Posts Tagged 'algae'

Inorganic carbon physiology underpins macroalgal responses to elevated CO2

Beneficial effects of CO2 on photosynthetic organisms will be a key driver of ecosystem change under ocean acidification. Predicting the responses of macroalgal species to ocean acidification is complex, but we demonstrate that the response of assemblages to elevated CO2 are correlated with inorganic carbon physiology. We assessed abundance patterns and a proxy for CO2:HCO3− use (δ13C values) of macroalgae along a gradient of CO2 at a volcanic seep, and examined how shifts in species abundance at other Mediterranean seeps are related to macroalgal inorganic carbon physiology. Five macroalgal species capable of using both HCO3− and CO2 had greater CO2 use as concentrations increased. These species (and one unable to use HCO3−) increased in abundance with elevated CO2 whereas obligate calcifying species, and non-calcareous macroalgae whose CO2 use did not increase consistently with concentration, declined in abundance. Physiological groupings provide a mechanistic understanding that will aid us in determining which species will benefit from ocean acidification and why.

Continue reading ‘Inorganic carbon physiology underpins macroalgal responses to elevated CO2’

Effects of elevated nutrients and CO2 emission scenarios on three coral reef macroalgae

Coral reef macroalgae are expected to thrive in the future under conditions that are deleterious to the health of reef-building corals. Here we examined how macroalgae would be affected by exposure to future CO2 emission scenarios (pCO2 and temperature), enriched nutrients and combinations of both. The species tested, Laurencia intricata (Rhodophyta), Turbinaria ornata and Chnoospora implexa (both Phaeophyceae), have active carbon-concentrating mechanisms but responded differently to the treatments. L. intricata showed high mortality under nutrient enriched RCP4.5 (“reduced” CO2 emission) and RCP8.5 (“business-as-usual” CO2 emission) and grew best under pre-industrial (PI) conditions, where it could take up carbon using external carbonic anhydrase combined, potentially, with proton extrusion. T. ornata’s growth rate showed a trend for reduction under RCP8.5 but was unaffected by nutrient enrichment. In C. implexa, highest growth was observed under PI conditions, but highest net photosynthesis occurred under RCP8.5, suggesting that under RCP8.5, carbon is stored and respired at greater rates while it is directed to growth under PI conditions. None of the species showed growth enhancement under future scenarios, nutrient enrichment or combinations of both. This leads to the conclusion that under such conditions these species are unlikely to pose an increasing threat to coral reefs.

Continue reading ‘Effects of elevated nutrients and CO2 emission scenarios on three coral reef macroalgae’

Altered epiphyte community and sea urchin diet in Posidonia oceanica meadows in the vicinity of submarine volcanic CO2 vents

Ocean acidification (OA) predicted for 2100 is expected to shift seagrass epiphyte communities towards the dominance of more tolerant non-calcifying taxa. However, little is known about the indirect effects of such changes on food provision to key seagrass consumers. We found that epiphyte communities of the seagrass Posidonia oceanica in two naturally acidified sites (i.e. north and south sides of a volcanic CO2 vent) and in a control site away from the vent at the Ischia Island (NW Mediterranean Sea) significantly differed in composition and abundance. Such differences involved a higher abundance of non-calcareous crustose brown algae and a decline of calcifying polychaetes in both acidified sites. A lower epiphytic abundance of crustose coralline algae occurred only in the south side of the vents, thus suggesting that OA may alter epiphyte assemblages in different ways due to interaction with local factors such as differential fish herbivory or hydrodynamics. The OA effects on food items (seagrass, epiphytes, and algae) indirectly propagated into food provision to the sea urchin Paracentrotus lividus, as reflected by a reduced P. oceanica exploitation (i.e. less seagrass and calcareous epiphytes in the diet) in favour of non-calcareous green algae in both vent sites. In contrast, we detected no difference close and outside the vents neither in the composition of sea urchin diet nor in the total abundance of calcareous versus non-calcareous taxa. More research, under realistic scenarios of predicted pH reduction (i.e. ≤ 0.32 units of pH by 2100), is still necessary to better understand cascading effects of this altered urchin exploitation of food resources under acidified conditions on ecosystem diversity and function.

Continue reading ‘Altered epiphyte community and sea urchin diet in Posidonia oceanica meadows in the vicinity of submarine volcanic CO2 vents’

Effects of in situ CO2 enrichment on Posidonia oceanica epiphytic community composition and mineralogy

Alterations in seagrass epiphytic communities are expected under future ocean acidification conditions, yet this hypothesis has been little tested in situ. A Free Ocean Carbon Dioxide Enrichment system was used to lower pH by a ~0.3 unit offset within a partially enclosed portion (1.7 m3) of a Posidonia oceanica meadow (11 m depth) between June 21 and November 3, 2014. Leaf epiphytic community composition (% cover) and bulk epiphytic mineralogy were compared every 4 weeks within three treatments, located in the same meadow: a pH-manipulated (experimental enclosure) and a control enclosure, as well as a nearby ambient area. Percent coverage of invertebrate calcifiers and crustose coralline algae (CCA) did not appear to be affected by the lowered pH. Furthermore, fleshy algae did not proliferate at lowered pH. Only Foraminifera, which covered less than 3% of leaf surfaces, declined in manner consistent with ocean acidification predictions. Bulk epiphytic magnesium carbonate composition was similar between treatments and percentage of magnesium appeared to increase from summer to autumn. CCA did not exhibit any visible skeleton dissolution or mineral alteration at lowered pH and carbonate saturation state. Negative impacts from ocean acidification on P. oceanica epiphytic communities were smaller than expected. Epiphytic calcifiers were possibly protected from the pH treatment due to host plant photosynthesis inside the enclosure where water flow is slowed. The more positive outcome than expected suggests that calcareous members of epiphytic communities may find refuge in some conditions and be resilient to environmentally relevant changes in carbonate chemistry.

Continue reading ‘Effects of in situ CO2 enrichment on Posidonia oceanica epiphytic community composition and mineralogy’

Acclimation of bloom-forming and perennial seaweeds to elevated pCO2 conserved across levels of environmental complexity

Macroalgae contribute approximately 15% of the primary productivity in coastal marine ecosystems, fix up to 27.4 Tg of carbon per year, and provide important structural components for life in coastal waters. Despite this ecological and commercial importance, direct measurements and comparisons of the short-term responses to elevated pCO2 in seaweeds with different life-history strategies are scarce. Here, we cultured several seaweed species (bloom-forming/non-bloom-forming/perennial/annual) in the laboratory, in tanks in an in-door mesocosm facility, and in coastal mesocosms under pCO2 levels ranging from 400 μatm to 2000 μatm. We find that, across all scales of the experimental set-up, ephemeral species of the genus Ulva increase their photosynthesis and growth rates in response to elevated pCO2 the most, whereas longer-lived perennial species show a smaller increase or a decrease. These differences in short-term growth- and photosynthesis rates are likely to give bloom-forming green seaweeds a competitive advantage in mixed communities, and our results thus suggest that coastal seaweed assemblages in eutrophic waters may undergo an initial shift toward communities dominated by bloom-forming, short-lived seaweeds.

Continue reading ‘Acclimation of bloom-forming and perennial seaweeds to elevated pCO2 conserved across levels of environmental complexity’

Coralline algae elevate pH at the site of calcification under ocean acidification

Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species-specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pHcf) using δ11B as a proxy. Declines in δ11B for all three species are consistent with shifts in δ11B expected if B(OH)4− was incorporated during precipitation. In particular, the δ11B ratio in Amphiroa anceps was too low to allow for reasonable pHcf values if B(OH)3 rather than B(OH)4− was directly incorporated from the calcifying fluid. This points towards δ11B being a reliable proxy for pHcf for coralline algal calcite and that if B(OH)3 is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH)4−. We thus show that pHcf is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO2, as did their pHcf. Neogoniolithon sp. had the highest pHcf, and most constant calcification rates, with the decrease in pHcf being ¼ that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pHcf under ocean acidification is physiologically important and should be included in future models involving calcification.

Continue reading ‘Coralline algae elevate pH at the site of calcification under ocean acidification’

Differential growth responses to water flow and reduced pH in tropical marine macroalgae

The physical environment plays a key role in facilitating the transfer of nutrients and dissolved gases to marine organisms and can alter the rate of delivery of dissolved inorganic carbon. For non-calcifying macroalgae, water motion can influence the physiological and ecological responses to various environmental changes such as ocean acidification (OA). We tested the effects of lowered pH under three different flow speeds on three dominant non-calcifying macroalgal species differing in their carbon-use and are commonly found in the back reefs of Moorea, French Polynesia. Relative growth rates (RGR) of two phaeophytes, Dictyota bartayresiana and Lobophora variegata (HCO3− users), and a rhodophyte, Amansia rhodantha (CO2 user) were measured to examine how the combined effects of OA and flow can affect algal growth. Growth rates were affected independently by pCO2 and flow treatments but there was no significant interactive effect. Additionally, growth rates among species varied within the different flow regimes. Of the three species, L. variegata had the overall greatest increase in RGR across all three flow speeds while A. rhodantha exhibited the greatest negative impact under elevated pCO2 at 0.1 cm·s− 1. These differential responses among algal species demonstrate the importance of flow when examining responses to a changing environment, and if the responses of macroalgae differ based on their carbon-use strategies, it may provide advantages to some macroalgal species in a future, more acidic ocean.

Continue reading ‘Differential growth responses to water flow and reduced pH in tropical marine macroalgae’


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Ocean acidification in the IPCC AR5 WG II

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