Seawater carbonate chemistry, calcification, primary production and respiration of a temperate rhodolith Lithothamnion corallioides in a laboratory experiment

Coralline algae are considered among the most sensitive species to near future ocean acidification. We tested the effects of elevated pCO2 on the metabolism of the free-living coralline alga Lithothamnion corallioides ("maerl") and the interactions with changes in temperature. Specimens were collected in North Brittany (France) and grown for 3 months at pCO2 of 380 (ambient pCO2), 550, 750, and 1000 µatm (elevated pCO2) and at successive temperatures of 10°C (ambient temperature in winter), 16°C (ambient temperature in summer), and 19°C (ambient temperature in summer +3°C). At each temperature, gross primary production, respiration (oxygen flux), and calcification (alkalinity flux) rates were assessed in the light and dark. Pigments were determined by HPLC. Chl a, carotene, and zeaxanthin were the three major pigments found in L. corallioides thalli. Elevated pCO2 did not affect pigment content while temperature slightly decreased zeaxanthin and carotene content at 10°C. Gross production was not affected by temperature but was significantly affected by pCO2 with an increase between 380 and 550 µatm. Light, dark, and diel (24 h) calcification rates strongly decreased with increasing pCO2 regardless of the temperature. Although elevated pCO2 only slightly affected gross production in L. corallioides, diel net calcification was reduced by up to 80% under the 1,000 µatm treatment. Our findings suggested that near future levels of CO2 will have profound consequences for carbon and carbonate budgets in rhodolith beds and for the sustainability of these habitats.

Effects of feeding and light intensity on the response of the coral Porites rus to ocean acidification

Recently, it has been suggested that there are conditions under which some coral species appear to be resistant to the effects of ocean acidification. To test if such resistance can be explained by environmental factors such as light and food availability, the present study investigated the effect of 3 feeding regimes crossed with 2 light levels on the response of the coral Porites rus to 2 levels of pCO2 at 28 °C. After 1, 2, and 3 weeks of incubation under experimental conditions, none of the factors-including pCO2-significantly affected area-normalized calcification and biomass-normalized calcification. Biomass also was unaffected during the first 2 weeks, but after 3 weeks, corals that were fed had more biomass per unit area than starved corals. These results suggest that P. rus is resistant to short-term exposure to high pCO2, regardless of food availability and light intensity. P. rus might therefore represent a model system for exploring the genetic basis of tolerance to OA.

Ocean acidification alleviates low-temperature effects on growth and photosynthesis of the red alga Neosiphonia harveyi (Rhodophyta)

This study aimed to examine interactive effects between ocean acidification and temperature on the photosynthetic and growth performance of Neosiphonia harveyi. N. harveyi was cultivated at 10 and 17.5 °C at present (~380 µatm), expected future (~800 µatm), and high (~1500 µatm) pCO2. Chlorophyll a fluorescence, net photosynthesis, and growth were measured. The state of the carbon-concentrating mechanism (CCM) was examined by pH-drift experiments (with algae cultivated at 10 °C only) using ethoxyzolamide, an inhibitor of external and internal carbonic anhydrases (exCA and intCA, respectively). Furthermore, the inhibitory effect of acetazolamide (an inhibitor of exCA) and Tris (an inhibitor of the acidification of the diffusive boundary layer) on net photosynthesis was measured at both temperatures. Temperature affected photosynthesis (in terms of photosynthetic efficiency, light saturation point, and net photosynthesis) and growth at present pCO2, but these effects decreased with increasing pCO2. The relevance of the CCM decreased at 10 °C. A pCO2 effect on the CCM could only be shown if intCA and exCA were inhibited. The experiments demonstrate for the first time interactions between ocean acidification and temperature on the performance of a non-calcifying macroalga and show that the effects of low temperature on photosynthesis can be alleviated by increasing pCO2. The findings indicate that the carbon acquisition mediated by exCA and acidification of the diffusive boundary layer decrease at low temperatures but are not affected by the cultivation level of pCO2, whereas the activity of intCA is affected by pCO2. Ecologically, the findings suggest that ocean acidification might affect the biogeographical distribution of N. harveyi.

Growth, respiration and photophysiology of coral massive Porites spp. in the experiment of Moorea

I tested the hypothesis that high pCO2 (76.6 Pa and 87.2 Pa vs. 42.9 Pa) has no effect on the metabolism of juvenile massive Porites spp. after 11 days at 28 °C and 545 µmol quanta/m**2/s. The response was assessed as aerobic dark respiration, skeletal weight (i.e., calcification), biomass, and chlorophyll fluorescence. Corals were collected from the shallow (3-4 m) back reef of Moorea, French Polynesia (17°28.614'S, 149°48.917'W), and experiments conducted during April and May 2011. An increase in pCO2 to 76.6 Pa had no effect on any dependent variable, but 87.2 Pa pCO2 reduced area-normalized (but not biomass-normalized) respiration 36 %, as well as maximum photochemical efficiency (Fv/Fm) of open RCIIs and effective photochemical efficiency of RCIIs in actinic light (Delta F/F'm ); neither biomass, calcification, nor the energy expenditure coincident with calcification (J/g) was effected. These results do not support the hypothesis that high pCO2 reduces coral calcification through increased metabolic costs and, instead, suggest that high pCO2 causes metabolic depression and photochemical impairment similar to that associated with bleaching. Evidence of a pCO2 threshold between 76.6 and 87.2 Pa for inhibitory effects on respiration and photochemistry deserves further attention as it might signal the presence of unpredictable effects of rising pCO2.

Seawater carbonate chemistry and calcification during a study of barrier reef flat in Moorea, French Polynesia, 1998

The relative contribution of soft bottoms to the community metabolism (primary production, respiration and net calcification) of a barrier reef flat has been investigated at Moorea (French Polynesia). Community metabolism of the sedimentary area was estimated using in situ incubations in perspex chambers, and compared with estimates of community metabolism of the whole reef flat obtained using a Lagrangian technique (Gattuso et al., 1996. Carbon flux in coral reefs. 1. Lagrangian measurement of community metabolism and resulting air-sea CO2 disequilibrium. Mar. Ecol. Prog. Ser. 145, 109-121). Net organic carbon production (E), respiration (R) and net calcification (G) of sediments were measured by seven incubations performed in triplicate at different irradiance. Respiration and environmental parameters were also measured at four randomly selected additional stations. A model of Photosynthesis-irradiance allowed to calculate oxygen (O2), organic carbon (CO2) and calcium carbonate (CaCO3) evolution from surface irradiance during a diel cycle. As chlorophyll a content of the sediment was not significantly different between stations, primary production of the sediment was considered as homogeneous for the whole lagoon. Thus, carbon production at the test station can be modelled from surface light irradiance. The modelled respiration was two times higher at the test station than the mean respiration of the barrier reef, and thus underestimated sediment contribution to excess production. Sediments cover 40-60% of the surface and accounted for 2.8-4.1% of organic carbon excess production estimated with the modelled R and 21-32% when mean R value was considered. The sedimentary CaCO3 budget was a very minor component of the whole reef budget.