Effects of elevated pCO2 and irradiance on growth, photosynthesis and calcification in Halimeda discoidea

Abstract

Ocean acidification (OA) effects on photophysiology and calcification were examined in Halimeda discoidea, a calcifying macroalga that produces tropical reef sediments. Photosynthetic parameters, including maximum photosynthetic rate (P max), photosynthetic efficiency (α) and compensating irradiance (I c) were determined in short-term assays on live thalli after a 10 d exposure to 4 levels of CO 2 partial pressures (pCO 2 ; 491, 653, 982 and 1201 µatm) under satura ting (300 µmol photons m −2 s −1) and sub-saturating (90 µmol photons m −2 s −1) irradiance in an aquaria study. Morphology of aragonite crystals produced in segments formed on adult thalli was characterized using scanning electron microscopy (SEM). Further, we examined crystal morphology and changes in inorganic content of non-living segments exposed to elevated (1201 µatm) and ambient pCO 2 for 27 d to assess OA effects on carbonate sediments generated from H. discoidea. Even though P max was higher under elevated pCO 2 , this photophysiological response did not result in higher calcification rates. Based on crystal measurements and SEM imagery, aragonite crystals within new segments were indistinguishable across pCO 2 and irradiance treatments. Under high irradiance, new segments showed a greater investment in organic versus inorganic production. Non-living segments contained narrower crystals after 27 d exposure to elevated pCO 2 relative to controls, but differences were small (0.03 µm) and did not contribute significantly to changes in normalized biomass or inorganic content. Based on these results, H. discoidea will likely produce new calcified segments with intact aragonite crystals under year 2100 pCO 2 levels at high and low irradiance, while aragonite crystals of the sediment may produce thinner needle carbonate muds.

Primary utricle structure of six Halimeda species and potential relevance for ocean acidification tolerance

“…Narrow needles in new segments grown in CO 2 -enriched (> 650 μatm) seawater were suggested to be formed under rapid calcification, potentially catalyzed by an increase in external carbonic anhydrase activity at low pH (Hofmann et al 2014, Wizemann et al 2015. Although a few studies observed thinner needles, in our previous work on seven Halimeda species we found no changes in crystal morphology under year 2100 (~ 1000 μatm) compared to ambient pCO 2 levels (Peach et al 2016(Peach et al , 2017.…”

Biotic Control of Surface pH and Evidence of Light-Induced H+ Pumping and Ca2+-H+ Exchange in a Tropical Crustose Coralline Alga

“…While Adey et al [ 12 ] advocate that cell wall calcification is internally controlled, there is secondary calcification between cell walls within the perithallial region that may be associated with photosynthesis, evidenced by the fact that there is a larger area of disorganized secondary crystals under greater irradiance and temperature. The pattern of CCA calcification appears analogous to the highly oriented aragonite crystals associated with the cell wall of the Chlorophyte macroalgal genus, Halimeda , in contrast to the adjacent randomly oriented crystals in spaces between filaments [ 28 – 30 ]. Different modes of calcification driven by diverse processes complicate identifying mechanisms of CCA calcification and may account for some discrepancies (i.e.…”

“…Thus the DBL may ameliorate potential effects of high pCO 2 , particularly under low flow regimes where a thick DBL develops [ 32 – 33 ]. The photosynthesis-induced increase in pH is thought to favor calcification due to the associated increase in aragonite saturation state and increase in CO 3 2- ions [ 30 , 33 ]. This positive relationship between photosynthesis and calcification has been observed in a diversity of calcareous macroalgae at ambient seawater pH, including CCA [ 38 – 41 ].…”

Benthic ctenophore (Order Platyctenida) reproduction, recruitment, and seasonality in south Florida

“…Samples were dried in three changes of hexamethyldisilazane (Electron Microscopy Sciences). Dried samples were coated with ~20 nm Pd in a plasma sputter coater (Peach, Koch, & Blackwelder, ). Samples were imaged and micrographs digitally collected using a Philips XL‐30 field emission scanning electron microscope (FEI XL‐30 Field Emission ESEM/ SEM , Thermo Fisher Scientific, Inc.) located at the University of Miami Center for Advanced Microscopy (UMCAM).…”