents and their offspring to their physiological performance (e.g. growth, respiration, photosynthetic activity, energy reserves) under the various conditions. The goal is to determine whether inheritance of such non-genetic processes can pre-condition the offspring to fare better in the altered environment.
See also Greg Torda’s article in The Conversation: Will the Great Barrier Reef recover from its worse-ever bleaching? (31 October 2016).
The following images show fluorescent proteins that provide photoprotection to corals, much like sunscreen for humans. A higher expression rate of fluorescent protein genes may be one of the adaptive traits that survivors of coral bleaching events posess. Depending on their position in the coral, fluorescent proteins may also enhance the efficiency of photosynthesis, which would be an adaptive trait in deeper habitats.