Influence of drainage basin topography and elevation on carbon dioxide and methane supersaturation of stream water

The partial pressures of CO₂ (pCO₂) andCH₄ (pCH₄) in streams are not only governed byinstream processes, but also by transformations occurring in soil andgroundwater ecosystems. As such, stream water pCO₂ andpCH₄ can provide a tool to assess ecosystem respiration andanaerobic metabolism throughout drainage basins. We conducted three surveyssampling the gas content of streams in eastern Tennessee and western NorthCarolina to assess factors regulating ecosystem metabolism in catchmentswith contrasting geomorphologies, elevations and soil organic matterstorage. In our first survey, the influence of drainage basin geomorphologyon ecosystem respiration was examined by sampling streams drainingcatchments underlain by either shale or dolomite. Geomorphology isinfluenced by geology with shale catchments having shallower soils, broader,unconstrained valley floors compared with dolomite catchments.pCO₂ varied little between catchment types but increased froman average of 3340 ppmv in spring to 9927 ppmv in summer or 9.3 and 28 timesatmospheric equilibrium (pCO_2(equilib)), respectively. Incontrast, pCH₄ was over twice as high in streams drainingshale catchments (306 ppmv; pCH_4(equilib) = 116) compared withmore steeply incised dolomite basins (130 ppmv; pCH_4(equilib)= 51). Using the ratio of pCH₄:pCO₂ as an indexof anaerobic metabolism, shale catchments had nearly twice as muchanaerobiosis (pCH₄:pCO₂ = 0.046) than dolomitedrainages (pCH₄:pCO₂ = 0.024). In our secondsurvey, streams were sampled along an elevational gradient (525 to 1700 m)in the Great Smoky Mountains National Park, USA where soil organic matterstorage increases with elevation. pCO₂ did not vary betweenstreams but increased from 5340 ppmv (pCO_2(equilib) = 15) to8565 ppmv (pCO_2(equilib) = 24) from spring to summer,respectively. During spring pCH₄ was low and constant acrossstreams, but during summer increased with elevation ranging from 17 to 2068ppmv (pCH_4(equilib) = 10 to 1216). The contribution ofanaerobiosis to total respiration was constant during spring(pCH₄:pCO₂ = 0.017) but during summer increasedwith elevation from 0.002 at 524 m to 0.289 at 1286 m. In our last survey,we examined how pCO₂ and pCH₄ changed withcatchment size along two rivers (ca. 60 km stretches in both riverscorresponding to increases in basin size from 1.7–477km² and 2.5–275 km²). pCO₂and pCH₄ showed opposite trends, with pCO₂decreasing ca. 50% along the rivers, whereas pCH₄roughly doubled in concentration downstream. These opposing shifts resultedin a nearly five-fold increase of pCH₄:pCO₂along the rivers from a low of 0.012 in headwaters to a high of 0.266 65-kmdownstream. pCO₂ likely declines moving downstream asgroundwater influences on stream chemistry decreases, whereaspCH₄ may increase as the prevalence of anoxia in riversexpands due to finer-grained sediments and reduced hydrologic exchange withoxygenated surface water.     

End product inhibition in methane fermentations: Effects of carbon dioxide and methane on methanogenic bacteria utilizing acetate

Summary The effects of pCO₂ and pCH₄ in the interval 0–1 bar on rates of acetate degradation and methane formation by methanogens as well as methane yields were studied in enrichment cultures in batch and continuous fermentations.In batch fermentations the rate of acetate utilization by methanogens was 1,000–1,500 mg/l · d at low levels of pCO₂. CO₂ was inhibitory and degradation rates were around 350 mg/l · d in 1 bar CO₂. The degradation of acetate was almost linear. In continuous culture maximum rates of acetate utilization around 2,500 mg/l · d were obtained and the acetate concentration in the substrate was reduced by 98–99%.The yields of methane on acetate substrates were close to the theoretical value (1 mole CH₄ per mole HAc) in the interval pCO₂-0–0.5 bar. In 1 bar CO₂ yields decreased by 20–30%.CH₄ was found to be only slightly inhibitory; the inhibiting effects of 1 bar CH₄ on acetate degradation rates were comparable to the effects of 0.3 bar CO₂. Also gas sparging and rapid mixing had small effects compared with a non-sparged, slowly mixed culture.The redox potential was usually around –200 mV during fermentations and no connection was found between acetate degradation rate, E_h and pCO₂.Acetate and propionate degradation were the reactions most sensitive to pCO₂ and to obtain maximum rates as well as maximum methane yields pCO₂-levels around 0.2 bar were found to be optimal.     

Extreme Variations of pCO2 and pH in a Macrophyte Meadow of the Baltic Sea in Summer: Evidence of the Effect of Photosynthesis and Local Upwelling

The impact of ocean acidification on benthic habitats is a major preoccupation of the scientific community. However, the natural variability of pCO₂ and pH in those habitats remains understudied, especially in temperate areas. In this study we investigated temporal variations of the carbonate system in nearshore macrophyte meadows of the western Baltic Sea. These are key benthic ecosystems, providing spawning and nursery areas as well as food to numerous commercially important species. In situ pCO₂, pH (total scale), salinity and PAR irradiance were measured with a continuous recording sensor package dropped in a shallow macrophyte meadow (Eckernförde bay, western Baltic Sea) during three different weeks in July (pCO₂ and PAR only), August and September 2011.The mean (± SD) pCO₂ in July was 383±117 µatm. The mean (± SD) pCO₂ and pH_tot in August were 239±20 µatm and 8.22±0.1, respectively. The mean (± SD) pCO₂ and pH_tot in September were 1082±711 µatm and 7.83±0.40, respectively. Daily variations of pCO₂ due to photosynthesis and respiration (difference between daily maximum and minimum) were of the same order of magnitude: 281±88 µatm, 219±89 μatm and 1488±574 µatm in July, August and September respectively. The observed variations of pCO₂ were explained through a statistical model considering wind direction and speed together with PAR irradiance. At a time scale of days to weeks, local upwelling of elevated pCO₂ water masses with offshore winds drives the variation. Within days, primary production is responsible. The results demonstrate the high variability of the carbonate system in nearshore macrophyte meadows depending on meteorology and biological activities. We highlight the need to incorporate these variations in future pCO₂ scenarios and experimental designs for nearshore habitats.     

Growth Attenuation with Developmental Schedule Progression in Embryos and Early Larvae of Sterechinus neumayeri Raised under Elevated CO2

The Southern Ocean, a region that will be an ocean acidification hotspot in the near future, is home to a uniquely adapted fauna that includes a diversity of lightly-calcified invertebrates. We exposed the larvae of the echinoid Sterechinus neumayeri to environmental levels of CO₂ in McMurdo Sound (control: 410 µatm, Ω = 1.35) and mildly elevated pCO₂ levels, both near the level of the aragonite saturation horizon (510 µatm pCO₂, Ω = 1.12), and to under-saturating conditions (730 µatm, Ω = 0.82). Early embryological development was normal under these conditions with the exception of the hatching process, which was slightly delayed. Appearance of the initial calcium carbonate (CaCO₃) spicule nuclei among the primary mesenchyme cells of the gastrulae was synchronous between control and elevated pCO₂ treatments. However, by prism (7 days after the initial appearance of the spicule nucleus), elongating arm rod spicules were already significantly shorter in the highest CO₂ treatment. Unfed larvae in the 730 µatm pCO₂ treatment remained significantly smaller than unfed control larvae at days 15–30, and larvae in the 510 µatm treatment were significantly smaller at day 20. At day 30, the arm lengths were more differentiated between 730 µatm and control CO₂ treatments than were body lengths as components of total length. Arm length is the most plastic morphological aspect of the echinopluteus, and appears to exhibit the greatest response to high pCO₂/low pH/low carbonate, even in the absence of food. Thus, while the effects of elevated pCO₂ representative of near future climate scenarios are proportionally minor on these early developmental stages, the longer term effects on these long-lived invertebrates is still unknown.     

Interactive Effects of CO2, Temperature,Irradiance, and Nutrient Limitation on the Growth and Physiology of the Marine Diatom Thalassiosira pseudonana (Coscinodiscophyceae)

The marine diatom Thalassiosira pseudonana was grown in continuous culture systems to study the interactive effects of temperature, irradiance, nutrient limitation, and the partial pressure of CO₂ (pCO₂) on its growth and physiological characteristics. The cells were able to grow at all combinations of low and high irradiance (50 and 300 μmol photons · m⁻² · s⁻¹, respectively, of visible light), low and high pCO₂ (400 and 1,000 μatm, respectively), nutrient limitation (nitrate-limited and nutrient-replete conditions), and temperatures of 10–32°C. Under nutrient-replete conditions, there was no adverse effect of high pCO₂ on growth rates at temperatures of 10–25°C. The response of the cells to high pCO₂ was similar at low and high irradiance. At supraoptimal temperatures of 30°C or higher, high pCO₂ depressed growth rates at both low and high irradiance. Under nitrate-limited conditions, cells were grown at 38 ± 2.4% of their nutrient-saturated rates at the same temperature, irradiance, and pCO₂. Dark respiration rates consistently removed a higher percentage of production under nitrate-limited versus nutrient-replete conditions. The percentages of production lost to dark respiration were positively correlated with temperature under nitrate-limited conditions, but there was no analogous correlation under nutrient-replete conditions. The results suggest that warmer temperatures and associated more intense thermal stratification of ocean surface waters could lower net photosynthetic rates if the stratification leads to a reduction in the relative growth rates of marine phytoplankton, and at truly supraoptimal temperatures there would likely be a synergistic interaction between the stresses from temperature and high pCO₂ (lower pH).     

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.     

Effects of ocean acidification on growth and physiology of Ulva lactuca (Chlorophyta) in a rockpool‐scenario

Rising atmospheric CO₂‐concentrations will have severe consequences for a variety of biological processes. We investigated the responses of the green alga Ulva lactuca (Linnaeus) to rising CO₂‐concentrations in a rockpool scenario. U. lactuca was cultured under aeration with air containing either preindustrial pCO₂ (280 μatm) or the pCO₂ predicted by the end of the 21st century (700 μatm) for 31 days. We addressed the following question: Will elevated CO₂‐concentrations affect photosynthesis (net photosynthesis, maximum relative electron transport rate (rETR(max)), maximum quantum yield (Fv/Fm), pigment composition) and growth of U. lactuca in rockpools with limited water exchange? Two phases of the experiment were distinguished: In the initial phase (day 1–4) the Seawater Carbonate System (SWCS) of the culture medium could be adjusted to the selected atmospheric pCO₂ condition by continuous aeration with target pCO₂ values. In the second phase (day 4–31) the SWCS was largely determined by the metabolism of the growing U. lactuca biomass. In the initial phase, Fv/Fm and rETR(max) were only slightly elevated at high CO₂‐concentrations, whereas growth was significantly enhanced. After 31 days the Chl a content of the thalli was significantly lower under future conditions and the photosynthesis of thalli grown under preindustrial conditions was not dependent on external carbonic anhydrase. Biomass increased significantly at high CO₂‐concentrations. At low CO₂‐concentrations most adult thalli disintegrated between day 14 and 21, whereas at high CO₂‐concentrations most thalli remained integer until day 31. Thallus disintegration at low CO₂‐concentrations was mirrored by a drastic decline in seawater dissolved inorganic carbon and HCO₃^?. Accordingly, the SWCS differed significantly between the treatments. Our results indicated a slight enhancement of photosynthetic performance and significantly elevated growth of U. lactuca at future CO₂‐concentrations. The accelerated thallus disintegration at high CO₂‐concentrations under conditions of limited water exchange indicates additional CO₂ effects on the life cycle of U. lactuca when living in rockpools.     

Have we been underestimating the effects of ocean acidification in zooplankton?

Understanding how copepods may respond to ocean acidification (OA) is critical for risk assessments of ocean ecology and biogeochemistry. The perception that copepods are insensitive to OA is largely based on experiments with adult females. Their apparent resilience to increased carbon dioxide (pCO₂) concentrations has supported the view that copepods are ‘winners’ under OA. Here, we show that this conclusion is not robust, that sensitivity across different life stages is significantly misrepresented by studies solely using adult females. Stage‐specific responses to pCO₂ (385–6000 μatm) were studied across different life stages of a calanoid copepod, monitoring for lethal and sublethal responses. Mortality rates varied significantly across the different life stages, with nauplii showing the highest lethal effects; nauplii mortality rates increased threefold when pCO₂ concentrations reached 1000 μatm (year 2100 scenario) with LC₅₀ at 1084 μatm pCO₂. In comparison, eggs, early copepodite stages, and adult males and females were not affected lethally until pCO₂ concentrations ≥3000 μatm. Adverse effects on reproduction were found, with >35% decline in nauplii recruitment at 1000 μatm pCO₂. This suppression of reproductive scope, coupled with the decreased survival of early stage progeny at this pCO₂ concentration, has clear potential to damage population growth dynamics in this species. The disparity in responses seen across the different developmental stages emphasizes the need for a holistic life‐cycle approach to make species‐level projections to climate change. Significant misrepresentation and error propagation can develop from studies which attempt to project outcomes to future OA conditions solely based on single life history stage exposures.     

Methane production from glucose and fatty acids at 55–85°C: Adaption of cultures and effects of pCO2

Summary Thermophilic cultures producing methane from glucose at 55 °C were developed from mesophilic and thermophilic inocula. Rates of fatty acid degradation and methane yields were compared. A high pCO₂ was found to decrease the temperature maximum for acetate degradation. In a glucose-enrichment in N₂-atomosphere methane production from glucose was possible at 80°C.     

Surface water CO2 concentration influences phytoplankton production but not community composition across boreal lakes

Recent experimental evidence suggests that changes in the partial pressure of CO₂ (pCO₂), in concert with nutrient fertilisation, may result in increased primary production and shifted phytoplankton community composition that favours species lacking adaptations to low CO₂ environments. It is not clear whether these results apply in ambient freshwaters, which are already often supersaturated in CO₂, and where phytoplankton structure and activity are under complex control of diverse local and regional factors. Here, we use a large‐scale comparative study of 69 boreal lakes to explore the influence of existing CO₂ gradients (c. 50–2300 μatm) on phytoplankton community composition and biomass production. While community composition did not respond to pCO₂ gradients, gross primary production was enhanced, but only in lakes already supersaturated in CO₂, demonstrating that environmental context is key in determining pCO₂–phytoplankton interactions. We further argue that increased atmospheric CO₂ is unlikely to influence phytoplanktonic composition and production in northern lakes.     

Pressure and Temperature Effects on Growth and Methane Production of the Extreme Thermophile Methanococcus jannaschii

The marine archaebacterium Methanococcus jannaschii was studied at high temperatures and hyperbaric pressures of helium to investigate the effect of pressure on the behavior of a deep-sea thermophile. Methanogenesis and growth (as measured by protein production) at both 86 and 90°C were accelerated by pressure up to 750 atm (1 atm = 101.29kPa), but growth was not observed above 90°C at either 7.8 or 250 atm. However, growth and methanogenesis were uncoupled above 90°C, and the high-temperature limit for methanogenesis was increased by pressure. Substantial methane formation was evident at 98°C and 250 atm, whereas no methane formation was observed at 94°C and 7.8 atm. In contrast, when argon was substituted for helium as the pressurizing gas at 250 atm, no methane was produced at 86°C. Methanogenesis was also suppressed at 86°C and 250 atm when the culture was pressurized with a 4:1 mix of H₂ and CO₂, although limited methanogenesis did occur when the culture was pressurized with H₂.     

The effect of pCO2 on carbon acquisition and intracellular assimilation in four marine diatoms

The effect of pCO₂ on carbon acquisition and intracellular assimilation was investigated in the three bloom-forming diatom species, Eucampia zodiacus (Ehrenberg), Skeletonema costatum (Greville) Cleve, Thalassionema nitzschioides (Grunow) Mereschkowsky and the non-bloom-forming Thalassiosira pseudonana (Hust.) Hasle and Heimdal. In vivo activities of carbonic anhydrase (CA), photosynthetic O₂ evolution, CO₂ and HCO₃⁻ uptake rates were measured by membrane-inlet mass spectrometry (MIMS) in cells acclimated to pCO₂ levels of 370 and 800 μatm. To investigate whether the cells operate a C₄-like pathway, activities of ribulose-1,5-bisphosphate carboxylase (RubisCO) and phosphoenolpyruvate carboxylase (PEPC) were measured at the mentioned pCO₂ levels and a lower pCO₂ level of 50 μatm. In the bloom-forming species, extracellular CA activities strongly increased with decreasing CO₂ supply while constantly low activities were obtained for T. pseudonana. Half-saturation concentrations (K_1/2) for photosynthetic O₂ evolution decreased with decreasing CO₂ supply in the two bloom-forming species S. costatum and T. nitzschioides, but not in T. pseudonana and E. zodiacus. With the exception of S. costatum, maximum rates (V_max) of photosynthesis remained constant in all investigated diatom species. Independent of the pCO₂ level, PEPC activities were significantly lower than those for RubisCO, averaging generally less than 3%. All examined diatom species operate highly efficient CCMs under ambient and high pCO₂, but differ strongly in the degree of regulation of individual components of the CCM such as C_i uptake kinetics and extracellular CA activities. The present data do not suggest C₄ metabolism in the investigated species.     

Is the response of coral calcification to seawater acidification related to nutrient loading?

The effect of decreasing aragonite saturation state (Ω_Arag) of seawater (elevated pCO₂) on calcification rates of Acropora muricata was studied using nubbins prepared from parent colonies located at two sites of La Saline reef (La Réunion Island, western Indian Ocean): a back-reef site (BR) affected by nutrient-enriched groundwater discharge (mainly nitrate), and a reef flat site (RF) with low terrigenous inputs. Protein and chlorophyll a content of the nubbins, as well as zooxanthellae abundance, were lower at RF than BR. Nubbins were incubated at ~27°C over 2 h under sunlight, in filtered seawater manipulated to get differing initial pCO₂ (1,440–340 μatm), Ω_Arag (1.4–4.0), and dissolved inorganic carbon (DIC) concentrations (2,100–1,850 μmol kg⁻¹). Increasing DIC concentrations at constant total alkalinity (A_T) resulted in a decrease in Ω_Arag and an increase in pCO₂. A_T at the beginning of the incubations was kept at a natural level of 2,193 ± 6 μmol kg⁻¹ (mean ± SD). Net photosynthesis (NP) and calcification were calculated from changes in pH and A_T during the incubations. Calcification decrease in response to doubling pCO₂ relative to preindustrial level was 22% for RF nubbins. When normalized to surface area of the nubbins, (1) NP and calcification were higher at BR than RF, (2) NP increased in high pCO₂ treatments at BR compared to low pCO₂ treatments, and (3) calcification was not related to Ω_Arag at BR. When normalized to NP, calcification was linearly related to Ω_Arag at both sites, and the slopes of the relationships were not significantly different. The increase in NP at BR in the high pCO₂ treatments may have increased calcification and thus masked the negative effect of low Ω_Arag on calcification. Removing the effect of NP variations at BR showed that calcification declined in a similar manner with decreased Ω_Arag (increased pCO₂) whatever the nutrient loading.     

Stoichiometric impacts of increased carbon dioxide on a planktonic herbivore

The partial pressure of carbon dioxide (pCO₂) in lake ecosystems varies over four orders of magnitude and is affected by local and global environmental perturbations associated with both natural and anthropogenic processes. Little is known, however, about how changes in pCO₂ extend into the function and structure of food webs in freshwater ecosystems. To fill this gap, we performed laboratory experiments using the ecologically important planktonic herbivore Daphnia and its algal prey under a natural range of pCO₂ with low light and phosphorus supplies. The experiment showed that increased pCO₂ stimulated algal growth but reduced algal P : C ratio. When feeding on algae grown under high pCO₂, herbivore growth decreased regardless of algal abundance. Thus, high CO₂‐raised algae were poor food for Daphnia. Short‐term experimental supplementation of PO₄ raised the P content of the high CO₂‐raised algae and improved Daphnia growth, indicating that low Daphnia growth rates under high pCO₂ conditions were due to lowered P content in the algal food. These results suggest that, in freshwater ecosystems with low nutrient supplies, natural processes as well as anthropogenic perturbations resulting in increased pCO₂ enhance algal production but reduce energy and mass transfer efficiency to herbivores by decreasing algal nutritional quality.     

Seasonal changes of pCO2 over a subantarctic Macrocystis kelp bed

The partial pressure of carbon dioxide (pCO₂), calculated from pH and total alkalinity measurements, was monitored together with chlorophyll a and bacterioplankton biomass in shallow coastal water located inside and outside a giant kelp bed (Macrocystis pyrifera) situated in the Kerguelen Archipelago, Southern Ocean. In spite of large changes over a short time-scale, pCO₂ variations over the year are large and exhibit a seasonal pattern in which the different stages of the annual biological turnover are well marked. The overall pattern of pCO₂ variations is related to biological activity (development of both photosynthesis and respiration) during almost the whole year. However, physical and thermodynamical constraints exert a strong influence on pCO₂ at meso time-scale (10 days) and/or when biological activity is weak. Macrocystis acts to maintain pCO₂ below saturation almost the whole year and large undersaturations (pCO₂ as low as 20 μatm) were observed within the kelp bed. Furthermore, primary production of Macrocystis covers a period of 8 ∼ 9 months a year from winter to late summer and the kelp bed seems to favour the spring phytoplanktonic bloom. The buffer factor β indicates that, outside the kelp bed, inorganic carbon dynamics are mainly influenced by air-sea exchange and photosynthesis without calcification. Inside the kelp bed, β suggests calcification by the epiphytic community. Accepted: 1 April 2000     

End product inhibition in methane fermentations: Effects of carbon dioxide on fermentative and acetogenic bacteria

Summary The rates of glucose utilization by fermentative bacteria and propionate and butyrate utilization by acetogenic bacteria were studied and their dependence of pCO₂ in the interval 0–1 bar was determined. A batch fermentation method was used permitting good control of fermentation parameters and rapid experiments.The rate of glucose fermentation to acids, CO₂ and H₂ was in the order of 12,000 mg glucose/l · day which was about two orders of magnitude faster than the utilization of propionic and butyric acid by acetogenic bacteria. The rate of glucose utilization was about 30% greater at low values of pCO₂ compared with 1 bar CO₂.Propionate degradation was strongly affected by pCO₂; rates were 60 mg/l · day at pCO₂=1 bar and 200 mg/l · day at pCO₂=0.2 bar. Some CO₂ was required since the rate of propionate utilization dropped rapidly below pCO₂=0.2 bar. The rate of butyric acid utilization was constant at 170 mg/l · day; slightly lower at pCO₂=1 bar.Yields of methane from glucose or acids were close to the theoretical value 50% of degraded substrate-carbon. Yields were 20–30% higher at low values of pCO₂ compared with 1 bar CO₂.The redox potential was usually between –200 and –250 mV, slowly increasing to between –150 and –200 mV during fermentation. No clear connection between rates of substrate utilization, pCO₂ and E_h was detected.     

Elevated CO2 and Phosphate Limitation Favor Micromonas pusilla through Stimulated Growth and Reduced Viral Impact

Growth and viral infection of the marine picoeukaryote Micromonas pusilla was studied under a future-ocean scenario of elevated partial CO₂ (pCO₂; 750 μatm versus the present-day 370 μatm) and simultaneous limitation of phosphorus (P). Independent of the pCO₂ level, the ratios of M. pusilla cellular carbon (C) to nitrogen (N), C:P and N:P, increased with increasing P stress. Furthermore, in the P-limited chemostats at growth rates of 0.32 and 0.97 of the maximum growth rate (μ_max), the supply of elevated pCO₂ led to an additional rise in cellular C:N and C:P ratios, as well as a 1.4-fold increase in M. pusilla abundance. Viral lysis was not affected by pCO₂, but P limitation led to a 150% prolongation of the latent period (6 to 12 h) and an 80% reduction in viral burst sizes (63 viruses per cell) compared to P-replete conditions (4 to 8 h latent period and burst size of 320). Growth at 0.32 μ_max further prolonged the latent period by another 150% (12 to 18 h). Thus, enhanced P stress due to climate change-induced strengthened vertical stratification can be expected to lead to reduced and delayed virus production in picoeukaryotes. This effect is tempered, but likely not counteracted, by the increase in cell abundance under elevated pCO₂. Although the influence of potential P-limitation-relieving factors, such as the uptake of organic P and P utilization during infection, is unclear, our current results suggest that when P limitation prevails in future oceans, picoeukaryotes and grazing will be favored over larger-sized phytoplankton and viral lysis, with increased matter and nutrient flow to higher trophic levels.     

One‐year experiment on the physiological response of the Mediterranean crustose coralline alga,Lithophyllum cabiochae,to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO₂ and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near‐ambient conditions of irradiance, at ambient or elevated temperature (+3°C), and at ambient (ca. 400 μatm) or elevated pCO₂ (ca. 700 μatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO₂ but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO₂. Conversely, photosynthesis was strongly affected by pCO₂ with a decline under elevated pCO₂ in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO₂. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO₂ but decreased with rising temperature under elevated pCO₂. Surprisingly, the highest rates in summer were found under elevated pCO₂ and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO₂. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO₂. However, there is likely a trade‐off with other physiological processes. For example, photosynthesis declines in response to increased pCO₂ under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.     

Quantifying the indirect effects of nitrogen deposition on grassland litter chemical traits

The quantitative contribution of tropical estuaries to the atmospheric CO₂ budget has large uncertainties, both spatially and seasonally. We investigated the seasonal and spatial variations of carbon biogeochemistry downstream of Ho Chi Minh City (Southern Vietnam). We sampled four sites distributed from downstream of a highly urbanised watershed through mangroves to the South China Sea coast during the dry and wet seasons. Measured partial pressure of CO₂ (pCO₂) ranged from 660 to 3000 μatm during the dry season, and from 740 to 5000 μatm during the wet season. High organic load, dissolved oxygen saturation down to 17%, and pCO₂ up to 5000 μatm at the freshwater endmember of the estuary reflected the intense human pressure on this ecosystem. We show that releases from mangrove soils affect the water column pCO₂ in this large tropical estuary (~600 m wide and 10–20 m deep). This study is among the few to report direct measurements of both water pCO₂ and CO₂ emissions in a Southeast Asian tropical estuary located in a highly urbanised watershed. It shows that the contribution of such estuaries may have been previously underestimated, with CO₂ emissions ranging from 74 to 876 mmol m^?2 day^?1 at low current velocity (< 0.2 m s^?1). Corresponding gas transfer velocities k₆₀₀, ranging from 1.7 to 11.0 m day^?1, were about 2 to 4 times of k₆₀₀ estimated using published literature equations.     

The Response of Antarctic Sea Ice Algae to Changes in pH and CO2

Ocean acidification substantially alters ocean carbon chemistry and hence pH but the effects on sea ice formation and the CO₂ concentration in the enclosed brine channels are unknown. Microbial communities inhabiting sea ice ecosystems currently contribute 10–50% of the annual primary production of polar seas, supporting overwintering zooplankton species, especially Antarctic krill, and seeding spring phytoplankton blooms. Ocean acidification is occurring in all surface waters but the strongest effects will be experienced in polar ecosystems with significant effects on all trophic levels. Brine algae collected from McMurdo Sound (Antarctica) sea ice was incubated in situ under various carbonate chemistry conditions. The carbon chemistry was manipulated with acid, bicarbonate and bases to produce a pCO₂ and pH range from 238 to 6066 µatm and 7.19 to 8.66, respectively. Elevated pCO₂ positively affected the growth rate of the brine algal community, dominated by the unique ice dinoflagellate, Polarella glacialis. Growth rates were significantly reduced when pH dropped below 7.6. However, when the pH was held constant and the pCO₂ increased, growth rates of the brine algae increased by more than 20% and showed no decline at pCO₂ values more than five times current ambient levels. We suggest that projected increases in seawater pCO₂, associated with OA, will not adversely impact brine algal communities.