Differential modification of seawater carbonate chemistry by major coral reef benthic communities

Ocean acidification (OA) resulting from uptake of anthropogenic CO₂ may negatively affect coral reefs by causing decreased rates of biogenic calcification and increased rates of CaCO₃ dissolution and bioerosion. However, in addition to the gradual decrease in seawater pH and Ω _a resulting from anthropogenic activities, seawater carbonate chemistry in these coastal ecosystems is also strongly influenced by the benthic metabolism which can either exacerbate or alleviate OA through net community calcification (NCC = calcification – CaCO₃ dissolution) and net community organic carbon production (NCP = primary production ? respiration). Therefore, to project OA on coral reefs, it is necessary to understand how different benthic communities modify the reef seawater carbonate chemistry. In this study, we used flow-through mesocosms to investigate the modification of seawater carbonate chemistry by benthic metabolism of five distinct reef communities [carbonate sand, crustose coralline algae (CCA), corals, fleshy algae, and a mixed community] under ambient and acidified conditions during summer and winter. The results showed that different communities had distinct influences on carbonate chemistry related to the relative importance of NCC and NCP. Sand, CCA, and corals exerted relatively small influences on seawater pH and Ω _a over diel cycles due to closely balanced NCC and NCP rates, whereas fleshy algae and mixed communities strongly elevated daytime pH and Ω _a due to high NCP rates. Interestingly, the influence on seawater pH at night was relatively small and quite similar across communities. NCC and NCP rates were not significantly affected by short-term acidification, but larger diel variability in pH was observed due to decreased seawater buffering capacity. Except for corals, increased net dissolution was observed at night for all communities under OA, partially buffering against nighttime acidification. Thus, algal-dominated areas of coral reefs and increased net CaCO₃ dissolution may partially counteract reductions in seawater pH associated with anthropogenic OA at the local scale.     

Framework of barrier reefs threatened by ocean acidification

To date, studies of ocean acidification (OA) on coral reefs have focused on organisms rather than communities, and the few community effects that have been addressed have focused on shallow back reef habitats. The effects of OA on outer barrier reefs, which are the most striking of coral reef habitats and are functionally and physically different from back reefs, are unknown. Using 5‐m long outdoor flumes to create treatment conditions, we constructed coral reef communities comprised of calcified algae, corals, and reef pavement that were assembled to match the community structure at 17 m depth on the outer barrier reef of Moorea, French Polynesia. Communities were maintained under ambient and 1200 μatm pCO₂ for 7 weeks, and net calcification rates were measured at different flow speeds. Community net calcification was significantly affected by OA, especially at night when net calcification was depressed ~78% compared to ambient pCO₂. Flow speed (2–14 cm s^?1) enhanced net calcification only at night under elevated pCO₂. Reef pavement also was affected by OA, with dissolution ~86% higher under elevated pCO₂ compared to ambient pCO₂. These results suggest that net accretion of outer barrier reef communities will decline under OA conditions predicted within the next 100 years, largely because of increased dissolution of reef pavement. Such extensive dissolution poses a threat to the carbonate foundation of barrier reef communities.     

Climate change and ocean acidification effects on seagrasses and marine macroalgae

Although seagrasses and marine macroalgae (macro‐autotrophs) play critical ecological roles in reef, lagoon, coastal and open‐water ecosystems, their response to ocean acidification (OA) and climate change is not well understood. In this review, we examine marine macro‐autotroph biochemistry and physiology relevant to their response to elevated dissolved inorganic carbon [DIC], carbon dioxide [CO₂], and lower carbonate [CO₃^2?] and pH. We also explore the effects of increasing temperature under climate change and the interactions of elevated temperature and [CO₂]. Finally, recommendations are made for future research based on this synthesis. A literature review of >100 species revealed that marine macro‐autotroph photosynthesis is overwhelmingly C₃ (≥ 85%) with most species capable of utilizing HCO₃^?; however, most are not saturated at current ocean [DIC]. These results, and the presence of CO₂‐only users, lead us to conclude that photosynthetic and growth rates of marine macro‐autotrophs are likely to increase under elevated [CO₂] similar to terrestrial C₃ species. In the tropics, many species live close to their thermal limits and will have to up‐regulate stress‐response systems to tolerate sublethal temperature exposures with climate change, whereas elevated [CO₂] effects on thermal acclimation are unknown. Fundamental linkages between elevated [CO₂] and temperature on photorespiration, enzyme systems, carbohydrate production, and calcification dictate the need to consider these two parameters simultaneously. Relevant to calcifiers, elevated [CO₂] lowers net calcification and this effect is amplified by high temperature. Although the mechanisms are not clear, OA likely disrupts diffusion and transport systems of H⁺ and DIC. These fluxes control micro‐environments that promote calcification over dissolution and may be more important than CaCO₃ mineralogy in predicting macroalgal responses to OA. Calcareous macroalgae are highly vulnerable to OA, and it is likely that fleshy macroalgae will dominate in a higher CO₂ ocean; therefore, it is critical to elucidate the research gaps identified in this review.     

Seasonal upwelling conditions promote growth and calcification in reef-building coralline algae

Crustose coralline algae (CCA) are important components of reef ecology contributing to reef framework construction. However, little is known about how seasonal upwelling systems influence growth and calcification of tropical CCA. We assessed marginal and vertical growth and net calcification rates of two dominant but morphologically different reef-building CCA, Porolithon antillarum and Lithophyllum cf. kaiseri, in a shallow coral reef of the Colombian Caribbean during upwelling and non-upwelling seasons. Growth and calcification rates varied seasonally with higher values during the upwelling compared to the non-upwelling (rainy) season. Annual vertical growth showed rates of 4.48 ± 1.58 and 4.31 ± 2.17 mm · y⁻¹, net calcification using crust growth estimates of 0.75 ± 0.30 g and 0.68 ± 0.60 g CaCO₃ · cm⁻² · y⁻¹ and net calcification using the buoyant weight method of 1.49 ± 0.57 and 0.52 ± 0.11 g CaCO₃ · cm⁻² · y⁻¹ in P. antillarum and L. kaiseri, respectively. Seawater temperature was inversely related with growth and calcification; however, complex oceanographic interactions between temperature and resource availability (e.g., light, nutrients, and CO₂) are proposed to modulate CCA vital rates. Although CCA calcification rates are comparable to hard corals, CCA vertical accretion is much lower, suggesting that the main contribution of CCA to reef construction is via cementation processes. These results provide baseline data on CCA in the region and generate useful information for monitoring the impacts of environmental changes on tropical upwelling environments.     

Combined effects of global climate change and nutrient enrichment on the physiology of three temperate maerl species

Made up of calcareous coralline algae, maerl beds play a major role as ecosystem engineers in coastal areas throughout the world. They undergo strong anthropogenic pressures, which may threaten their survival. The aim of this study was to gain insight into the future of maerl beds in the context of global and local changes. We examined the effects of rising temperatures (+3°C) and ocean acidification (?0.3 pH units) according to temperature and pH projections (i.e., the RCP 8.5 scenario), and nutrient (N and P) availability on three temperate maerl species (Lithothamnion corallioides, Phymatolithon calcareum, and Lithophyllum incrustans) in the laboratory in winter and summer conditions. Physiological rates of primary production, respiration, and calcification were measured on all three species in each treatment and season. The physiological response of maerl to global climate change was species‐specific and influenced by seawater nutrient concentrations. Future temperature–pH scenario enhanced maximal gross primary production rates in P. calcareum in winter and in L. corallioides in both seasons. Nevertheless, both species suffered an impairment of light harvesting and photoprotective mechanisms in winter. Calcification rates at ambient light intensity were negatively affected by the future temperature–pH scenario in winter, with net dissolution observed in the dark in L. corallioides and P. calcareum under low nutrient concentrations. Nutrient enrichment avoided dissolution under future scenarios in winter and had a positive effect on L. incrustans calcification rate in the dark in summer. In winter conditions, maximal calcification rates were enhanced by the future temperature–pH scenario on the three species, but P. calcareum suffered inhibition at high irradiances. In summer conditions, the maximal calcification rate dropped in L. corallioides under the future global climate change scenario, with a potential negative impact on CaCO₃ budget for maerl beds in the Bay of Brest where this species is dominant. Our results highlight how local changes in nutrient availability or irradiance levels impact the response of maerl species to global climate change and thus point out how it is important to consider other abiotic parameters in order to develop management policies capable to increase the resilience of maerl beds under the future global climate change scenario.     

Calcification by juvenile corals under heterotrophy and elevated CO2

Ocean acidification (OA) threatens the existence of coral reefs by slowing the rate of calcium carbonate (CaCO₃) production of framework-building corals thus reducing the amount of CaCO₃ the reef can produce to counteract natural dissolution. Some evidence exists to suggest that elevated levels of dissolved inorganic nutrients can reduce the impact of OA on coral calcification. Here, we investigated the potential for enhanced energetic status of juvenile corals, achieved via heterotrophic feeding, to modulate the negative impact of OA on calcification. Larvae of the common Atlantic golf ball coral, Favia fragum, were collected and reared for 3 weeks under ambient (421 μatm) or significantly elevated (1,311 μatm) CO₂ conditions. The metamorphosed, zooxanthellate spat were either fed brine shrimp (i.e., received nutrition from photosynthesis plus heterotrophy) or not fed (i.e., primarily autotrophic). Regardless of CO₂ condition, the skeletons of fed corals exhibited accelerated development of septal cycles and were larger than those of unfed corals. At each CO₂ level, fed corals accreted more CaCO₃ than unfed corals, and fed corals reared under 1,311 μatm CO₂ accreted as much CaCO₃ as unfed corals reared under ambient CO₂. However, feeding did not alter the sensitivity of calcification to increased CO₂; ? calcification/?Ω was comparable for fed and unfed corals. Our results suggest that calcification rates of nutritionally replete juvenile corals will decline as OA intensifies over the course of this century. Critically, however, such corals could maintain higher rates of skeletal growth and CaCO₃ production under OA than those in nutritionally limited environments.     

Sponge erosion under acidification and warming scenarios: differential impacts on living and dead coral

Ocean acidification will disproportionately impact the growth of calcifying organisms in coral reef ecosystems. Simultaneously, sponge bioerosion rates have been shown to increase as seawater pH decreases. We conducted a 20‐week experiment that included a 4‐week acclimation period with a high number of replicate tanks and a fully orthogonal design with two levels of temperature (ambient and +1 °C), three levels of pH (8.1, 7.8, and 7.6), and two levels of boring sponge (Cliona varians, present and absent) to account for differences in sponge attachment and carbonate change for both living and dead coral substrate (Porites furcata). Net coral calcification, net dissolution/bioerosion, coral and sponge survival, sponge attachment, and sponge symbiont health were evaluated. Additionally, we used the empirical data from the experiment to develop a stochastic simulation of carbonate change for small coral clusters (i.e., simulated reefs). Our findings suggest differential impacts of temperature, pH and sponge presence for living and dead corals. Net coral calcification (mg CaCO₃ cm^?2 day^?1) was significantly reduced in treatments with increased temperature (+1 °C) and when sponges were present; acidification had no significant effect on coral calcification. Net dissolution of dead coral was primarily driven by pH, regardless of sponge presence or seawater temperature. A reevaluation of the current paradigm of coral carbonate change under future acidification and warming scenarios should include ecologically relevant timescales, species interactions, and community organization to more accurately predict ecosystem‐level response to future conditions.     

Deposition and cycling of carbon and nitrogen in carbonate mud of the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef

The dynamics of carbon and nitrogen in carbonate mud were examined in the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef. Most (89–93%) of the organic carbon and total nitrogen depositing to the carbonate mud zones was mineralized over a sediment depth of 1 m, with ∼50% of CO₂ produced during microbial decomposition involved in carbonate precipitation/dissolution reactions. There was proportionally little burial of organic carbon (10–11%) or nitrogen (7–10%). Nitrogen budgets suggest rapid turnover of porewater inorganic N pools on the order of hours to a few days. Incubation experiments indicate carbonate dissolution in surface deposits (≤20 cm depth) and carbonate precipitation in deeper sediments. Depth-integrated reaction rates indicate net carbonate precipitation of 7–10 mol CaCO₃ m² year⁻¹ over a depth of 1 m. Budget calculations at the whole-reef scale imply that deposition of CaCO₃ in the mud zones of both lagoons may equate to 50–90% of total reef carbonate production, with organic carbon fluxes equating to nearly all net primary production on each reef. These biogeochemical estimates point to the functional importance of carbonate mud zones in the lagoons of the shelf reefs of the Great Barrier Reef.     

Inorganic nutrient and oxygen fluxes across the sediment–water interface in the inshore macrophyte areas of a shallow estuary (Lake Illawarra,Australia)

Rates of inorganic nutrient and oxygen fluxes, and gross community primary productivity were investigated using incubated cores in July, August and September 2001, in a seagrass meadow of Lake Illawarra, a barrier estuary in New South Wales, Australia. The results indicated that rates of gross primary productivity were high, varying from C = 0.62 to 1.89 g m^–2 d^–1; low P/R ratios of 0.28–0.48 define the system as heterotrophic and indicate that more carbon is respired than is produced. In order to determine the effect of macroalgae on O₂ and nutrient fluxes, measurements were also conducted on cores from which the macroalgae had been removed. The results showed that the O₂ fluxes during light incubations were significantly lower in the cores without macroalgae (P<0.01), indicating that macroalgae could be a significant contributor to the primary production in the lake. In general, nutrient fluxes showed a typical diurnal variation with an efflux from sediments in the dark and a reduced efflux (or uptake) in the light. Dissolved inorganic nitrogen (NO₂ ^–+ NO₃ ^–+NH₄ ⁺) net fluxes were directed from the sediments towards the water column and dominated by the NH₄ ⁺ fluxes (>80%). NO₂ ^–+ NO₃ ^– and o-P fluxes were always very low during the sampling period. The increasing tendency of net nutrient effluxes, especially NH₄ ⁺ from July to September, is consistent with the increase of the water temperature and seagrass biomasses. However, in September, significantly lower light, dark and net NH₄ ⁺ effluxes were found in the cores with macroalgae (SA-sediments) compared with the cores without macroalgae (S-sediments). These results support the hypothesis that actively-growing dense macroalgal mats (i.e., algal blooms in September) may act as a filter reducing the flux of nutrients to the water column.     

Effects of High Dissolved Inorganic and Organic Carbon Availability on the Physiology of the Hard Coral Acropora millepora from the Great Barrier Reef

Coral reefs are facing major global and local threats due to climate change-induced increases in dissolved inorganic carbon (DIC) and because of land-derived increases in organic and inorganic nutrients. Recent research revealed that high availability of labile dissolved organic carbon (DOC) negatively affects scleractinian corals. Studies on the interplay of these factors, however, are lacking, but urgently needed to understand coral reef functioning under present and near future conditions. This experimental study investigated the individual and combined effects of ambient and high DIC (pCO₂ 403 μatm/ pH_Total 8.2 and 996 μatm/pH_Total 7.8) and DOC (added as Glucose 0 and 294 μmol L^-1, background DOC concentration of 83 μmol L^-1) availability on the physiology (net and gross photosynthesis, respiration, dark and light calcification, and growth) of the scleractinian coral Acropora millepora (Ehrenberg, 1834) from the Great Barrier Reef over a 16 day interval. High DIC availability did not affect photosynthesis, respiration and light calcification, but significantly reduced dark calcification and growth by 50 and 23%, respectively. High DOC availability reduced net and gross photosynthesis by 51% and 39%, respectively, but did not affect respiration. DOC addition did not influence calcification, but significantly increased growth by 42%. Combination of high DIC and high DOC availability did not affect photosynthesis, light calcification, respiration or growth, but significantly decreased dark calcification when compared to both controls and DIC treatments. On the ecosystem level, high DIC concentrations may lead to reduced accretion and growth of reefs dominated by Acropora that under elevated DOC concentrations will likely exhibit reduced primary production rates, ultimately leading to loss of hard substrate and reef erosion. It is therefore important to consider the potential impacts of elevated DOC and DIC simultaneously to assess real world scenarios, as multiple rather than single factors influence key physiological processes in coral reefs.     

Profiling coral reef productivity and calcification using pH and oxygen electrodes

An instrument package carrying pH and oxygen electrodes and a thermister was floated across a reef flat. Using equations associated with the alkalinity anomaly technique, and by making certain assumptions, the productivity and calcification rates of the reef flat were calculated. At an average light intensity of 800 μE · m^?2 · s^?1, the average net oxygen production was 21.5 mmol O₂ · m^?2 · h^?1 and the average rate of calcification was 11.0 mmol CaCO₃ · m^?2 · h^?1. Results showed three metabolic zones within the transect which corresponded to zonation seen in an aerial photograph and confirmed by benthic surveys.     

Productivity and calcification on a coral reef: A survey using pH and oxygen electrode techniques

An instrument package carrying pH and oxygen electrodes and a thermistor was floated across a reef flat at different times of the day and night. The data collected were used to obtain profiles of primary production, respiration, and calcification or solution of reef rock across the transect area. Average rates for these processes across the transect area, and rates at particular points along the transect, were related to light intensity and light response curves were constructed for productivity and calcification. Productivity tended to saturate at high light intensities and the amplitude and shape of the response curve changed with distance from the reef crest. Calcification showed no such saturation. Rates of calcification, and the dependence of calcification on light intensity, increased with distance from the reef crest. Average reef flat gross productivity and net calcification, based on the light response curves were 8.8 g C · m ^?2 · day^?1 and 9.4 g CaCO₃ · m^?2 · day^?1 (3.5 kg · m^?2· yr^?1), and the production: consumption ratio was 1.16:1. These results were obtained in late summer, and are for a cloudless day. The results suggest that the degree of stability in the reef surface, and the amount of disturbance experienced by different reef flat communities, probably exert major controls on reef flat community metabolism.     

Productivity gains do not compensate for reduced calcification under near‐future ocean acidification in the photosynthetic benthic foraminifer species Marginopora vertebralis

Changes in the seawater carbonate chemistry (ocean acidification) from increasing atmospheric carbon dioxide (CO₂) concentrations negatively affect many marine calcifying organisms, but may benefit primary producers under dissolved inorganic carbon (DIC) limitation. To improve predictions of the ecological effects of ocean acidification, the net gains and losses between the processes of photosynthesis and calcification need to be studied jointly on physiological and population levels. We studied productivity, respiration, and abundances of the symbiont‐bearing foraminifer species Marginopora vertebralis on natural CO₂ seeps in Papua New Guinea and conducted additional studies on production and calcification on the Great Barrier Reef (GBR) using artificially enhanced pCO₂. Net oxygen production increased up to 90% with increasing pCO₂; temperature, light, and pH together explaining 61% of the variance in production. Production increased with increasing light and increasing pCO₂ and declined at higher temperatures. Respiration was also significantly elevated (~25%), whereas calcification was reduced (16–39%) at low pH/high pCO₂ compared to present‐day conditions. In the field, M. vertebralis was absent at three CO₂ seep sites at pH_Total levels below ~7.9 (pCO₂ ~700 μatm), but it was found in densities of over 1000 m^?2 at all three control sites. The study showed that endosymbiotic algae in foraminifera benefit from increased DIC availability and may be naturally carbon limited. The observed reduction in calcification may have been caused either by increased energy demands for proton pumping (measured as elevated rates of respiration) or by stronger competition for DIC from the more productive symbionts. The net outcome of these two competing processes is that M. vertebralis cannot maintain populations under pCO₂ exceeding 700 μatm, thus are likely to be extinct in the next century.     

Reef corals, zooxanthellae and free-living algae: a microcosm study that demonstrates synergy between calcification and primary production

A mature, high-biodiversity coral reef microcosm and its chambered subsets were used to examine the relationship between calcification and photosynthesis and its most critical biotic components. Whole ecosystem calcification at 4.0±0.2 kg (40±2 mol) CaCO₃ m⁻² year⁻¹ is related to its primary components (stony coral 17.6%, Halimeda 7.4%, Tridacna 9.0%, algal turf, coralline and foraminifera 29.4%, and miscellaneous invertebrates 36%). Through analysis of the microcosm's daily carbonate system, it is demonstrated that bicarbonate ion, not carbonate ion, is the principal component of total alkalinity reduction in the water column (thus, bicarbonate ion is the principal measured component of calcification as normally measured on reef transects). While chamber-isolated free-living algae remove carbon dioxide, and raise pH and carbonate ion equivalent to that in the microcosm as a whole, no total alkalinity reduction (calcification) occurs. On the other hand, chamber isolated stony corals remove considerable bicarbonate, with very little pH or carbonate ion elevation. Combining the non-calcifying free-living macroalgae Chondria with stony corals in chamber subsets, it is possible to remove more carbon dioxide (elevating pH) and thereby increase coral calcification rates by 60 and 120% above zooxanthellae-mediated rates to 20.6 kg (206 mol) and 18.5 kg (185 mol) CaCO₃ m⁻² year⁻¹ for Acropora and Montipora, respectively. These findings, which support the McConnaughey and Whelan hypothesis of bicarbonate ion neutralization in coral calcification, are easily demonstrated in the controlled microcosm environment.     

Responses of coral gastrovascular cavity pH during light and dark incubations to reduced seawater pH suggest species-specific responses to the effects of ocean acidification on calcification

Coral polyps have a fluid-filled internal compartment, the gastrovascular cavity (GVC). Respiration and photosynthesis cause large daily excursions in GVC oxygen concentration (O₂) and pH, but few studies have examined how this correlates with calcification rates. We hypothesized that GVC chemistry can mediate and ameliorate the effects of decreasing seawater pH (pH_SW) on coral calcification. Microelectrodes were used to monitor O₂ and pH within the GVC of Montastraea cavernosa and Duncanopsammia axifuga (pH only) in both the light and the dark, and three pH_SW levels (8.2, 7.9, and 7.6). At pH_SW 8.2, GVC O₂ ranged from ca. 0 to over 400% saturation in the dark and light, respectively, with transitions from low to high (and vice versa) within minutes of turning the light on or off. For all three pH_SW treatments and both species, pH_GVC was always significantly above and below pH_SW in the light and dark, respectively. For M. cavernosa in the light, pH_GVC reached levels of pH 8.4–8.7 with no difference among pH_SW treatments tested; in the dark, pH_GVC dropped below pH_SW and even below pH 7.0 in some trials at pH_SW 7.6. For D. axifuga in both the light and the dark, pH_GVC decreased linearly as pH_SW decreased. Calcification rates were measured in the light concurrent with measurements of GVC O₂ and pH_GVC. For both species, calcification rates were similar at pH_SW 8.2 and 7.9 but were significantly lower at pH_SW 7.6. Thus, for both species, calcification was protected from seawater acidification by intrinsic coral physiology at pH_SW 7.9 but not 7.6. Calcification was not correlated with pH_GVC for M. cavernosa but was for D. axifuga. These results highlight the diverse responses of corals to changes in pH_SW, their varying abilities to control pH_GVC, and consequently their susceptibility to ocean acidification.

     

Primary Production and Calcification by the Soritid Foraminifer Archais angulatus (Fichtel & Moll)*

SYNOPSIS. Symbiosis between Chlamydomonas hedleyi (Lee, Crockett, Hagen & Stone) and Archais angulatus (Fichtel & Moll) was examined during laboratory studies of primary production and light-enhanced calcification. Photosynthesis and calcification are directly proportional to light intensity in the range of 0–200 μEinsteins m^-2 sec^-1. Calcification in the light is directly proportional to photosynthesis and proceeds at rates that are 2–3 times that observed in the dark. The herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), in concentrations of 1–100 μM, completely inhibits photosynthesis and light-enhanced calcification. Calcification of the foraminiferan test is therefore due to the photosynthetic activity of the symbiote. Calcification rates for foraminifers incubated in the dark or with DCMU are not significantly different from the calcification rates obtained for dead foraminifers. Rates of calcification obtained with ⁴⁵Ca are twice that obtained with ¹⁴C.     

Photosynthesis,Respiration and Exopolymer Calcium-Binding in Biofilm Calcification (Westerhöfer and Deinschwanger Creek,Germany)

The impact of microbial activity on biofilm calcification in aquatic environments is still a matter of debate, especially in settings where ambient water has high CaCO₃ mineral supersaturation. In this study, biofilms of two CO₂-degassing karst-water creeks in Germany, which attain high calcite supersaturation during their course downstream, were investigated with regard to water chemistry of the biofilm microenvironment. The biofilms mainly consisted of filamentous cyanobacteria (Phormidium morphotype) and heterotrophic bacteria (including sulfate-reducing bacteria), which affect the microenvironment and produce acidic exopolymers. In situ and ex situ microelectrode measurements showed that a strong pH increase, coupled with Ca^{2 +} consumption, occurred in light conditions at the biofilm surface, while the opposite occurred in the dark. Calcite supersaturation at the biofilm surface, calculated from ex situ Ca^{2 +} and CO₃ ^2? microelectrode measurements, showed that photosynthesis resulted in high omega values during illumination, while respiration slightly lowered supersaturation values in the dark, compared to values in the water column. Dissociation calculation demonstrated that the potential amount of Ca^{2 +} binding by exopolymers would be insufficient to explain the Ca^{2 +} loss observed, although Ca^{2 +} complexation to exopolymers might be crucial for calcite nucleation. No spontaneous precipitation occurred on biofilm-free limestone substrates under the same condition, regardless of high supersaturation. These facts indicate that photosynthesis is a crucial mechanism to overcome the kinetic barrier for CaCO₃ precipitation, even in highly supersaturated settings.     

Decline in growth of foraminifer Marginopora rossi under eutrophication and ocean acidification scenarios

The combination of global and local stressors is leading to a decline in coral reef health globally. In the case of eutrophication, increased concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) are largely attributed to local land use changes. From the global perspective, increased atmospheric CO₂ levels are not only contributing to global warming but also ocean acidification (OA). Both eutrophication and OA have serious implications for calcium carbonate production and dissolution among calcifying organisms. In particular, benthic foraminifera precipitate the most soluble form of mineral calcium carbonate (high‐Mg calcite), potentially making them more sensitive to dissolution. In this study, a manipulative orthogonal two‐factor experiment was conducted to test the effects of dissolved inorganic nutrients and OA on the growth, respiration and photophysiology of the large photosymbiont‐bearing benthic foraminifer, Marginopora rossi. This study found the growth rate of M. rossi was inhibited by the interaction of eutrophication and acidification. The relationship between M. rossi and its photosymbionts became destabilized due to the photosymbiont's release from nutrient limitation in the nitrate‐enriched treatment, as shown by an increase in zooxanthellae cells per host surface area. Foraminifers from the OA treatments had an increased amount of Chl a per cell, suggesting a greater potential to harvest light energy, however, there was no net benefit to the foraminifer growth. Overall, this study demonstrates that the impacts of OA and eutrophication are dose dependent and interactive. This research indicates an OA threshold at pH 7.6, alone or in combination with eutrophication, will lead to a decline in M. rossi calcification. The decline in foraminifera calcification associated with pollution and OA will have broad ecological implications across their ubiquitous range and suggests that without mitigation it could have serious implications for the future of coral reefs.     

PHOSPHORUS LIMITATION AND CARBON METABOLISM IN A UNICELLULAR ALGA: INTERACTION BETWEEN GROWTH RATE AND THE MEASUREMENT OF NET AND GROSS PHOTOSYNTHESIS1

Chlamydomonas reinhardii Dangeard was grown in continuous culture under P limitation at a range of dilution rates. Carbon uptake measurements were performed using double isotope (¹²C/¹⁴C) techniques and the fluxes of carbon in the light and dark were analysed over the range of growth rates. ¹⁴C uptake was shown to be equal to gross photosynthesis only at maximum relative growth rates; at low relative growth rates ¹⁴C uptake approximated net photosynthesis. The altered pattern of C uptake was found to be due to the suppression of dark respiration in the light and the release of ¹⁴C0₂ from respiratory pathways at low relative growth rates. Metabolic channelling of ¹⁴C from photosynthetic pathways to respiratory pathways occurred at low growth rates as the specific activity of the respired CO₂ reached 45% of the input gas mixture. These data are discussed in the light of the controversy concerning the measurement of gross and net photosynthesis in natural populations and in the light of models of ¹⁴C uptake in single celled algae. Existing models are shown to be adequate for high relative growth rates but not for low relative growth rates under P limitation.     

Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform

Experimental studies have shown that coral calcification rates are dependent on light, nutrients, food availability, temperature, and seawater aragonite saturation (Ω _arag), but the relative importance of each parameter in natural settings remains uncertain. In this study, we applied Calcein fluorescent dyes as time indicators within the skeleton of coral colonies (n = 3) of Porites astreoides and Diploria strigosa at three study sites distributed across the northern Bermuda coral reef platform. We evaluated the correlation between seasonal average growth rates based on coral density and extension rates with average temperature, light, and seawater Ω _arag in an effort to decipher the relative importance of each parameter. The results show significant seasonal differences among coral calcification rates ranging from summer maximums of 243 ± 58 and 274 ± 57 mmol CaCO₃ m^?2 d^?1 to winter minimums of 135 ± 39 and 101 ± 34 mmol CaCO₃ m^?2 d^?1 for P. astreoides and D. strigosa, respectively. We also placed small coral colonies (n = 10) in transparent chambers and measured the instantaneous rate of calcification under light and dark treatments at the same study sites. The results showed that the skeletal growth of D. strigosa and P. astreoides, whether hourly or seasonal, was highly sensitive to Ω _arag. We believe this high sensitivity, however, is misleading, due to covariance between light and Ω _arag, with the former being the strongest driver of calcification variability. For the seasonal data, we assessed the impact that the observed seasonal differences in temperature (4.0 °C), light (5.1 mol photons m^?2 d^?1), and Ω _arag (0.16 units) would have on coral growth rates based on established relationships derived from laboratory studies and found that they could account for approximately 44, 52, and 5 %, respectively, of the observed seasonal change of 81 ± 14 mmol CaCO₃ m^?2 d^?1. Using short-term light and dark incubations, we show how the covariance of light and Ω _arag can lead to the false conclusion that calcification is more sensitive to Ω _arag than it really is.