Ocean Acidification: Comparative Impacts on the Photophysiology of a Temperate Symbiotic Sea Anemone and a Tropical Coral

<p>Ocean acidification has the potential to drastically alter the coral reef ecosystem by reducing the calcification rate of corals and other reef-builders, and hence a considerable amount of research is now focused on this issue. It also is conceivable that acidification may affect other physiological processes of corals. In particular, acidification may alter photosynthetic physiology and hence the productivity of the coraldinoflagellate symbiosis that is pivotal to the reef's survival and growth. However, very little is known about the impacts of acidification on the photophysiology of corals or, indeed, other invertebrate-algal symbioses. This gap in our knowledge was addressed here by measuring the impacts of acidification (pH 7.6 versus pH 8.1) on the photophysiology and health of the tropical coral Stylophora pistillata and its isolated dinoflagellate symbionts ('zooxanthellae'), and the temperate sea anemone Anthopleura aureoradiata. The comparative nature of this study allowed for any differences between tropical and temperate symbioses, and zooxanthellae in a symbiotic or free-living state, to be assessed. Corals, anemones and cultured zooxanthellae were maintained in flowthrough seawater systems, and treated either with non-acidified (control) seawater at pH 8.1, or seawater acidified with CO2 or HCl to pH 7.6. A variety of parameters, including zooxanthellar density, chlorophyll content, photosynthetic health (Yi), and the ratio of gross photosynthetic production to respiration (P:R) were measured via cell counts, spectrophotometry, respirometry and PAM fluorometry, at a series of time-points up to a maximum of 42 days. Acidification generated by the addition of CO2 had no discernible effect on Yi of either the corals or anemones. However, in the coral, chlorophyll content per zooxanthella cell increased by 25%, which was countered by a near-significant decline (22%) in the rate of gross photosynthesis per unit chlorophyll; as zooxanthellar density remained unchanged, this led to a constant P:R ratio. When acidified via CO2, the isolated zooxanthellae exhibited no impacts in recorded Yi or chlorophyll levels. The response of the anemone to acidification via CO2 was different to that observed in the coral, as the density of zooxanthellae increased, rather than the chlorophyll content per cell, leading to an increased rate of gross photosynthesis. However P:R again remained constant as the increased photosynthesis was matched by an increased rate of respiration. In contrast to the impacts of CO2, HCl adversely impacted the chlorophyll content per cell in both the isolated zooxanthellae and sea anemone, and Yi, gross photosynthesis per cell, and overall gross photosynthesis in the sea anemone; however, despite the decline in gross photosynthesis, P:R remained constant due to the concurrent decline in respiration. Unfortunately, the corals in the HCl experiment died due to technical issues. There are two plausible reasons for this difference between CO2 and HCl. Firstly, HCl may have caused intracellular acidosis which damaged chloroplast structure and photosynthetic function. Secondly, the increased levels of aqueous CO2 stimulated photosynthetic function and hence mitigated for the effects of lowered pH. In addition, evidence is presented for a pH threshold for A. aureoradiata of between pH 6 and pH 6.75 (acidified with HCl), at which point photosynthesis 'shuts-down'. This suggests that, even without the potentially beneficial effects from increased CO2 levels, it is likely that oceanic pH would need to decrease to less than pH 6.75 for any acidosis effects to compromise the productivity of this particular symbiosis. Since acidification will have the benefits of increased CO2 and will reach nowhere near such low pH levels as those extremes tested here, it is proposed that ocean acidification via increased dissolution of CO2 into our oceans will have no impact on the photosynthetic production of symbiotic cnidarians. Indeed, it is entirely likely that increased CO2 will add some benefit to the usually carbon-limited symbiotic zooxanthellae. Ocean acidification is not likely to benefit corals however, with compromised calcification rates likely to undermine the viability of the coral. Symbiotic sea anemones, which do not bio-mineralise CaCO3, are better placed to take advantage of the increased CO2 as we move toward more acidic oceans.</p>

Ocean Acidification: Comparative Impacts on the Photophysiology of a Temperate Symbiotic Sea Anemone and a Tropical Coral

<p>Ocean acidification has the potential to drastically alter the coral reef ecosystem by reducing the calcification rate of corals and other reef-builders, and hence a considerable amount of research is now focused on this issue. It also is conceivable that acidification may affect other physiological processes of corals. In particular, acidification may alter photosynthetic physiology and hence the productivity of the coraldinoflagellate symbiosis that is pivotal to the reef's survival and growth. However, very little is known about the impacts of acidification on the photophysiology of corals or, indeed, other invertebrate-algal symbioses. This gap in our knowledge was addressed here by measuring the impacts of acidification (pH 7.6 versus pH 8.1) on the photophysiology and health of the tropical coral Stylophora pistillata and its isolated dinoflagellate symbionts ('zooxanthellae'), and the temperate sea anemone Anthopleura aureoradiata. The comparative nature of this study allowed for any differences between tropical and temperate symbioses, and zooxanthellae in a symbiotic or free-living state, to be assessed. Corals, anemones and cultured zooxanthellae were maintained in flowthrough seawater systems, and treated either with non-acidified (control) seawater at pH 8.1, or seawater acidified with CO2 or HCl to pH 7.6. A variety of parameters, including zooxanthellar density, chlorophyll content, photosynthetic health (Yi), and the ratio of gross photosynthetic production to respiration (P:R) were measured via cell counts, spectrophotometry, respirometry and PAM fluorometry, at a series of time-points up to a maximum of 42 days. Acidification generated by the addition of CO2 had no discernible effect on Yi of either the corals or anemones. However, in the coral, chlorophyll content per zooxanthella cell increased by 25%, which was countered by a near-significant decline (22%) in the rate of gross photosynthesis per unit chlorophyll; as zooxanthellar density remained unchanged, this led to a constant P:R ratio. When acidified via CO2, the isolated zooxanthellae exhibited no impacts in recorded Yi or chlorophyll levels. The response of the anemone to acidification via CO2 was different to that observed in the coral, as the density of zooxanthellae increased, rather than the chlorophyll content per cell, leading to an increased rate of gross photosynthesis. However P:R again remained constant as the increased photosynthesis was matched by an increased rate of respiration. In contrast to the impacts of CO2, HCl adversely impacted the chlorophyll content per cell in both the isolated zooxanthellae and sea anemone, and Yi, gross photosynthesis per cell, and overall gross photosynthesis in the sea anemone; however, despite the decline in gross photosynthesis, P:R remained constant due to the concurrent decline in respiration. Unfortunately, the corals in the HCl experiment died due to technical issues. There are two plausible reasons for this difference between CO2 and HCl. Firstly, HCl may have caused intracellular acidosis which damaged chloroplast structure and photosynthetic function. Secondly, the increased levels of aqueous CO2 stimulated photosynthetic function and hence mitigated for the effects of lowered pH. In addition, evidence is presented for a pH threshold for A. aureoradiata of between pH 6 and pH 6.75 (acidified with HCl), at which point photosynthesis 'shuts-down'. This suggests that, even without the potentially beneficial effects from increased CO2 levels, it is likely that oceanic pH would need to decrease to less than pH 6.75 for any acidosis effects to compromise the productivity of this particular symbiosis. Since acidification will have the benefits of increased CO2 and will reach nowhere near such low pH levels as those extremes tested here, it is proposed that ocean acidification via increased dissolution of CO2 into our oceans will have no impact on the photosynthetic production of symbiotic cnidarians. Indeed, it is entirely likely that increased CO2 will add some benefit to the usually carbon-limited symbiotic zooxanthellae. Ocean acidification is not likely to benefit corals however, with compromised calcification rates likely to undermine the viability of the coral. Symbiotic sea anemones, which do not bio-mineralise CaCO3, are better placed to take advantage of the increased CO2 as we move toward more acidic oceans.</p>

Photosynthesis and respiration of the soft coral Xenia umbellata respond to warming but not to organic carbon eutrophication

Eutrophication with dissolved organic carbon (DOC) as a far under-investigated stressor, and ocean warming, can strongly affect coral reefs and hard corals as major reefs ecosystem engineers. However, no previous studies have investigated the metabolic responses of soft corals to DOC eutrophication, or its interaction with ocean warming. Thus, we investigated respiration and photosynthesis response of Xenia umbellata, a common mixotrophic soft coral from the Indo-pacific, to (1) three levels of DOC eutrophication simulated by glucose addition over the first 21 days of experiment and (2) ocean warming scenarios where the temperature was gradually increased from 26 °C (control condition) to 32 °C over another 24 days in an aquarium experiment. We found no significant difference in response to DOC treatments and all corals survived regardless of the DOC concentrations, whilst subsequent exposure to simulated ocean warming significantly decreased gross photosynthesis by approximately 50% at 30 °C, and 65% at 32 °C, net photosynthesis by 75% at 30 °C and 79% at 32 °C, and respiration by a maximum of 75% at 30 °C; with a slight increase at 32 °C of 25%. The ratio between gross photosynthesis and respiration decreased by the end of the warming period but remained similar between controls and colonies previously exposed to DOC. Our findings suggest that soft corals may be more resistant than hard corals to DOC eutrophication and in consequence, may potentially experiment in less magnitude the negative effects of increased temperature or subsequently both stressors. The results of this study may contribute to explain the successful role of soft corals in phase shifts as reported from many coral reefs. Where predicted declines in reef ecosystems health due to increased eutrophication levels can be exacerbated by future warming.

Controls on Nutrient Cycling in Estuarine Mangrove Lake Sediments

We estimated the net exchange of nitrogen and phosphorus species using core incubations under light and dark conditions in estuarine lakes that are the aquatic interface between the freshwater Everglades and marine Florida Bay. These lakes and adjacent shallow water Florida Bay environments are sites where the restoration of hydrological flows will likely have the largest impact on salinity. Sediment respiration, measured by oxygen uptake, averaged (±S.D.) −2400 ± 1300, −300 ± 1000, and 1900 ± 1400 μmol m−2 h−1 for dark incubations, light incubations, and gross photosynthesis estimates, respectively, with dark incubations consistent with oxygen uptake measured by microelectrode profiles. Although most fluxes of soluble reactive phosphorus, nitrate, and N2–N were low under both light and dark incubation conditions, we observed a number of very high efflux events of NH4+ during dark incubations. A significant decrease in NH4+flux was observed in the light. The largest differences between light and dark effluxes of NH4+ occurred in lakes during periods of low coverage of the aquatic macrophyte Chara hornemannii Wallman, with NH4+ effluxes > 200 μmol m−2 h−1. Increasing freshwater flow from the Everglades is expected to expand lower salinity environments suitable for Chara, and therefore, diminish the sediment NH4+ effluxes that may fuel algal blooms.

Potential local adaptation of corals at acidified and warmed Nikko Bay, Palau

Ocean warming and acidification caused by increases of atmospheric carbon dioxide are now thought to be major threats to coral reefs on a global scale. Here we evaluated the environmental conditions and benthic community structures in semi-closed Nikko Bay at the inner reef area in Palau, which has high pCO2 and seawater temperature conditions with high zooxanthellate coral coverage. Nikko Bay is a highly sheltered system with organisms showing low connectivity with surrounding environments, making this bay a unique site for evaluating adaptation and acclimatization responses of organisms to warmed and acidified environments. Seawater pCO2/Ωarag showed strong gradation ranging from 380 to 982 µatm (Ωarag: 1.79–3.66), and benthic coverage, including soft corals and turf algae, changed along with Ωarag while hard coral coverage did not change. In contrast to previous studies, net calcification was maintained in Nikko Bay even under very low mean Ωarag (2.44). Reciprocal transplantation of the dominant coral Porites cylindrica showed that the calcification rate of corals from Nikko Bay did not change when transplanted to a reference site, while calcification of reference site corals decreased when transplanted to Nikko Bay. Corals transplanted out of their origin sites also showed the highest interactive respiration (R) and lower gross photosynthesis (Pg) to respiration (Pg:R), indicating higher energy acquirement of corals at their origin site. The results of this study give important insights about the potential local acclimatization and adaptation capacity of corals to different environmental conditions including pCO2 and temperature.

High photosynthetic plasticity may reinforce invasiveness of upside-down zooxanthellate jellyfish in Mediterranean coastal waters

Ecological profiling of non-native species is essential to predict their dispersal and invasiveness potential across different areas of the world. Cassiopea is a monophyletic taxonomic group of scyphozoan mixotrophic jellyfish including C. andromeda, a recent colonizer of sheltered, shallow-water habitats of the Mediterranean Sea, such as harbors and other light-limited, eutrophic coastal habitats. To assess the ecophysiological plasticity of Cassiopea jellyfish and their potential to spread across the Mare Nostrum by secondary introductions, we investigated rapid photosynthetic responses of jellyfish to irradiance transitions—from reduced to increased irradiance conditions (as paradigm of transition from harbors to coastal, meso/oligotrophic habitats). Laboratory incubation experiments were carried out to compare oxygen fluxes and photobiological variables in Cassiopea sp. immature specimens pre-acclimated to low irradiance (PAR = 200 μmol photons m−2 s−1) and specimens rapidly exposed to higher irradiance levels (PAR = 500 μmol photons m−2 s−1). Comparable photosynthetic potential and high photosynthetic rates were measured at both irradiance values, as also shown by the rapid light curves. No significant differences were observed in terms of symbiont abundance between control and treated specimens. However, jellyfish kept at the low irradiance showed a higher content in chlorophyll a and c (0.76±0.51SD vs 0.46±0.13SD mg g-1 AFDW) and a higher Ci (amount of chlorophyll per cell) compared to jellyfish exposed to higher irradiance levels. The ratio between gross photosynthesis and respiration (P:R) was >1, indicating a significant input from the autotrophic metabolism. Cassiopea sp. specimens showed high photosynthetic performances, at both low and high irradiance, demonstrating high potential to adapt to sudden changes in light exposure. Such photosynthetic plasticity, combined with Cassiopea eurythermal tolerance and mixotrophic behavior, jointly suggest the upside-down jellyfish as a potentially successful invader in the scenario of a warming Mediterranean Sea.

Photosynthetic responses of Halimeda scabra (Chlorophyta, Bryopsidales) to interactive effects of temperature, pH, and nutrients and its carbon pathways

In this study, we evaluated the interactive effects of temperature, pH, and nutrients on photosynthetic performance in the calcareous tropical macroalga Halimeda scabra. A significant interaction among these factors on gross photosynthesis (Pgross) was found. The highest values of Pgross were reached at the highest temperature, pH, and nutrient enrichment tested and similarly in the control treatment (no added nutrients) at 33 °C at the lowest pH. The Q10 Pgross values confirmed the effect of temperature only under nutrient enrichment scenarios. Besides the above, bicarbonate (HCO3−) absorption was assessed by the content of carbon stable isotope (δ13C) in algae tissue and by its incorporation into photosynthetic products, as well as by carbonic anhydrase (CA) inhibitors (Acetazolamide, AZ and Ethoxyzolamide, EZ) assays. The labeling of δ13C revealed this species uses both, CO2 and HCO3− forms of Ci relying on a CO2 Concentration Mechanism (CCM). These results were validated by the EZ-AZ inhibition assays in which photosynthesis inhibition was observed, indicating the action of internal CA, whereas AZ inhibitor did not affect maximum photosynthesis (Pmax). The incorporation of 13C isotope into aspartate in light and dark treatments also confirmed photosynthetic and non-photosynthetic the HCO3−uptake.

Nutrient cycling and productivity of a Himalayan Glacier Surface

&lt;p&gt;Nutrients deposited and cycled on the glacier surfaces are important not only because of their role in the global biogeochemical cycling in the downstream environments, but also because of their importance as a primary food source for microbes inhabiting glacial surfaces, their ice surface darkening properties, and the consequent potential for enhancing glacier melt. The present study focuses on the Chhota Shigri (CS) Glacier in the North-Western (NW) Indian Himalayan region. The dissolved organic carbon (DOC) concentration in the bare ice is relatively higher than the other aspects studied in the glacial environment of CS indicating that much of the active microbial activity occurring in the bare ice. Total inorganic nitrogen (TIN) is typically concentrated in the snow, which is the major contributor of NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;. The rapid declining of TIN in the bare ice as compare to other aspects and its enrichment in DOC suggests for a more active microbial activity occurrence in the exposed ice rather than in isolated cryoconites holes in the Chhota Shigri Glacier. The net biological productivity indicates the dominance of net gross photosynthesis over respiration, suggesting a net autotrophic production in CS.&lt;/p&gt;

Impacts of long-term warming and enhanced nitrogen and sulfur deposition on carbon sink potential in a boreal peatland

&lt;p&gt;In order to assess peatland carbon sink potential under multiple global change perturbations, we examined the individual and combined effects of long-term warming and enhanced nitrogen (N) and sulfur (S) deposition on ecosystem CO&lt;sub&gt;2 &lt;/sub&gt;exchange at one of the longest-running experiments on peatlands, Deger&amp;#246; Stormyr poor fen, Sweden. The site has been treated with NH&lt;sub&gt;4&lt;/sub&gt;NO&lt;sub&gt;3&lt;/sub&gt; (15 times ambient annual wet deposition), Na&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; (6 times ambient annual wet deposition) and elevated temperature (air +3.6 C) for 23 years. Gross photosynthesis, ecosystem respiration and net CO&lt;sub&gt;2&lt;/sub&gt; exchange were measured weekly during June-August using chambers. After 23 years, two of the experimental perturbations: N addition and warming individually reduced net CO&lt;sub&gt;2&lt;/sub&gt; uptake potential down to 0.3-0.4 fold compared to the control mainly due to lower gross photosynthesis. Under S only treatment ecosystem CO&lt;sub&gt;2&lt;/sub&gt; fluxes were largely unaltered. In contrast, the combination of S and N deposition and warming led to a more pronounced effect and close to zero net CO&lt;sub&gt;2&lt;/sub&gt; uptake potential or net C source. Our study emphasizes the value of the long-term multifactor experiments in examining the ecosystem responses: simultaneous perturbations can have nonadditive interactions that cannot be predicted based on individual responses and thus, must be studied in combination when evaluating feedback mechanisms to ecosystem C sink potential under global change.&lt;/p&gt;