Bibliothek

Notizen: Many macroalgae have significant spatial differentiation involving higher rate resource use at a site than of acquisition of that resource from the environment at that site. Long-distance symplasmic transport of solutes occurs in some large green algae where the solutes are moved in streaming cytoplasm. In some large brown algae there is evidence of long-distance symplasmic transport of organic C and other solutes. Structural and physiological data suggest that while the transport in ‘sieve tubes’ of Macrocystis might be by a Munch pressure flow mechanism the transport in many other brown algae is less likely to be by this mechanism. Less is known of long-distance symplasmic transport in red algae. In terrestrial bryophytes transpiration occurs and in some liverworts and many mosses (but not in hornworts) there are files of dead cells in their tissues which may, and in some cases certainly, function in long-distance apoplasmic water transport. The hydraulic conductivity of these conduits is poorly characterized. Long-distance symplasmic transport in some mosses have been characterized both structurally and physiologically, but in other mosses and in liverworts the evidence is only structural. Most of these symplasmic transport pathways seem to have a high resistance to flow.

Notizen: There is approximately 50 times more inorganic carbon in the global ocean than in the atmosphere. On time scales of decades to millions of years, the interaction between these two geophysical fluids determines atmospheric CO2 levels. During glacial periods, for example, the ocean serves as the major sink for atmospheric CO2, while during glacial–interglacial transitions, it is a source of CO2 to the atmosphere. The mechanisms responsible for determining the sign of the net exchange of CO2 between the ocean and the atmosphere remain unresolved. There is evidence that during glacial periods, phytoplankton primary productivity increased, leading to an enhanced sedimentation of particulate organic carbon into the ocean interior. The stimulation of primary production in glacial episodes can be correlated with increased inputs of nutrients limiting productivity, especially aeolian iron. Iron directly enhances primary production in high nutrient (nitrate and phosphate) regions of the ocean, of which the Southern Ocean is the most important. This trace element can also enhance nitrogen fixation, and thereby indirectly stimulate primary production throughout the low nutrient regions of the central ocean basins. While the export flux of organic carbon to the ocean interior was enhanced during glacial periods, this process does not fully account for the sequestration of atmospheric CO2. Heterotrophic oxidation of the newly formed organic carbon, forming weak acids, would have hydrolyzed CaCO3 in the sediments, increasing thereby oceanic alkalinity which, in turn, would have promoted the drawdown of atmospheric CO2. This latter mechanism is consistent with the stable carbon isotope pattern derived from air trapped in ice cores. The oceans have also played a major role as a sink for up to 30% of the anthropogenic CO2 produced during the industrial revolution. In large part this is due to CO2 solution in the surface ocean; however, some, poorly quantified fraction is a result of increased new production due to anthropogenic inputs of combined N, P and Fe. Based on ‘circulation as usual’, models predict that future anthropogenic CO2 inputs to the atmosphere will, in part, continue to be sequestered in the ocean. Human intervention (large-scale Fe fertilization; direct CO2 burial in the deep ocean) could increase carbon sequestration in the oceans, but could also result in unpredicted environmental perturbations. Changes in the oceanic thermohaline circulation as a result of global climate change would greatly alter the predictions of C sequestration that are possible on a ‘circulation as usual’ basis.

Notizen: Submerged aquatic macrophytes growing in water where free CO2 is unavailable (above pH 8·2) must use mechanisms to supply external dissolved inorganic carbon in a form available to chloroplasts (CO2). Active transport of HCO3– across the plasmalemma has not been proven to be widespread in aquatic macrophytes and catalytic conversion of HCO3– to CO2 is the usual supply mechanism in submerged macrophytes. The interaction of leaf form and function in this respect was investigated in the linear, submerged leaves of Ranunculus penicillatus (Dumort.) Bab ssp. pseudofluitans (Syme) S.Webster. Viable protoplasts were isolated using a mixture of cell wall degrading enzymes optimized for this species. Protoplast viabilities greater than 80% after 5 h of isolation were achieved. Photosynthetic rates of isolated protoplasts were comparable with that of intact plant tissue. Results of carbon isotopic disequilibrium experiments showed that CO2 was the preferred species of dissolved inorganic carbon for photosynthesis by protoplasts and that HCO3– which predominates in the plant’s natural environment mainly contributes by supplying CO2 outside the cells.

Notizen: Primary producers in aquatic environments cover a range of living biomasses from 10−13 to 104 g dw. Benthic plants at the high end of this range contribute 5 Gt C year,1 to global primary productivity. Plankton at the lower end (up to 10−4 g dw) contributes about 30 Gt C year−1. While many problems of interpretation remain, in general terms the size of the organisms which dominate particular habitats can be related to the physics of water movement and its interaction with the availability of light and nutrients, the generation time of the organism, and the attentions of grazers. A second scaling problem is that of methods of studying the global energy flow and nutrient cycling roles of aquatic primary producers. Problems with scaling up from small-scale and mesoscale to regional or global scale, and the prospects of more direct estimates of large-scale productivity, are discussed.

Notizen: We present a mechanism of regulation of growth and activity of legume root nodules which is consistent with published experimental observations. The concentration of reduced nitrogen compounds, probably amino acids, flowing into the nodules from the phloem, is sensed by the nodules; growth and activity of the nodules is adjusted accordingly. In many legumes this response may involve changes in the oxygen diffusion resistance of the nodule cortex. A straightforward feedback mechanism in which nodule activity is lowered when reduced N in the phloem is high and increased when it is low is envisaged. Almost all import into nodules is via the phloem sap originating in the lower leaves. As a plant develops, these mature leaves no longer utilize nitrogen delivered in the xylem and so export it in the phloem. In plants with an adequate nitrogen supply (from nodules or combined nitrogen in soil), a high concentration of nitrogen containing compounds in the phloem from the lower leaves may inhibit nodule growth as well as activity. This suggestion is an alternative to the hypotheses of carbohydrate deprivation or nitrate inhibition which are commonly used to explain the effects of combined nitrogen on nodule growth and activity.

Notizen: Nitrogen stable isotope (15N, 14N) natural abundance has been much less used than carbon isotopes (13C, 12C) in plant physiology and ecology. Analytical problems, the lower fractional abundance of 15N than of 13C in the biosphere, the greater complexity of the N cycle relative to the C cycle, and smaller expressed discriminations in nature, are contributing factors. The major N pools, globally, have different isotope signatures: atmospheric N2 is 15N-depleted relative to organic N (including sedimentary N), a situation resulting from a greater expressed discrimination in the organic N to N2 (via denitrification) reaction than of diazotrophy during accumulation of the reduced N. Essentially all of the enzymes except nitrogenase which transform N compounds show discrimination against 15N, although for glutamine synthetase, and the amination of 2-oxoglutarate and pyruvate, this is only seen in terms of NH4+ rather than the true substrate, NH3. Discrimination is expressed in various N interconversions within plants, leading to substantial differences in δ15N (up to 12‰) among N compounds and macroscopic plant parts. N isotope fractionation during assimilation of exogenous combined N is often much lower than that expected from studies of isolated enzymes due to processes which show very little discrimination, such as limitation by transport through aqueous solution and membranes. Application of 15N/14N discrimination studies to plant ecology have concentrated largely on distinguishing diazotrophy from N supplied from combined N, based on the lower 15N/14N in diazotrophs due to the higher 15N/14N of combined N sources not being offset by fractionation during uptake. While potentially very useful, a number of pitfalls are discussed in its ecological use in both terrestrial and aquatic systems. N isotope discrimination is also useful in tracking N through food webs, and hence, back to combined N sources for plants.

Notizen: The use of stable isotope natural abundance measurements in plant ecophysiological research is discussed in the context of studies of 13C/12C ratios in marine plants, with emphasis on the uniqueness of the information given by natural abundance measurements and of the importance of complementary data obtained by other techniques in making full use of the natural abundance data. (1) Inorganic C acquisition and assimilation in marine plants can involve diffusive entry of CO2, or the occurrence of a CO2-concentrating mechanism frequently involving active HCO3− influx. For diffusive CO2 entry, the δ13C measurements can give unique information on the fractional limitation of photosynthesis by CO2 transport which, with photosynthetic rate measurements, can be used to compute transport conductances. For active HCO3−, influx, the δ13C values uniquely permit computation of the ratio of the bidirection fluxes (influx/efflux) which, with photon yield data, can be used to given information on the mechanism of the efflux. The analyses are absolutely dependent on external (non-δ13C) data distinguishing between diffusive CO2 entry and the occurrence of a CO2 concentrating mechanism. (2) δ13C measurements on marine photolithotrophs and on members of other trophic levels collected from the sea can give unique data on food webs, with measurements of δ values for other isotopes and compositional data adding precision to the interpretations. (3) Measurements of in situδ13C values for extant marine photolithotrophs, compared with δ13C values for ancient atmospheric CO2, can give unique information on the mechanism of atmospheric CO2 draw-down at the start of glacials; other information permits more concrete conclusions to be drawn.

Notizen: Abstract. Photosynthesis by many marine phytoplankton algae is saturated by the inorganic C concentration in air-equilibrated sea water. These organisms appear to use an active inorganic C transport process (CO2-concentrating mechanism) which increases the CO2 concentration around rubisco and saturates this enzyme with CO2 and suppresses its oxygenase activity. A minority of marine phytoplankton algae have photosynthetic characteristics more suggestive of diffusive CO2 entry; the inorganic C concentration present in sea water does not saturate photosynthesis by these organisms. Theoretical considerations, tested when possible against observation, suggest that the organisms with a CO2-concentrating mechanism could have a lower cost of photons, nitrogen, iron, manganese and molybdenum to achieve a given rate of carbon accumulation by the cells than is the case for the organisms with diffusive CO2 entry. Zinc and selenium costs may show the reverse effect. The increased sea-surface inorganic C, and CO2 concentrations which will result from anthropogenic increases in atmospheric CO2 content are predicted to increase the rate of photosynthesis, and of growth when other resources are abundant, and to reduce, or reverse, the higher resource (photons, nitrogen, iron, manganese and molybdenum) cost of a given rate of CO2 assimilation in organisms with CO2 diffusion relative to those which have CO2 concentrating mechanisms and do not repress them at higher inorganic C concentrations. These effects may well alter species composition, and overall resource cost of growth, of phytoplankton; any influence that these effects may have on CO2 removal from the atmosphere are severely constrained by other trophic levels and, especially, oceanic circulation patterns. Changed sea-surface temperatures are unlikely to qualitatively alter these conclusions.

Notizen: Abstract. pH is an all-pervasive variable in the environment of phototrophs. Phototrophs as a whole, and even the single genus Dunaliella, can grow over essentially the whole range of pH values found in nature. Such a large range of pH values, combined with other chemical variations in the environment, impose a range of constraints on plant behaviour related to intracellular pH regulation, nutrient acquisition, and avoidance of toxic effects. No single genotype can grow well over the whole pH range compatible with growth of phototrophs as a whole, although some deliberately alter surface pH so as to create a 5–6 unit pH gradient over the surface related to nutrient acquisition and avoidance of toxic influences. The regulation of these various processes does not, on current evidence, involve pH-sensing by any extracellular sensor which is not part of the catalytic or regulatory mechanism of a membrane protein such as a porter.