Peach, Katherine

We hypothesized that ocean acidification OA effects on calcifying benthic macroalgae are inextricably
linked to light availability. To address this supposition, we examined OA and light effects on the
calcifying green macroalga Halimeda discoidea, an important carbonate sediment producer in tropical
reef ecosystems. An aquaria experiment was conducted to examine elevated pCO2 and light saturating
vs. sub-saturating effects on photosynthesis, growth, calcification and crystal morphology.
Photosynthetic rates were enhanced under elevated pCO2 and saturating light. This upregulation of
photosynthesis did not enhance calcification of new segments or the number of new segments produced. The lowest number of new segments was produced in the saturating light treatment.
Scanning electron microscopy of new segments confirmed net calcification and production of aragonite
crystals at 2100 pCO2 levels. Aragonite crystal size and abundance were unaffected by CO2 or light
treatments. We also examined dissolution of nonliving segments and observed that without
photosynthesis dead segments experienced greater dissolution and possessed smaller crystals under
elevated pCO2. Our findings support the hypothesis that light plays a crucial role in calcifying
macroalgal responses to OA. We further suggest that H. discoidea will maintain its role as a dominant
reef sediment producer under year 2100 pCO2 levels.

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).