Seawater carbonate chemistry and sea urchin larval size in a laboratory experiment

Ocean acidification results from an increase in the concentrations of atmospheric carbon dioxide (CO2) impacts on marine calcifying species, which is predicted to become more pronounced in the future. By the end of this century, atmospheric pCO2 levels will have doubled relative to the pre-industrial levels of 280 ppm. However, the effects of pre-industrial pCO2 levels on marine organisms remain largely unknown. In this study, we investigated the effects of pre-industrial pCO2 conditions on the size of the pluteus larvae of sea urchins, which are known to be vulnerable to ocean acidification. The larval size of Hemicentrotus pulcherrimus significantly increased when reared at pre-industrial pCO2 level relative to the present one, and the size of Anthocidaris crassispina larvae decreased as the pCO2 levels increased from the pre-industrial level to the near future ones after 3 days' exposure. In this study, it is suggested that echinoid larvae responded to pre-industrial pCO2 levels. Ocean acidification may be affecting some sensitive marine calcifiers even at the present pCO2 level.

Seawater carbonate chemistry and calcification of the crustose coralline alga Hydrolithon onkodes in a laboratory experiment

Anthropogenic CO2 emissions have exacerbated two environmental stressors, global climate warming and ocean acidification (OA), that have serious implications for marine ecosystems. Coral reefs are vulnerable to climate change yet few studies have explored the potential for interactive effects of warming temperature and OA on an important coral reef calcifier, crustose coralline algae (CCA). Coralline algae serve many important ecosystem functions on coral reefs and are one of the most sensitive organisms to ocean acidification. We investigated the effects of elevated pCO2 and temperature on calcification of Hydrolithon onkodes, an important species of reef-building coralline algae, and the subsequent effects on susceptibility to grazing by sea urchins. H. onkodes was exposed to a fully factorial combination of pCO2 (420, 530, 830 µatm) and temperature (26, 29 °C) treatments, and calcification was measured by the change in buoyant weight after 21 days of treatment exposure. Temperature and pCO2 had a significant interactive effect on net calcification of H. onkodes that was driven by the increased calcification response to moderately elevated pCO2. We demonstrate that the CCA calcification response was variable and non-linear, and that there was a trend for highest calcification at ambient temperature. H. onkodes then was exposed to grazing by the sea urchin Echinothrix diadema, and grazing was quantified by the change in CCA buoyant weight from grazing trials. E. diadema removed 60% more CaCO3 from H. onkodes grown at high temperature and high pCO2 than at ambient temperature and low pCO2. The increased susceptibility to grazing in the high pCO2 treatment is among the first evidence indicating the potential for cascading effects of OA and temperature on coral reef organisms and their ecological interactions.

Svalbard 2010 mesocosm experiment: Protistan diversity

Hierarchical clustering. Taxonomic assignment of reads was performed using a preexisting database of SSU rDNA sequences from including XXX reference sequences generated by Sanger sequencing. Experimental amplicons (reads), sorted by abundance, were then concatenated with the reference extracted sequences sorted by decreasing length. All sequences, experimental and referential, were then clustered to 85% identity using the global alignment clustering option of the uclust module from the usearch v4.0 software (Edgar, 2010). Each 85% cluster was then reclustered at a higher stringency level (86%) and so on (87%, 88%,.) in a hierarchical manner up to 100% similarity. Each experimental sequence was then identified by the list of clusters to which it belonged at 85% to 100% levels. This information can be viewed as a matrix with the lines corresponding to different sequences and the columns corresponding to the cluster membership at each clustering level. Taxonomic assignment for a given read was performed by first looking if reference sequences clustered with the experimental sequence at the 100% clustering level. If this was the case, the last common taxonomic name of the reference sequence(s) within the cluster was used to assign the environmental read. If not, the same procedure was applied to clusters from 99% to 85% similarity if necessary, until a cluster was found containing both the experimental read and reference sequence(s), in which case sequences were taxonomically assigned as described above.

(Table 1) Climate characteristics of the four North American Tundra Experiment (ITEX) sites

Climate warming is expected to differentially affect CO2 exchange of the diverse ecosystems in the Arctic. Quantifying responses of CO2 exchange to warming in these ecosystems will require coordinated experimentation using standard temperature manipulations and measurements. Here, we used the International Tundra Experiment (ITEX) standard warming treatment to determine CO2 flux responses to growing-season warming for ecosystems spanning natural temperature and moisture ranges across the Arctic biome. We used the four North American Arctic ITEX sites (Toolik Lake, Atqasuk, and Barrow [USA] and Alexandra Fiord [Canada]) that span 10° of latitude. At each site, we investigated the CO2 responses to warming in both dry and wet or moist ecosystems. Net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem photosynthesis (GEP) were assessed using chamber techniques conducted over 24-h periods sampled regularly throughout the summers of two years at all sites. At Toolik Lake, warming increased net CO2 losses in both moist and dry ecosystems. In contrast, at Atqasuk and Barrow, warming increased net CO2 uptake in wet ecosystems but increased losses from dry ecosystems. At Alexandra Fiord, warming improved net carbon uptake in the moist ecosystem in both years, but in the wet and dry ecosystems uptake increased in one year and decreased the other. Warming generally increased ER, with the largest increases in dry ecosystems. In wet ecosystems, high soil moisture limited increases in respiration relative to increases in photosynthesis. Warming generally increased GEP, with the notable exception of the Toolik Lake moist ecosystem, where warming unexpectedly decreased GEP >25%. Overall, the respiration response determined the effect of warming on ecosystem CO2 balance. Our results provide the first multiple-site comparison of arctic tundra CO2 flux responses to standard warming treatments across a large climate gradient. These results indicate that (1) dry tundra may be initially the most responsive ecosystems to climate warming by virtue of strong increases in ER, (2) moist and wet tundra responses are dampened by higher water tables and soil water contents, and (3) both GEP and ER are responsive to climate warming, but the magnitudes and directions are ecosystem-dependent.

Seawater conditions during the experiment in May 2011 at the sampling sites off Vulcano Island

The impacts of ocean acidification on coastal biofilms are poorly understood. Carbon dioxide vent areas provide an opportunity to make predictions about the impacts of ocean acidification. We compared biofilms that colonised glass slides in areas exposed to ambient and elevated levels of pCO2 along a coastal pH gradient, with biofilms grown at ambient and reduced light levels. Biofilm production was highest under ambient light levels, but under both light regimes biofilm production was enhanced in seawater with high pCO2. Uronic acids are a component of biofilms and increased significantly with high pCO2. Bacteria and Eukarya denaturing gradient gel electrophoresis profile analysis showed clear differences in the structures of ambient and reduced light biofilm communities, and biofilms grown at high pCO2 compared with ambient conditions. This study characterises biofilm response to natural seabed CO2 seeps and provides a baseline understanding of how coastal ecosystems may respond to increased pCO2 levels.

Experiment: Ultraviolet radiation modulates the physiological responses of the calcified rhodophyte Corallina officinalis to elevated CO2

Ocean acidification reduces the concentration of carbonate ions and increases those of bicarbonate ions in seawater compared with the present oceanic conditions. This altered composition of inorganic carbon species may, by interacting with ultraviolet radiation (UVR), affect the physiology of macroalgal species. However, very little is known about how calcareous algae respond to UVR and ocean acidification. Therefore, we conducted an experiment to determine the effects of UVR and ocean acidification on the calcified rhodophyte Corallina officinalis using CO2-enriched cultures with and without UVR exposure. Low pH increased the relative electron transport rates (rETR) but decreased the CaCO3 content and had a miniscule effect on growth. However, UVA (4.25 W m-2) and a moderate level of UVB (0.5 W m-2) increased the rETR and growth rates in C. officinalis, and there was a significant interactive effect of pH and UVR on UVR-absorbing compound concentrations. Thus, at low irradiance, pH and UVR interact in a way that affects the multiple physiological responses of C. officinalis differently. In particular, changes in the skeletal content induced by low pH may affect how C. officinalis absorbs and uses light. Therefore, the light quality used in ocean acidification experiments will affect the predictions of how calcified macroalgae will respond to elevated CO2.