Bicarbonate uptake via an anion exchange protein is the main mechanism of inorganic carbon acquisition by the giant kelp Macrocystis pyrifera (Laminariales,Phaeophyceae) under variable pH

Macrocystis pyrifera is a widely distributed, highly productive, seaweed. It is known to use bicarbonate (HCO₃^?) from seawater in photosynthesis and the main mechanism of utilization is attributed to the external catalyzed dehydration of HCO₃^? by the surface‐bound enzyme carbonic anhydrase (CA_ext). Here, we examined other putative HCO₃^? uptake mechanisms in M. pyrifera under pH_T 9.00 (HCO₃^?: CO₂ = 940:1) and pH_T 7.65 (HCO₃^?: CO₂ = 51:1). Rates of photosynthesis, and internal CA (CA_int) and CA_ext activity were measured following the application of AZ which inhibits CA_ext, and DIDS which inhibits a different HCO₃^? uptake system, via an anion exchange (AE) protein. We found that the main mechanism of HCO₃^? uptake by M. pyrifera is via an AE protein, regardless of the HCO₃^?: CO₂ ratio, with CA_ext making little contribution. Inhibiting the AE protein led to a 55%–65% decrease in photosynthetic rates. Inhibiting both the AE protein and CA_ext at pH_T 9.00 led to 80%–100% inhibition of photosynthesis, whereas at pH_T 7.65, passive CO₂ diffusion supported 33% of photosynthesis. CA_int was active at pH_T 7.65 and 9.00, and activity was always higher than CA_ext, because of its role in dehydrating HCO₃^? to supply CO₂ to RuBisCO. Interestingly, the main mechanism of HCO₃^? uptake in M. pyrifera was different than that in other Laminariales studied (CA_ext‐catalyzed reaction) and we suggest that species‐specific knowledge of carbon uptake mechanisms is required in order to elucidate how seaweeds might respond to future changes in HCO₃^?:CO₂ due to ocean acidification.     

Plant and frond dynamics of the giant kelp,Macrocystis pyrifera,forming a fringing zone in the Falkland Islands

Fluctuations in plant and frond characteristics are described for Macrocystis pyrifera (L.) C. Agardh (Laminariales, Phaeophyta) forming a fringing zone in the Falkland Islands. Giant kelp plants were sampled along a transect in the austral autumn (May 1986) and late spring (December 1986) which, according to previous frond weight analysis, were the times when extremes in population parameters were expected. Plant density and holdfast wet weights were similar for both seasons, but plants had more fronds and the fronds weighed more in spring than in autumn. Consequently, in autumn the frond biomass (1·1 wet kg m^?2) and productivity (34·1 wet g m^?2 d^?1) were lower than in spring, when a biomass of 5·0 wet kg m^?2 and a productivity of 72·4 wet g m^?2 d^?1 were recorded. Production of new fronds and loss of old fronds were determined at monthly intervals between April 1986 and March 1987. New frond production rates followed fluctuations in the quantity of light and varied between 0·08 and 0·48 fronds per plant per day. Frond loss rates did not show a seasonal pattern and fluctuated between 0·05 and 0·42 fronds per plant per day. It is suggested that the Falkland Islands Macrocystis population is more stable than most other giant kelp beds at high latitudes, because of the absence of winter storms.     

Ocean acidification and seaweed reproduction: increased CO2 ameliorates the negative effect of lowered pH on meiospore germination in the giant kelp Macrocystis pyrifera (Laminariales,Phaeophyceae)

The worldwide effects of ocean acidification (OA) on marine species are a growing concern. In temperate coastal seas, seaweeds are dominant primary producers that create complex habitats and supply energy to higher trophic levels. Studies on OA and macroalgae have focused on calcifying species and adult stages, but critically, they have overlooked the microscopic stages of the reproductive life cycle, which, for other anthropogenic stressors, e.g., UV‐B radiation, are the most susceptible life‐history phase. Also, environmental cues and stressors can cause changes in the sex ratio, which has implications for the mating system and recruitment success. Here, we report the effects of pH (7.59–8.50) on meiospore germination and sex determination for the giant kelp, Macrocystis pyrifera (Laminariales), in the presence and absence of additional dissolved inorganic carbon (DIC). Lowered pH (7.59–7.60, using HCl‐only) caused a significant reduction in germination, whereas added DIC had the opposite effect, indicating that increased CO₂ at lower pH ameliorates physiological stress. This finding also highlights the importance of appropriate manipulation of seawater carbonate chemistry when testing the effects of OA on photosynthetic organisms. The proportion of male to female gametophytes did not vary significantly between treatments, suggesting that pH was not a primary environmental modulator of sex. Relative to the baseline (pH 8.19), gametophytes were 32% larger under moderate OA (pH 7.86) and 10% larger under extreme OA (pH 7.61). We suggest that metabolically active cells can compensate for the acidification of seawater. This homeostatic function minimizes the negative effects of lower pH (high H⁺ ions) on cellular activity. The 6–9% reduction in germination success under extreme OA suggests that meiospores of M. pyrifera may be resistant to future OA.     

Short‐ and long‐term acclimation patterns of the giant kelp Macrocystis pyrifera (Laminariales,Phaeophyceae) along a depth gradient

The giant kelp, Macrocystis pyrifera, is exposed to highly variable irradiance and temperature regimes across its geographic and vertical depth gradients. The objective of this study was to extend our understanding of algal acclimation strategies on different temporal scales to those varying abiotic conditions at various water depths. Different acclimation strategies to various water depths (0.2 and 4 m) between different sampling times (Jan/Feb and Aug/Sept 2012; long‐term acclimation) and more rapid adjustments to different depths (0.2, 2 and 4 m; short‐term acclimation) during 14 d of transplantation were found. Adjustments of variable Chl a fluorescence, pigment composition (Chl c, fucoxanthin), and the de‐epoxidation state of the xanthophyll cycle pigments were responsible for the development of different physiological states with respect to various solar radiation and temperature climates. Interestingly, the results indicated that phlorotannins are important during long‐term acclimation while antioxidants have a crucial role during short‐term acclimation. Furthermore, the results suggested that modifications in total lipids and fatty acid compositions apparently also might play a role in depth acclimation. In Aug/Sept (austral winter), M. pyrifera responded to the transplantation from 4 m to 0.2 m depth with a rise in the degree of saturation and a switch from shorter‐ to longer‐chain fatty acids. These changes seem to be essential for the readjustment of thylakoid membranes and might, thus, facilitate efficient photosynthesis under changing irradiances and temperatures. Further experiments are needed to disentangle the relative contribution of solar radiation, temperature and also other abiotic parameters in the observed physiological changes.     

Dependence of phytoplankton assimilation quotients on light and nitrogen source: implications for oceanic primary productivity

Photosynthetic production of oxygen by phytoplankton assemblagedominated by Peridinium in Lake Kinneret, Israel, generallyexceeds the molar equivalent rate of carbon assimilation. Carbonassimilation occurs only if oxygenic photosynthesis exceedsa light-dependent threshold. Assimilation quotients (mol C molO₂^–1) are a variable function of irradiance, and typicallyonly about one-half of the photoreductant produced during oxygenicphotosynthesis is used for reduction of carbon dioxide. Mostof the residual oxygenic photoreductant probably is used forlight-dependent reduction of nitrate, which competes with carbondioxide for oxygenic photoreductant. Nitrate is an importantsource of nitrogen for this algal assemblage, and light-dependentnitrate reduction probably is much larger than carbon dioxidereduction at lowest irradiances in the euphotic zone. Oxygenproduction also may be much larger than carbon assimilationat low light levels in other environments where oxidized formsof nitrogen are important nitrogenous nutrients for phytoplankton,as in the lower euphotic zone of the sea, where low rates ofcarbon assimilation by phytoplankton have been thought to beinconsistent with the amount of oxygen that accumulates duringsummer.     

Microbial responses to a changing environment: implications for the future functioning of terrestrial ecosystems

In this review, we present a conceptual model which links plant communities and saprotrophic microbial communities through the reciprocal exchange of growth-limiting resources. We discuss the numerous ways human-induced environmental change has directly and indirectly impacted this relationship, and review microbial responses that have occurred to date. We argue that compositional shifts in saprotrophic microbial communities underlie functional responses to environmental change that have ecosystem-level implications. Drawing on a long-term, large-scale, field experiment, we illustrate how and why chronic atmospheric N deposition can alter saprotrophic communities in the soil of a wide-spread sugar maple (Acer saccharum) ecosystem in northeastern North America, resulting in the slowing of plant litter decay, the rapid accumulation of soil organic matter, and the accelerated production and loss of dissolved organic carbon (DOC). Compositional shifts in soil microbial communities, mediated by ecological interactions among soil saprotrophs, appear to lie at the biogeochemical heart of ecosystem response to environmental change.     

Amelioration of plant responses to drought under elevated CO2 by rejuvenating photosynthesis and nitrogen use efficiency: implications for future climate-resilient crops

Photosynthesis Research - The contemporary global agriculture is beset with serious threats from diverse eco-environmental conditions causing decreases in crop yields by?~?15%. These...     

Liquid-culture pH,temperature, and carbon (not nitrogen) source regulate phenazine productivity of the take-all biocontrol agentPseudomonas fluorescens 2-79

 Strain 2-79 is a biocontrol agent againsttake-all, an important disease of wheat caused by Gaeumannomyces graminis var. tritici. In the rhizosphere, it produces the antibiotic phenazine 1-carboxylic acid (PCA) as the primary means of disease suppression. One barrier to commercial use of phenazine-producing pseudomonads, like strain 2-79, is the lack of liquid-culture technology for mass production. For instance, there is little published research concerning the impact of liquid-culture secondary metabolism on the biocontrol qualities of the cell harvest, i.e., efficacy, phytotoxicity, and storage survival. Yet it is important to know whether the fermentation process should be designed to enhance or eliminate secondary metabolite accumulation. To enable future exploration of this issue, we identified liquid-culture parameters that could be manipulated to controlthe phenazine productivity of strain 2-79. Our results indicated that PCA accumulation was very sensitive to the culture pH and temperature. It was possible to produce large cell populations with either high or low phenazine productivity by choosing to control culture pH at 7 and 8 respectively. Although high cell accumulations were achieved over the broad 25–34°C range studied, high, moderate, or low PCA productivities were observed at 25–27°C, 29–32.5°C, or 34°C respectively. When pH was controlled at 7, specific PCAproductions at 25°C could be modulated by the choice of carbon source supplied. PCA accumulation per unit biomass reached 0.31 g/g on glucose, 0.16 g/g on glycerol and xylose, and only 0.09 g/g on fructose. Although the nitrogen source was also tested as a variable, it had little influence on culture PCA productivity under controlled pH. Received: 12 July 1994 / Received revision: 8 October 1994/Accepted: 22 November 1994     

Regulated externalization of phosphatidylserine at the cell surface: implications for apoptosis

The regulated loss of plasma membrane phosphatidylserine (PS) asymmetry is critical to many biological processes. In particular, the appearance of PS at the cell surface, a hallmark of apoptosis, prepares the dying cell for engulfment and elimination by phagocytes. While it is well established that PS externalization is regulated by activation of a calcium-dependent phospholipid scramblase activity in concert with inactivation of the aminophospholipid translocase, there is no evidence indicating that these processes are triggered and regulated by apoptotic regulatory mechanisms. Using a novel model system, we show that PS externalization is inducible, reversible, and independent of cytochrome c release, caspase activation, and DNA fragmentation. Additional evidence is presented indicating that the outward movement of plasma membrane PS requires sustained elevation in cytosolic Ca2+ in concert with inactivation of the aminophospholipid translocase and is inhibited by calcium channel blockers.     

Vegetative growth and reproduction of Fossombronia brasiliensis Steph.: the influence of photoperiod,temperature and inorganic nitrogen source

Abstract

Monoclonal cultures of Fossombronia brasiliensis were grown with different photoperiods, temperatures and inorganic nitrogen sources. The subsequent vegetative growth and production of gametangia is described. A quantitative analysis of the numbers of antheridia and archegonia and their relative proportions is given. At 18°C, F. brasiliensis was found to be a short-day plant, with a critical night length of between six and twelve hours, while at 10°C it exhibited quantitative SD plant properties. Plants produced more male gametangia at 18°C and more female gametangia at 10°C. Nitrogen as nitrate consistently produced more gametangia than the ammonium source.     

The Pochonia chlamydosporia serine protease gene vcp1 is subject to regulation by carbon, nitrogen and pH: implications for nematode biocontrol

The alkaline serine protease VCP1 of the fungus Pochonia chlamydosporia belongs to a family of subtilisin-like enzymes that are involved in infection of nematode and insect hosts. It is involved early in the infection process, removing the outer proteinaceous vitelline membrane of nematode eggs. Little is known about the regulation of this gene, even though an understanding of how nutrients and other factors affect its expression is critical for ensuring its efficacy as a biocontrol agent. This paper provides new information on the regulation of vcp1 expression. Sequence analysis of the upstream regulatory region of this gene in 30 isolates revealed that it was highly conserved and contained sequence motifs characteristic of genes that are subject to carbon, nitrogen and pH-regulation. Expression studies, monitoring enzyme activity and mRNA, confirmed that these factors affect VCP1 production. As expected, glucose reduced VCP1 expression and for a few hours so did ammonium chloride. Surprisingly, however, by 24 h VCP1 levels were increased in the presence of ammonium chloride for most isolates. Ambient pH also regulated VCP1 expression, with most isolates producing more VCP1 under alkaline conditions. There were some differences in the response of one isolate with a distinctive upstream sequence including a variant regulatory-motif profile. Cryo-scanning electron microscopy studies indicated that the presence of nematode eggs stimulates VCP1 production by P. chlamydosporia, but only where the two are in close contact. Overall, the results indicate that readily-metabolisable carbon sources and unfavourable pH in the rhizosphere/egg-mass environment may compromise nematode parasitism by P. chlamydosporia. However, contrary to previous indications using other nematophagous and entomopathogenic fungi, ammonium nitrate (e.g. from fertilizers) may enhance biocontrol potential in some circumstances.     

Self-association and stability of the ApoE isoforms at low pH: implications for ApoE-lipid interactions

Apolipoprotein E (apoE) isoforms are known to differentially accumulate in the lysosomes of neuronal cells, and the deleterious effects of the apoE4 isoform in Alzheimer's disease may relate to its properties at the low lysosomal pH. However, the effect of pH on the molecular properties of full-length apoE is unclear. Here we examine the pH dependence of the monomer-dimer-tetramer reaction, of lipid binding, and of the stability of the three major apoE isoforms. Using FRET measurements, we find that the association-dissociation behavior of apoE proteins changes dramatically with changes in pH. At pH 4.5, approximating the pH of the lysosome, rate constants for association and dissociation are 2-10 times faster than those at pH 7.4. Aggregation beyond the tetrameric form is also more evident at lower pH values. Stability, as measured by urea denaturation at pH 4.5, is found to be considerably greater than that at neutral pH and to be isoform dependent. Lipid binding, as measured by turbidity clearance of unilamellar vesicles of DMPC, is faster at acidic pH values and consistent with our previous hypothesis that it is only the monomeric form of apoE that binds lipid tightly. Since apoE is more stable at pH 4.5 than at neutral pH, the more rapid apoE-lipid interactions at low pH are not correlated with the stability of the apoE isoforms, but rather to the faster association-dissociation behavior. Our results indicate that pathological behavior of apoE4 may arise from altered molecular properties of this protein at the acidic pH of the lysosome.     

Tree responses to rising CO2 in field experiments: implications for the future forest

The need to assess the role of forests in the global cycling of carbon and how that role will change as the atmospheric concentration of CO₂ increases has spawned many experiments over a range of scales. Experiments using open-top chambers have been established at many sites to test whether the short-term responses of tree seedlings described in controlled environments would be sustained over several growing seasons under field conditions. Here we review the results of those experiments, using the framework of the interacting cycles of carbon, water and nutrients, because that is the framework of the ecosystem models that are being used to address the decades-long response of forests. Our analysis suggests that most of what was learned in seedling studies was qualitatively correct. The evidence from field-grown trees suggests a continued and consistent stimulation of photosynthesis of about 60% for a 300 p.p.m. increase in [CO₂], and there is little evidence of the long-term loss of sensitivity to CO₂ that was suggested by earlier experiments with tree seedlings in pots. Despite the importance of respiration to a tree's carbon budget, no strong scientific consensus has yet emerged concerning the potential direct or acclimation response of woody plant respiration to CO₂ enrichment. The relative effect of CO₂ on above-ground dry mass was highly variable and greater than that indicated by most syntheses of seedling studies. Effects of CO₂ concentration on static measures of response are confounded with the acceleration of ontogeny observed in elevated CO₂. The trees in these open-top chamber experiments were in an exponential growth phase, and the large growth responses to elevated CO₂ resulted from the compound interest associated with an increasing leaf area. This effect cannot be expected to persist in a closed-canopy forest where growth potential is constrained by a steady-state leaf area index. A more robust and informative measure of tree growth in these experiments is the annual increment in wood mass per unit leaf area, which increased 27% in elevated CO₂. There is no support for the conclusion from many studies of seedlings that root-to-shoot ratio is increased by elevated CO₂; the production of fine roots may be enhanced, but it is not clear that this response would persist in a forest. Foliar nitrogen concentrations were lower in CO₂-enriched trees, but to a lesser extent than was indicated in seedling studies and only when expressed on a leaf mass basis. The prediction that leaf litter C/N ratio would increase was not supported in field experiments. Also contrasting with seedling studies, there is little evidence from the field studies that stomatal conductance is consistently affected by CO₂; however, this is a topic that demands more study. Experiments with trees in open-top chambers under field conditions have provided data on longer-term, larger-scale responses of trees to elevated CO₂ under field conditions, confirmed some of the conclusions from previous seedling studies, and challenged other conclusions. There remain important obstacles to using these experimental results to predict forest responses to rising CO₂, but the studies are valuable nonetheless for guiding ecosystem model development and revealing the critical questions that must be addressed in new, larger-scale CO₂ experiments.     

Narrow range of temperature and irradiance supports optimal development of Lessonia corrugata (Ochrophyta) gametophytes: implications for kelp aquaculture and responses to climate change

The kelp Lessonia corrugata (Ochrophyta, Laminariales) is being developed for integrated multi-trophic aquaculture (IMTA) trials in the vicinity of salmon cages in Tasmania, Australia. Gametophytes are vegetally maintained before seeding on hatchery twine; however, the optimal temperature and light conditions for growth and sexual development are unknown. We measured vegetative size of female and male gametophytes and sexual development of females over a range of temperatures and irradiances using a temperature gradient table and neutral density light filters. Over a 4-week experiment, gametophytes were exposed to a combination of thermal (5.7–24.9 °C) and irradiance (10–100 μmol photons m^?2 s^?1) gradients, to assess biological performance. At the temperature extremes (hottest = 24.9 °C, coldest = 5.7 °C), we observed the critical thermal limits for this species and the results reveal a narrow optimal temperature range for growth and sexual development between 15.7 and 17.9 °C, with irradiances between 40 and 100 μmol photons m^?2 s^?1 resulting in fertile female gametophytes. Lessonia corrugata inhabits a small geographic range, found only around Tasmania, south of the Australian mainland, hence oceanic changes such as ongoing increases in sea surface temperatures (SSTs), and altered irradiance regimes may limit recruitment of the early microscopic life stages in the future. Our findings provide optimised culture conditions for aquaculture and information to predict the future geographic range of L. corrugata under ocean global change.

     

Multidecadal to millennial-scale shifts in drought conditions on the Canadian prairies over the past six millennia: implications for future drought assessment

Three high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the Canadian prairies, show that shifts in drought conditions have prevailed on centennial to millennial time scales for at least the past six millennia. These shifts in mean aridity exhibit broad regional synchrony, with particularly pronounced shifts at all sites between ∼1700–2000 cal. yr bp and ∼3600–3900 cal. yr bp , as well as at ∼5400–5500 cal. yr bp for the two sites which extend back to at least 6000 cal. yr bp . The two Saskatchewan lakes exhibited significant coherence in both the timing and direction of these shifts, whereas inferred changes at the westernmost site in Alberta were significantly correlated to the Saskatchewan sites, but opposite in sign, and exhibited more high-frequency variation on the scale of centuries. The mechanisms behind these abrupt shifts in aridity are poorly understood, but may be linked to changes in oceanic–atmospheric interactions that influence the mean position of the jetstream and the associated storm tracks. Natural shifts in mean climatic conditions may accelerate with increasing carbon dioxide levels intensifying the likelihood of extreme droughts in North American prairies.