Posts Tagged 'light'

The influence of CO2 enrichment on net photosynthesis of seagrass Zostera marina in a brackish water environment

Seagrasses are distributed across the globe and their communities may play key roles in the coastal ecosystems. Seagrass meadows are expected to benefit from the increased carbon availability which might be used in photosynthesis in a future high CO2 world. The main aim of this study was to examine the effect of elevated pCO2 on the net photosynthesis of seagrass Zostera marina in a brackish water environment. The short-term mesocosm experiments were conducted in Kõiguste Bay (northern part of Gulf of Riga, the Baltic Sea) in June–July 2013 and 2014. As the levels of pCO2 naturally range from ca. 150 μatm to well above 1000 μatm under summer conditions in Kõiguste Bay we chose to operate in mesocosms with the pCO2 levels of ca. 2000, ca. 1000, and ca. 200 μatm. Additionally, in 2014 the photosynthesis of Z. marina was measured outside of the mesocosm in the natural conditions. In the shallow coastal Baltic Sea seagrass Z. marina lives in a highly variable environment due to seasonality and rapid changes in meteorological conditions. This was demonstrated by the remarkable differences in water temperatures between experimental years of ca. 8°C. Thus, the current study also investigated the effect of elevated pCO2 in combination with short-term natural fluctuations of environmental factors, i.e., temperature and PAR on the photosynthesis of Z. marina. Our results show that elevated pCO2 alone did not enhance the photosynthesis of the seagrass. The photosynthetic response of Z. marina to CO2 enrichment was affected by changes in water temperature and light availability.

Continue reading ‘The influence of CO2 enrichment on net photosynthesis of seagrass Zostera marina in a brackish water environment’

Light availability and temperature, not increased CO2, will structure future meadows of Posidonia oceanica

We evaluated the photosynthetic performance of Posidonia oceanica during short-term laboratory exposures to ambient and elevated temperatures (24–25 °C and 29–30 °C) warming and pCO2 (380, 750 and 1000 ppm pCO2) under normal and low light conditions (200 and 40 μmol photons m−2 s−1 respectively). Plant growth was measured at the low light regime and showed a negative response to warming. Light was a critical factor for photosynthetic performance, although we found no evidence of compensation of photosynthetic quantum efficiency in high light. Relative Electron Rate Transport (rETRmax) was higher in plants incubated in high light, but not affected by pCO2 or temperature. The saturation irradiance (Ik) was negatively affected by temperature. We conclude that elevated CO2 does not enhance photosynthetic activity and growth, in the short term for P. oceanica, while temperature has a direct negative effect on growth. Low light availability also negatively affected photosynthetic performance during the short experimental period examined here. Therefore increasing concentrations of CO2 may not compensate for predicted future conditions of warmer water and higher turbidity for seagrass meadows.

Continue reading ‘Light availability and temperature, not increased CO2, will structure future meadows of Posidonia oceanica’

Kelp and carbon: pathways and barriers to acquisition and transport

Large brown algae in the class Phaeophyceae (Heterokontophyta) form the structural and energetic foundation of temperate and subtropical nearshore marine forests of high productivity and ecological diversity. This dissertation examines the carbon uptake and transport physiology of large brown algae with a particular focus on the plastic or adaptive responses of these physiological traits to their abiotic environment. Chapter 1 takes an anatomical and modeling approach to investigate the structure and function of photosynthate transport networks (analogous to phloem) in diverse members of the Laminariales. To evaluate the existence of scaling and optimization of the kelp vascular system, a model of optimized transport anatomy was developed and tested with a  diverse suite of kelp species in the Laminariales. Results revealed a surprising lack of universal scaling in the kelps and the presence of optimized transport anatomy in the giant kelp (Macrocystis pyrifera) only. Chapter 2 focuses on the dynamics of carbon uptake in M. pyrifera, which can acquire both carbon dioxide and bicarbonate as carbon substrates for photosynthesis. To evaluate whether the proportion of carbon dioxide and bicarbonate utilized by M. pyrifera is constant or a variable function of their fluctuating environment, oxygen evolution experiments were carried out  n entire blades from several targeted populations in the Monterey Bay. Results indicated that M. pyrifera possesses a plastic carbon uptake physiology in which proportionally more bicarbonate is used in high irradiance and high flow conditions, but that local populations have not yet developed fixed genetic differences. Chapter 3 investigates the mechanism and patterns of carbon stable isotope discrimination in M. pyrifera. Results of a dual field and laboratory incubation approach indicate that 13C discrimination patterns are determined by a complex interaction of light intensity, dissolved inorganic carbon limitation, and fractionation occurring during transport of polysaccharides. Overall, this dissertation informs patterns and mechanisms of carbon uptake and transport in kelps, and highlights the many ways in which kelps may impact and structure their ecosystems.

Continue reading ‘Kelp and carbon: pathways and barriers to acquisition and transport’

Light and temperature control physiological responses of Halimeda to ocean acidification

The oceans have absorbed excess carbon dioxide (CO2) resulting from anthropogenic activities such as the burning of fossil fuels and deforestation. As a result, seawater chemistry has shifted causing an increase in bicarbonate ions (HCO32-) and hydrogen ions (H+) and leading to a reduction in carbonate (CO32-) concentration. This shift in seawater chemistry leads to a decrease in aragonite saturation state and pH. Eventually, the ocean will accumulate most of the extra CO2 produced over many years resulting in extreme acidified conditions where aragonite saturation levels will not support the chemical process of calcification that is vital to marine calcifiers. This thesis investigates the combined effects of elevated pCO2 with temperature and light on the calcification and photosynthesis of the green calcareous algae Halimeda. Halimeda, is a major contributor to sediment production for coral reef accretion and island reef formation. Based on carbonate data from biologists and geologists it is estimated that vertical accretion of CaCO3 by Halimeda ranges between 0.18 to 5.9 m in 1000 years. The role that light plays in the coupling between photosynthesis and calcification in Halimeda macroloba was investigated experimentally through a combination of two pCO2 levels (360 and 1200 uatm) and three irradiances (80, 150, and 595 μmol quanta m-2 s-1). A decrease in calcification at low light intensity and elevated pCO2 suggests that light is a limiting factor for the physiology of H. macroloba. The effects of elevated pCO2 and temperature on the photosynthesis and calcification of Halimeda incrassata were tested through two experiments using two pCO2 levels (390 and 900 uatm) and four temperatures (26, 29, 30 and 34 °C). Elevated temperature can mitigate the effects Ocean Acidification (OA) in H. incrassata. An estimate of current carbonate production by H. incrassata in Key Biscayne Florida Lagoon was obtained from biomass, CaCO3 content and turnover rate. Calcification rates from laboratory experiments were used to estimate future (200 years from now) seasonal carbonate production rates, which were then compared against current summer carbonate production. Future summer carbonate production rates were not affected by elevated pCO2 in relationship to current summer carbonate production. Elevated temperatures ~2 °C above summer maximum average could promote calcification of H. incrassata under ocean acidification conditions and, therefore, overall carbonate production of the reef. Results throughout the thesis revealed that the tolerance of the green calcareous algae Halimeda to OA could change depending on light and temperature conditions. In a more acidic future ocean, growth rates and sediment production of Halimeda will be affected under low light and temperature and will be enhanced under high light and and moderate elevated temperatures.

Continue reading ‘Light and temperature control physiological responses of Halimeda to ocean acidification’

Conspecific aggregations mitigate the effects of ocean acidification on calcification of the coral Pocillopora verrucosa

In densely populated communities, such as coral reefs, organisms can modify the physical and chemical environment for neighbouring individuals. We tested the hypothesis that colony density (12 colonies each placed∼0.5 cm apart versus∼8 cm apart) can modulate the physiological response (measured through rates of calcification, photosynthesis, and respiration in the light and dark) of the coral Pocillopora verrucosa to pCO2 treatments (∼ 400 µatm and∼1200 µatm) by altering the seawater flow regimes experienced by colonies placed in aggregations within a flume at a single flow speed. While light calcification decreased 20% under elevated versus ambient pCO2 for colonies in low-density aggregations, light calcification of high-density aggregations increased 23% at elevated versus ambient pCO2. As a result, densely aggregated corals maintained calcification rates over 24 h that were comparable to those maintained under ambient pCO2, despite a 45% decrease in dark calcification at elevated versus ambient pCO2. Additionally, densely aggregated corals experienced reduced flow speeds and higher seawater retention times between colonies due to the formation of eddies. These results support recent indications that neighbouring organisms, such as the conspecific coral colonies in the present example, can create small-scale refugia from the negative effects of ocean acidification.

Continue reading ‘Conspecific aggregations mitigate the effects of ocean acidification on calcification of the coral Pocillopora verrucosa’

A key marine diazotroph in a changing ocean: the interacting effects of temperature, CO2 and light on the growth of Trichodesmium erythraeum IMS101

Trichodesmium is a globally important marine diazotroph that accounts for approximately 60 − 80% of marine biological N2 fixation and as such plays a key role in marine N and C cycles. We undertook a comprehensive assessment of how the growth rate of Trichodesmium erythraeum IMS101 was directly affected by the combined interactions of temperature, pCO2 and light intensity. Our key findings were: low pCO2 affected the lower temperature tolerance limit (Tmin) but had no effect on the optimum temperature (Topt) at which growth was maximal or the maximum temperature tolerance limit (Tmax); low pCO2 had a greater effect on the thermal niche width than low-light; the effect of pCO2 on growth rate was more pronounced at suboptimal temperatures than at supraoptimal temperatures; temperature and light had a stronger effect on the photosynthetic efficiency (Fv/Fm) than did CO2; and at Topt, the maximum growth rate increased with increasing CO2, but the initial slope of the growth-irradiance curve was not affected by CO2. In the context of environmental change, our results suggest that the (i) nutrient replete growth rate of Trichodesmium IMS101 would have been severely limited by low pCO2 at the last glacial maximum (LGM), (ii) future increases in pCO2 will increase growth rates in areas where temperature ranges between Tmin to Topt, but will have negligible effect at temperatures between Topt and Tmax, (iii) areal increase of warm surface waters (> 18°C) has allowed the geographic range to increase significantly from the LGM to present and that the range will continue to expand to higher latitudes with continued warming, but (iv) continued global warming may exclude Trichodesmium spp. from some tropical regions by 2100 where temperature exceeds Topt.

Continue reading ‘A key marine diazotroph in a changing ocean: the interacting effects of temperature, CO2 and light on the growth of Trichodesmium erythraeum IMS101’

Short-term interactive effects of ultraviolet radiation, carbon dioxide and nutrient enrichment on phytoplankton in a shallow coastal lagoon

The main goal of this study was to evaluate short-term interactions between increased CO2, UVR and inorganic macronutrients (N, P and Si) on summer phytoplankton assemblages in the Ria Formosa coastal lagoon (SW Iberia), subjected to intense anthropogenic pressures and highly vulnerable to climate change. A multifactorial experiment using 20 different nutrient-enriched microcosms exposed to different spectral and CO2 conditions was designed. Before and after a 24-h in situ incubation, phytoplankton abundance and composition were analysed. Impacts and interactive effects of high CO2, UVR and nutrients varied among different functional groups. Increased UVR had negative effects on diatoms and cyanobacteria and positive effects on cryptophytes, whereas increased CO2 inhibited cyanobacteria but increased cryptophyte growth. A positive synergistic interaction between CO2 and UVR was observed for diatoms; high CO2 counteracted the negative effects of UVR under ambient nutrient concentrations. Nutrient enrichments suppressed the negative effects of high CO2 and UVR on cyanobacteria and diatoms, respectively. Beneficial effects of CO2 were observed for diatoms and cryptophytes under combined additions of nitrate and ammonium, suggesting that growth may be limited by DIC availability when the primary limitation by nitrogen is alleviated. Beneficial effects of high CO2 and UVR in diatoms were also induced or intensified by ammonium additions.

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Ocean acidification in the IPCC AR5 WG II

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