Physiologically mediated susceptibilities of coralline algae to ocean change

Coralline algae are important foundation species in coastal ecosystems around the world but are threatened by ocean acidification (OA) and other anthropogenic stressors such as ocean warming (OW) and sedimentation. Physiological responses to ocean change are challenging to predict because the mechanisms that provide tolerance to different stressors are unknown and there is a lack of understanding of responses to multiple drivers. To address these issues, I conducted four experiments examining the physiological responses of multiple temperate coralline algal species to decreasing irradiance, declining pH, OW and marine heatwaves (MHWs), and a combination of OA, OW and reduced irradiance.

Coralline algal calcification generally responded parabolically to irradiance, but photosynthesis responded linearly. My results suggest that light enhanced calcification is the result of increased ion pumping rates to elevate the calcium carbonate saturation state in the calcifying fluid (CF) and a higher daytime pH in the diffusion boundary layer that raises pHCF. My results implied the existence of two calcification strategies in coralline algae that I discuss within the thesis.

Tolerance to OA was coupled to the species’ ability to maintain stable carbonate chemistry at the site of calcification to support calcification as seawater pH declined. Conversely, pronounced changes in internal calcium carbonate saturation state (ΩCF) and skeletal magnesium content were observed in the sensitive taxa. However, ΩCF did generally not decline but increase under OA. There was also slight OA-induced photodamage in sensitive taxa that could explain the inability to support ion-pumping and growth under OA.

Photo-physiology and calcification of coralline algae were generally unaffected by OW and MHWs implying a high thermo-tolerance. However, exposure to future ocean conditions (decreased irradiance+OW x OA) caused the most severe reductions in calcification. Single driver (OA and decreased irradiance+OW) impacts were smaller. Calcification responses were decoupled from ΩCF likely due to the effective control over the internal carbonate chemistry. However, calcification likely declined due to the increased energy expenditure of calcification or when energy supply was reduced. Indeed, variations in energy expenditure and photosynthesis could explain most of the observed calcification responses.

Overall, this thesis has increased our predictive understanding regarding the impact of ocean change on coralline algae by addressing several critical issues by providing a new mechanistic model that more accurately defines the role of irradiance in coralline algal calcification.

Krieger E., 2022. Physiologically mediated susceptibilities of coralline algae to ocean change. PhD thesis, Te Herenga Waka-Victoria University of Wellington, 225 p. Thesis.

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