Adaptation Recovery And Mitigation

It is difficult to predict if marine organisms and ecosystem will adapt to or recover from the rapid changes to ocean carbonate chemistry. An optimistic view may be that for organisms with short generation times micro-evolutionary adaptation could be rapid and that species adversely affected by high CO2 could be replaced by more CO2-tolerant strains or species, with minimal impacts up the food chain. The less optimistic view is that CO2-sensitive groups, such as the marine calcifiers, will be unable to compete ecologically, resulting in widespread extinctions with profound ramifications up the food chain.

6.1. Adaptation

There are periods within a coccolithophore life cycle that are non-calcifying. In addition, there are some species that appear to have lost the ability to form CaCO3 liths [82]. This suggests that the biochemical pathways involved in calcification in coccolithophores can be turned on and off. Should coccolitho-phores struggle to form their coccoliths in future high CO2 scenarios, as is suggested by experimental data, they may have the genetic diversity and capability to adapt. Indeed, this may have happened several times throughout the course of evolution [83] although they would have had more time to do this then than is available during the current acidification event.

Although tropical Scleractinian corals have adapted, over millions of years, to live in warm, sunlit waters highly saturated in aragonite they have survived, and even retained their algal symbionts and completed gametogen-esis, for a year in experiments at pH 7.4 although in a 'naked', decalcified form [84]. When transferred back to ambient pH conditions of 8.2, the soft-bodied corals calcified and reformed colonies. However, it should be noted that if this occurred in the wild the naked corals would be prone to greater grazing and they could not build reef structures which create important biodiversity hotspots.

A fossil coral from ^70 Ma ago had skeletal features identical to those observed in present-day Scleractinians but was made entirely of calcite rather than the aragonite of today's Scleractinian coral skeletons [85]. This implies that in geological times, some corals may have been able to switch between different carbonate forms to make their skeletons. However, the estimated rate of change during even the largest of these previous acidification events was an order of magnitude lower (over several thousand years) than our predicted current change (over a few hundreds of years) [86] so current corals may not have sufficient time to adapt.

Tropical coral migration to higher latitudes with more optimal sea surface temperature is unlikely, due both to latitudinally decreasing aragonite concentrations and projected atmospheric CO2 increases [6,57,87]. Coral migration is also limited by lack of available substrate.

It would therefore seem unlikely that coral reefs would be able to adapt to a high CO2 ocean sufficiently quickly in this current rapid anthropogenic perturbation, neither through switching to another carbonate form nor through migration.

The changes in current ecosystem composition caused by a natural CO2 vent systems emitted by a volcano have shown a lack of many calcifying organisms in the lower pH areas (pH < 7.8) and a shift to predominance of sea grass beds or invasive alien species [19]. This study demonstrates the inability of many calcifiers to adapt to longer term decline in pH and gives an unattractive in situ insight into future ecosystems in a high CO2 ocean.

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