Elevated pCO2 effects on the macroalgal genus Halimeda: Potential roles of photophysiology and morphology

While ocean acidification (OA) is predicted to inhibit calcification in marine macroalgae, species whose photosynthesis is limited by current dissolved inorganic carbon (DIC) levels may benefit. Furthermore, variations in macroalgal morphology will likely give rise to a range of OA tolerance in calcifying macroalgae. One genus of calcifying macroalgae that has shown varying species’ tolerance to OA is Halimeda, a major carbonate sediment producer on tropical reefs. Species within this genus occupy a range of habitats within tropical environments (reefs and lagoons), illustrating their ability to adapt to diverse environmental conditions (e.g. carbonate chemistry, irradiance). To date it is not clear if morphological and photophysiological diversity in Halimeda will translate to different tolerances to OA conditions (elevated pCO2 and lower pH).

In this dissertation, aquaria experiments were conducted to examine growth, calcification, and aragonite crystal formation in 7 Halimeda species using year 2100 pCO2 levels (∼1000 µatm) compared to controls (∼400 µatm) under low (sub-saturating) and high (saturating) irradiance. Elevated pCO2 effects on dissolution in non-living segments of 4 Halimeda species were also assessed. Each species’ photophysiology and primary utricle morphology were characterized to determine their potential role in experimental responses to OA conditions.

Net calcification rates varied among species, but were unaffected by elevated pCO2, regardless of irradiance. The lack of a pCO2 effect on calcification was attributed to photosynthesis that was maintained or enhanced at [DIC] above current levels. Low irradiance stimulated growth of new segments with aragonite crystals that were indistinguishable across pCO2 treatments. In contrast, aragonite needles in non-living segments became narrower under elevated pCO2 relative to controls in weakly calcified species, unlike those that were heavily cemented with micro-anhedral crystals. Inter-species’ variations in inorganic content and crystal microstructure were related to differences in primary utricle morphology. Although these diverse utricle morphologies were predicted to influence variable OA tolerances, this thesis was rejected based on experimental evidence that all 7 Halimeda species maintained their ability to calcify and precipitate aragonite crystals under elevated pCO2. Thus, it is proposed that these 7 Halimeda species will continue their roles as carbonate sediment producers on future tropical reefs.

Peach K. E., 2016. Elevated pCO2 effects on the macroalgal genus Halimeda: Potential roles of photophysiology and morphology. Ph.D thesis, Florida Atlantic University, 145 p. Thesis (restricted access).

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