title: The role of red coralline algal habitats as blue carbon stores
Creators: name: Mao, Jinhua
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Red coralline algae, blue carbon, climate change, calcification, biomineralisation, ocean acidification.

description: The ocean is the largest carbon sink on earth, accounting for about 30% of atmospheric carbon dioxide (CO2). Marine autotrophs and associated ecosystems photosynthetically fix atmospheric CO2 and contribute to long-term carbon sequestration in sediments. The term blue carbon refers to the carbon sequestration by marine vegetated habitats. Recent research has identified the significance of blue carbon storage worldwide, while particularly the coastal marine habitats have great contribution (~ 60-210 t C km-2 yr-1). Taking into account that oceans have absorbed ~48% of anthropogenic CO2 since the Industrial Revolution, the potential of mitigating anthropogenic carbon emissions by blue carbon can be significant. The majority of research has focused on three particular coastal habitats: mangrove forests, tidal saltmarshes and seagrass meadows. Red coralline algal habitats have been long overlooked for their potential role in blue carbon, despite their worldwide distribution in coastal waters and their critical role in supporting associated ecosystems with substantially high primary productivity. This is because their calcification releases CO2 and may alter the role of related habitat to a carbon source rather than a sink, although the mechanisms and impacts remain unclear to date. This research aimed to expand our understanding of the role of red coralline algal habitats in carbon burial (blue carbon) by investigating 1) the relative blue carbon storage at contemporary and historic time scales, 2) the effectiveness and efficiency of a carbon sink by red coralline algae and 3) resilience of red coralline algae to future climatic pressures. A combination of laboratory experiments and field-based studies was conducted to achieve these aims. Results based on sediment cores (Chapter 2) suggested that the red coralline algal habitat buries considerable organic carbon, which had no significant difference to seagrasses (global / Australia average). The buried carbon was derived from both autochthonous and allochthonous sources, with ratios (autochthonous : allochthonous) shifting from ~1 to ~2.5 by ~ 1520 AD. The role of red coralline algal thalli was determined as an effective carbon sink rather than a net carbon source that had previously been thought, as the algal photosynthesis recycled ~43.2% of the CO2 produced by their own calcification (Chapter 3). This mitigation may be even more profound at ecosystem level. The blue carbon storage by red coralline algal habitats showed specific susceptibility to ecosystem stability and to changes in environmental state (i.e. sediment grain size composition, primary carbon sources, North Atlantic Oscillation (NAO) phases); a positive relationship had been identified between the carbon storage and ecosystem primary production (Chapter 2). Ancient buried carbon can be released back to atmosphere due to disturbances on the associated ecosystems (Chapter 2). Natural variability of pH over the red coralline algal beds was dramatically high for diel and seasonal patterns; more importantly, the algae already spend ~46% of one-year living in extreme low pH conditions projected for 2100 (Chapter 4). Such exposure highly likely occurs for multiple years and may have profound influence to enhancing their tolerance to future ocean acidification and to shaping their genetic adaptation for future high pCO2 ocean. As key species of creating and supporting the blue carbon deposits as red coralline algae, the related blue carbon storage capacity will attribute to their enhanced survival strategy against future climate change (Chapter 4). A general discussion was given in Chapter 5, providing a comprehensive source of information on the blue carbon capacity in red coralline algae habitats. Prior to this research, there was very limited information on blue carbon storage by red coralline algal habitats. Results of this research provide critical insight into the blue carbon over a geological time scale, by assessing the quantification of carbon storage and relative variations, examining the role as an effective carbon sink for red coralline algae and predicting their continued ecological function of supporting blue carbon habitats under future climate change. This research shows the importance of blue carbon in red coralline algal habitats and highlights the significance of community primary production to blue carbon storage, necessitating the need for further research.