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Notes: 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.

Notes: 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.

Notes: 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.

Notes: A testable mechanism of CO2 accumulation in photolithotrophs, originally suggested by Pronina & Semenenko, is quantitatively analysed. The mechanism involves (as does the most widely accepted hypothesis) the delivery of HCO3− to the compartment containing Rubisco. It differs in proposing subsequent HCO3− entry (by passive uniport) to the thylakoid lumen, followed by carbonic anhydrase activity in the lumen; uncatalysed conversion of HCO3− to CO2, even at the low pH of the lumen, is at least 300 times too slow to account for the rate of inorganic C acquisition. Carbonic anhydrase converts the HCO3− to CO2 at the lower pH maintained in the illuminated thylakoid lumen by the light-driven H+ pump, generating CO2 at 10 times or more the thylakoid HCO3− concentration. Efflux of this CO2 can suppress Rubisco oxygenase activity and stimulate carboxylase activity in the stroma. This mechanism differs from the widely accepted hypotheses in the required location of carbonic anhydrase, i.e. in the thylakoid lumen rather than the stroma or pyrenoid, and in the need for HCO3− influx to thylakoids. The capacity for anion (assayed as Cl−) entry by passive uniport reported for thylakoid membranes is adequate for the proposed mechanism; if the Cl− channel does not transport HCO3−, HCO3− entry could be by combination of the Cl− channel with a Cl− HCO3− antiporter. This mechanism is particularly appropriate for organisms which lack overt accumulation of total inorganic C in cells, but which nevertheless have the gas exchange characteristics of an organism with a CO2-concentrating mechanism.

Notes: Abstract Rates of uptake of 14C-labelled inorganic carbon were measured for whole Chara hispida plants, detached parts of the shoot and isolated (split-chamber technique) apices, lateral branchlets and rhizoid—node complexes.The rates of inorganic carbon uptake by the rhizoid—node complex expressed per gram fresh weight whole plant were three to four orders of magnitude less than the uptake for the whole plant. Up to 70% of the carbon taken up by the rhizoid—node complex was translocated to the shoot. After 12 h exposure to 14C-labelled inorganic carbon the concentration of 14C was greater in apices than in uppermost or central internodal cells and in all lateral branchlets, regardless of whether label was supplied to the whole plant or isolated rhizoid—node complexes. Measurement of inorganic carbon uptake by detached internodal cells and detached and isolated apices and lateral branchlets showed that lateral branchlets had the greatest rates of inorganic carbon uptake. During 12 h exposure to 14C, isolated lateral branchlets translocated to the attached shoot 55% of the labelled carbon taken up; for isolated apices this value was only 13%.It is concluded that it is highly unlikely that the rhizoid of Chara hispida could acquire a significant fraction of the whole plant requirement for inorganic carbon and that apices are sink regions for photosynthate while lateral branchlets are source regions.

Notes: Abstract The acid-base balance during ammonium (used to mean NH 4+ and/or NH3) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m−3 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H+ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H+ accumulate in the medium per organic N synthesized. Granted a permeability (PNH3) of some 10−3 cm s−1, and a finite [NH3] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH 4+. This means that most of the H + appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H+ accumulates in the medium equivalent to any NH3 entry by requilibration from exogenous NH 4+. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO3+grown cells, suggesting that there is no action of a ‘biochemical pH stat’ during longterm assimilation of NO3+in H. africanum.Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO3+-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 μmol dm−3) the net influx was all attributable to NH4+influx via a uniporter, probably partly short-circuited by a passive NH3 efflux due to intrinsic membrane permeability to NH3. The net ammonium influx was in all cases associated with H+ accumulation in the medium. (1.3-1.7 H + per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated.Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a ‘pH-regulating’ change in H+ fluxes.

Notes: The variation in stomatal characters in leaves from one Alnus glutinosa (L.) Gaertn. tree is analysed. Measurements were taken from over 70 sites on the abaxial surfaces of representative ‘sun’ and ‘shade’ leaves having the same insertion point. The mean values of stomatal density and index in the shade leaf were significantly lower (71 and 93%, respectively) than those for the sun leaf. Within leaves, up to 2.5-fold differences in stomatal density values were observed. Contour maps derived from the data reveal non-random trends over the leaf surface. Correlations between stomatal density, epidermal cell density and stomatal index indicate that the variation in stomatal density within leaves arose primarily from local differences in stomatal differentiation, rather than from local differences in leaf expansion. This research demonstrates that a high level of variation in stomatal characters occurs both within and between leaves. We conclude that a well-defined sampling strategy should be used when estimating stomatal characters for (tree) leaves. Furthermore, the leaf's insertion point and situation within the tree crown should be taken into account. We discuss the implications of these findings for palaeoclimatic interpretations and emphasize the need for great caution when drawing conclusions based solely on stomatal characters.

Notes: Summary The inorganic carbon fixation patterns of Isoetes lacustris and Lobelia dortmanna from an oligotrophic Scottish loch have been examined by following titratable acidity changes in plant sap and light/dark 14CO2 incorporation by roots and shoots. The diurnal pattern of titratable acidity changes in I. lacustris suggests crassulacean acid metabolism (CAM) while the lack of any change in titratable acidity in L. dortmanna suggests C3 metabolism. Of the carbon fixed by L. dortmanna, 99.9% was taken up through the roots and fixation occurred primarily during the day. In Isoetes, CO2 was taken up by both roots and shoots and during both day and night. Regardless of the site of CO2 uptake, fixation occurred only in the shoots of both plants. Analysis of carbon isotope ratios of plant organic material was used to further investigate the photosynthetic mechanisms of these Isoetids. Considering the absence of a nighttime peak in titratable acidity in L. dortmanna, the Δ13C (Δ=δ13C plant-δ13C source) value of the shoots of L. dortmanna (-14.2‰) is indicative of C3 photosynthesis limited by the rate of CO2 diffusion. The less negative Δ of I. lacustris (-6.0‰) is consistent with both dark acidification of CAM and CO2 limited C3 photosynthesis. This is in contrast to the terrestrial Isoetes durieui which is shown to have a Δ value which is similar to a terrestrial C3 plant. The carbon fixation patterns of these Isoetids suggest that the CO2 concentration in the loch may be growth limiting, and that root uptake and/or dark acidification are means of optimising CO2 supply. However, in view of the relatively high levels of CO2 in sediment and bulk water, it is suggested that low levels of nutrients may also limit growth in these plants.

Notes: Abstract Seasonal variations and the effect of reproductive development on resource acquisition by two intertidal fucoid species, the iteroparous Fucus serratus L. and the semelparous Himanthalia elongata (L.) S. F. Gray were examined. The oxygen-exchange characteristics of vegetative apical tissue of both non-fertile and fertile plants and receptacle tissue were compared at monthly intervals throughout reproductive development. Respiratory rates in non-fertile F. serratus varied seasonally between 1.5 and 8.0 μmol g−1 fresh wt h−1; in fertile plants the receptacle had a significantly lower respiratory rate than the vegetative tissue. The respiratory rate of the vegetative button of fertile H. elongata displayed less seasonal variation and was lower than that of the receptacle, which varied from a maximum of 9.5 μmol g−1 fresh wt h−1 at receptacle initiation in October to a minimum of 2.0 μmol g−1 fresh wt h−1 in February. The maximum photosynthetic rate (P max) of non-fertile plants of both species did not vary in a distinct seasonal manner (∼60 μmol g−1 fresh wt h−1 for F. serratus and ∼12 μmol g−1 fresh wt h−1 for H. elongata). In fertile plants, the P max of the receptacle tissue was (∼50% lower in F. serratus, and at its peak three times higher in H. elongata, than that of vegetative tissue. The stable carbon-isotope ratio (δ13C) did not differ between different tissue types in F. serratus, but values did vary seasonally, being less negative in the summer than in the winter (−13.5‰ compared to −18‰). The receptacle tissue of H. elongata also displayed a distinct seasonal variation in δ13C values (−12‰ in summer, −16‰ in winter), whilst the δ13C of the vegetative button did not vary seasonally. The rate of uptake of inorganic nitrogen by the vegetative thallus was lower in H. elongata than in F. serratus. The receptacle tissue of F. serratus had lower uptake rates than the vegetative tissue, whilst the uptake rate by H. elongata receptacle tissue was higher than that of the vegetative button.