Response of two marine bacterial isolates to high CO2 concentration

Experimental results related to the effects of ocean acidification on planktonic marine microbes are still rather inconsistent and occasionally contradictory. Moreover, laboratory or field experiments that address the effects of changes in CO₂ concentrations on heterotrophic microbes are very scarce, despite the major role of these organisms in the marine carbon cycle. We tested the direct effect of an elevated CO₂ concentration (1000 ppmv) on the biomass and metabolic rates (leucine incorporation, CO₂ fixation and respiration) of 2 isolates belonging to 2 relevant marine bacterial families, Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217). Our results demonstrate that, contrary to some expectations, high pCO₂ did not negatively affect bacterial growth but increased growth efficiency in the case of MED217. The elevated partial pressure of CO₂ ( pCO₂) caused, in both cases, higher rates of CO₂ fixation in the dissolved fraction and, in the case of MED217, lower respiration rates. Both responses would tend to increase the pH of seawater acting as a negative feedback between elevated atmospheric CO₂ concentrations and ocean acidification.

Rising CO2 and increased light exposure synergistically reduce marine primary productivity

Carbon dioxide and light are two major prerequisites of photosynthesis. Rising CO₂ levels in oceanic surface waters in combination with ample light supply are therefore often considered stimulatory to marine primary production^{1, 2, 3}. Here we show that the combination of an increase in both CO₂ and light exposure negatively impacts photosynthesis and growth of marine primary producers. When exposed to CO₂ concentrations projected for the end of this century⁴, natural phytoplankton assemblages of the South China Sea responded with decreased primary production and increased light stress at light intensities representative of the upper surface layer. The phytoplankton community shifted away from diatoms, the dominant phytoplankton group during our field campaigns. To examine the underlying mechanisms of the observed responses, we grew diatoms at different CO₂ concentrations and under varying levels (5–100%) of solar radiation experienced by the phytoplankton at different depths of the euphotic zone. Above 22–36% of incident surface irradiance, growth rates in the high-CO₂-grown cells were inversely related to light levels and exhibited reduced thresholds at which light becomes inhibitory. Future shoaling of upper-mixed-layer depths will expose phytoplankton to increased mean light intensities⁵. In combination with rising CO₂ levels, this may cause a widespread decline in marine primary production and a community shift away from diatoms, the main algal group that supports higher trophic levels and carbon export in the ocean.

Effects of increased CO2 and nutrients on seagrass (Cymodocea nodosa) metabolism

Continuous global change leads to acidify ocean through increasing of atmospheric CO2 level which is major issue for our ecosysem now-a-days. Addressing this ocean acidification and ongoing anthropogenic problems of eutrophication with ocean temperature increase and teir cumulative impacts or interactive effects are still demanding a lot in research arena of oceanic environment. In this connection, this experiment conducted to investigate the effect of both nutrient and CO2 enrichment on the net community production (NCP) of Cymodocea nodosa beds collected from the western sector of the highly dynamic coastal lagoon Ria Formosa (south Portugal: 37° 01´ N, 7° 50´ W) in a mesocosm set up situated in Ramalhete Marine Station of University of Algarve where the open circulation of seawater exiss. To address the interaction with seagrass metabolism; two types of CO2 concentration (enriched: 700 ppm with pH 7.84 and control: existing 370 ppm with pH 8.12) and two types of nutrient concentration (enriched and control) were used with seawater. However, four types of different combinations from CO2 and nutrient concentration can explain effects of net community production for to complementary methods performed: light incubation and dark incubation. To estimate seagrass community metabolism, I measured change in calculated concentration of dissolved inorganic carbon (DIC) throughout photosynthesis and respiration from conducted twelve light incubations and nine dark incubations respectively in diferent days and times in order to catch possible wider range of underwater irradiances in case of light incubations. There were mild different trends suggesting increased production (± 38000 µmol C h-1 m-2) at underwater irradiance of ± 900 PAR µmol m-2 s-1 in the treatment of enriched nutrients and control CO2 concentration while decreased production (± 30000 µmol C h-1 m-2) found in the treatment with control CO2 and control nutrient at same irradiance. However, in consider to daytime, the net community production in afternoon found to differ a little bit after photoinhibition (observed at 13.30 h with ±1100 PAR µmol m-2 s-1) where maximum increased of NCP (± 35000 µmol C h-1 m-2) found at 17.00 h in the enriched (both in CO2 and nutrient) treatment. In all cases, average positive NCP values (from light) are found lower than the average negative NCP values (from dark) suggesting more community respiration in the equal day-night dates though the treatment with control CO2 and enriched nitrogen showed maximum net community production (around 60000 µmol C h-1 m-2) in the study place of south Portugal in the month of April-May when the daylight existed around 14 hours in a day. However, both CO2 and Nitrogen contents of seawater were not significantly affected yet in Cymodocea nodosa beds in generally even thouh there was significant difference (p = 0.002) among the daily average net community production of the four treatments. Further study should be carried out in order to better understand the underlying metabolic activities of C. nodosa leading to net community production in elevated CO2 and nutrients concentration to meet the upcoming global change.

Emiliania huxleyi shows identical responses to elevated pCO2 in TA and DIC manipulations

With respect to their sensitivity to ocean acidification, calcifiers such as the coccolithophore Emiliania huxleyi have received special attention, as the process of calcification seems to be particularly sensitive to changes in the marine carbonate system. For E. huxleyi, apparently conflicting results regarding its sensitivity to ocean acidification have been published (Iglesias-Rodriguez et al., 2008a; Riebesell et al., 2000). As possible causes for discrepancies, intra-specific variability and different effects of CO2 manipulation methods, i.e. the manipulation of total alkalinity (TA) or total dissolved inorganic carbon (DIC), have been discussed. While Langer et al. (2009) demonstrate a high degree of intra-specific variability between strains of E. huxleyi, the question whether different CO2 manipulation methods influence the cellular responses has not been resolved yet. In this study, closed TA as well as open and closed DIC manipulation methods were compared with respect to E. huxleyi’s CO2-dependence in growth rate, POC- and PIC-production. The differences in the carbonate chemistry between TA and DIC manipulations were shown not to cause any differences in response patterns, while the latter differed between open and closed DIC manipulation. The two strains investigated showed different sensitivities to acidification of seawater, RCC1256 being more negatively affected in growth rates and PIC production than NZEH.

Impacts of elevated CO2 on organic carbon dynamics in nutrient depleted Okhotsk Sea surface waters

Increasing CO2 in seawater (i.e. ocean acidification) may have various and potentially adverse effects on phytoplankton dynamics and hence the organic carbon dynamics. We conducted a CO2 manipulation experiment in the Sea of Okhotsk in summer 2006 to investigate the response of the organic carbon dynamics. During the 14-day incubation of nutrient depleted and 200 ?atm in situ pCO2 surface water with a natural plankton assemblage under 150, 280, 480, and 590 µatm pCO2, the amount of net dissolved organic carbon accumulation was significantly lower at N480 µatm pCO2 than at 150 µatm pCO2, while differences in net particulate organic carbon accumulation between the treatments were small and did not show a clear relationship with the pCO2. This is the first report to show a decreased net organic carbon production of natural plankton community under elevated pCO2. Phytoplankton pigment analysis suggests that the relative contribution of fucoxanthin-containing phytoplankton such as diatoms to the phytoplankton biomass was lower at N 280 µatm pCO2 than at 150 µatm pCO2. Different pCO2 conditions may alter the organic carbond ynamics through changes in plankton processes. We conclude that the continuing increase in atmospheric CO2 in a time scale from the last half century to the end of this century has potential to affect the carbon cycle in nutrient depleted subpolar surface waters.

Carbon concentrating mechanisms in eukaryotic marine phytoplankton

The accumulation of inorganic carbon from seawater by eukaryotic marine phytoplankton is limited by the diffusion of carbon dioxide (CO₂) in water and the dehydration kinetics of bicarbonate to CO₂ and by ribulose-1,5-bisphosphate carboxylase/oxygenase’s (RubisCO) low affinity for its inorganic carbon substrate, CO₂. Nearly all marine phytoplankton have adapted to these limitations and evolved inorganic carbon (or CO₂) concentrating mechanisms (CCMs) to support photosynthetic carbon fixation at the concentrations of CO₂ present in ocean surface waters (<10–30 μM). The biophysics and biochemistry of CCMs vary within and among the three dominant groups of eukaryotic marine phytoplankton and may involve the activity of external or intracellular carbonic anhydrase, HCO₃⁻ transport, and perhaps a C₄ carbon pump. In general, coccolithophores have low-efficiency CCMs, and diatoms and the haptophyte genus Phaeocystis have high-efficiency CCMs. Dinoflagellates appear to possess moderately efficient CCMs, which may be necessitated by the very low CO₂ affinity of their form II RubisCO. The energetic and nutrient costs of CCMs may modulate how variable CO₂ affects primary production, element composition, and species composition of phytoplankton in the ocean.

Effects of elevated CO2 and phosphorus supply on growth, photosynthesis and nutrient uptake in the marine macroalga Gracilaria lemaneiformis (Rhodophyta)

The red alga Gracilaria lemaneiformis was cultured under different CO2 and phosphorus conditions for 16 days, and its growth, photosynthesis and uptake of nitrate and phosphate were examined in order to establish the longer-term impacts of elevated CO2 and phosphorus supplies on this economically important seaweed. Enrichment with either CO2 or phosphorus in culture markedly increased the growth of G. lemaneiformis compared to the control. Light-saturated photosynthetic rate was enhanced significantly by phosphorus enrichment, but hardly affected by the elevation of CO2 when G. lemaneiformis was grown under low phosphorus conditions. High phosphorus stimulated photosynthetic inorganic carbon utilization and nitrogen uptake. Under low phosphorus conditions, the thalli grown at the high level of CO2 had a lower carbon utilization capacity and a higher nitrogen uptake rate compared to those grown under ambient CO2. Reversed results were found when the algae were grown under high phosphorus conditions. Hence, available phosphorus may regulate inorganic carbon utilization of G. lemaneiformis grown at different CO2 levels, and growth reflected a balance between carbon and nutrient metabolism.

Living in a high CO2 world: impacts of global climate change on marine phytoplankton

The planet is currently going through a period of global climate change, the pace of which is unprecedented in geological history. Marine phytoplankton contribute approximately 50% of the total global primary productivity and play a vital role in global carbon cycling. Consequently it is extremely important to understand the impact that global climate change will have on the ecological performance of these organisms. In this review we summarise current understanding of the influence that global climate change has on the physiological properties, productivity and assemblage composition of marine phytoplankton. While most phytoplankton are likely to show little direct effect of elevated CO2 on photosynthetic rates, some, including the ecologically important coccolithophorids, are likely to show significant stimulation of growth. The rise in temperature consequent upon the elevated atmospheric levels of CO2 and other greenhouse gases will stimulate growth of some species and increase ocean temperatures in some areas beyond their current optima. However, more importantly, increasing global temperatures will stimulate stratification of the water column which, in tropical and mid-latitudes will exacerbate nutrient limitation in surface waters. This in turn will lead to changes in phytoplankton assemblage composition, primary productivity and sensitivity to UVB radiation.

Testing the effects of ocean acidification on algal metabolism: considerations for experimental designs

Ocean acidification describes changes in the carbonate chemistry of the ocean due to the increased absorption of anthropogenically released CO2. Experiments to elucidate the biological effects of ocean acidification on algae are not straightforward because when pH is altered, the carbon speciation in seawater is altered, which has implications for photosynthesis and, for calcifying algae, calcification. Furthermore, photosynthesis, respiration, and calcification will themselves alter the pH of the seawater medium. In this review, algal physiologists and seawater carbonate chemists combine their knowledge to provide the fundamental information on carbon physiology and seawater carbonate chemistry required to comprehend the complexities of how ocean acidification might affect algae metabolism. A wide range in responses of algae to ocean acidification has been observed, which may be explained by differences in algal physiology, timescales of the responses measured, study duration, and the method employed to alter pH. Two methods have been widely used in a range of experimental systems: CO2 bubbling and HCl/NaOH additions. These methods affect the speciation of carbonate ions in the culture medium differently; we discuss how this could influence the biological responses of algae and suggest a third method based on HCl/NaHCO3 additions. We then discuss eight key points that should be considered prior to setting up experiments, including which method of manipulating pH to choose, monitoring during experiments, techniques for adding acidified seawater, biological side effects, and other environmental factors. Finally, we consider incubation timescales and prior conditioning of algae in terms of regulation, acclimation, and adaptation to ocean acidification.

Physiological responses of Mediterranean corals to temperature and pH perturbations

Understanding coral responses to environmental change is critical to predicting their health and future distribution. While tropical and subtropical corals generally experience limited environmental changes, temperate corals undergo pronounced seasonal cycles in irradiance, temperature and nutrients. Their ability to cope with environmental changes makes temperate organisms an ideal model to investigate the effects of global change on coral physiology.

Using the two symbiotic Mediterranean corals, Cladocora caespitosa and Oculina patagonica, we studied the effects of natural changes in light and temperature on their physiological responses both in laboratory and in situ. Temperature was shown to be the major factor affecting coral metabolism. Photosynthetic and growth rates were maximal under normal summer temperatures up to a physiological threshold after which very high temperatures induced, in both coral species, a severe decrease in the rates of photosynthesis and growth. Coral response to high temperatures was species-specific, and highly dependent on the amplitude and length of the temperature stress. These results, together with the observations of recurrent mass-mortalities during recent summers suggest that endemic Mediterranean corals are living near their upper thermal limits. A second study assessed the effects of seasonality, thermal stress (+ 3 °C) and ocean acidification (700 μatm; end-of-century prediction) on the coral C. caespitosa. Corals were collected at 30 m depth and maintained in aquaria over a one-year period under experimental conditions. Surprisingly, calcification and photosynthetic rates were not affected by the high pCO2 conditions, while the seasonal change in temperature had significant effects.