Inorganic C-sources for Lemanea, Cladophora and Ranunculus in a fast-flowing stream: Measurements of gas exchange and of carbon isotope ratio and their ecological implications

Carbon isotope fractionation in karst aquatic mosses

Radiocarbon activity (a¹⁴C) and ¹³C composition (δ¹³C) were measured in hygrophyte and mesophyte (land) mosses collected in the natural habitat of the Plitvice Lakes and along the Zrmanja and Krupa Rivers (typical continental and Mediterranean climates, respectively), Croatia. a¹⁴C and δ¹³C values of mosses, of atmospheric CO₂ and dissolved inorganic carbon (DIC) were compared with contemporary data and with data from 30 years ago at the Plitvice Lakes when ¹⁴C activity of atmospheric CO₂ was ∼30% higher. A positive correlation between a¹⁴C_moss and δ¹³C_moss was observed for all data reflecting the change of carbon isotopic composition in DIC along the water flows and in atmospheric CO₂ regardless of the climatic regions and historic period. Fraction of the atmospheric carbon in moss (ωatm.C) and carbon fractionation factor from aquatic CO₂ (DIC) to moss tissue (εmoss/g-aq) were calculated for each individual moss. Three species of mosses had ω_atm.C ∼ 0 % implying that they turn to anabiosis during dry periods. The relation εmoss/g-aqvs.ωatm.C differentiates true aquatic and amphiphyte mosses. The first had a statistically significant negative correlation between εmoss/g-aq and ωatm.C. The amphiphyte mosses had lower εmoss/g-aq with higher water flow rates.

Rubisco and carbon-concentrating mechanism co-evolution across chlorophyte and streptophyte green algae

Green algae expressing a carbon-concentrating mechanism (CCM) are usually associated with a Rubisco-containing micro-compartment, the pyrenoid. A link between the small subunit (SSU) of Rubisco and pyrenoid formation in Chlamydomonas reinhardtii has previously suggested that specific RbcS residues could explain pyrenoid occurrence in green algae. A phylogeny of RbcS was used to compare the protein sequence and CCM distribution across the green algae and positive selection in RbcS was estimated. For six streptophyte algae, Rubisco catalytic properties, affinity for CO₂ uptake (K_0.5 ), carbon isotope discrimination (δ¹³ C) and pyrenoid morphology were compared. The length of the βA-βB loop in RbcS provided a phylogenetic marker discriminating chlorophyte from streptophyte green algae. Rubisco kinetic properties in streptophyte algae have responded to the extent of inducible CCM activity, as indicated by changes in inorganic carbon uptake affinity, δ¹³ C and pyrenoid ultrastructure between high and low CO₂ conditions for growth. We conclude that the Rubisco catalytic properties found in streptophyte algae have coevolved and reflect the strength of any CCM or degree of pyrenoid leakiness, and limitations to inorganic carbon in the aquatic habitat, whereas Rubisco in extant land plants reflects more recent selective pressures associated with improved diffusive supply of the terrestrial environment.

Carbon acquisition characteristics of six microalgal species isolated from a subtropical reservoir: potential implications for species succession

CO₂ levels in freshwater systems can fluctuate widely, potentially influencing photosynthetic rates and growth of phytoplankton. Given the right conditions, this can lead to bloom formation and affect water quality. This study investigated the acquisition of dissolved inorganic carbon (DIC) by six species of microalgae, a cyanobacterium Cylindrospermopsis raciborskii, the diatoms Cyclotella sp., Nitzschia sp., and the green algae Stichococcus sp., Staurastrum sp., and Monoraphidium sp., all isolated from a subtropical reservoir in Australia. Carbon acquisition characteristics, specifically the affinity for DIC, internal pH, and internal DIC concentrations were measured. Affinities for CO₂ ( K0.5(CO2) ) ranged between 0.7 and 6 μM CO₂ . This was considerably lower than air-equilibrated surface water CO₂ concentrations, and below reported affinities for CO₂ of RuBisCO suggesting operation of active carbon dioxide concentrating mechanisms (CCMs) in all species. Internal pH was lowest for Cyclotella sp. at 7.19, and highest for Staurastrum sp., at 7.71. At 180 μM external DIC, ratios of internal:external CO₂ ranged from 2.5 for Nitzschia sp. to 14 in C. raciborskii. Internal HCO₃^- concentration showed a linear relationship with surface area to biovolume ratio (SA:Vol). We hypothesized that species with a higher SA:Vol suffer more from diffusive escape of CO₂ , thus storage of DIC as bicarbonate is favored in these strains. For C. raciborskii, under stratified summer conditions, its strong CCM, and resilient photosynthetic characteristics may contribute to its bloom forming capacity.

Contribution of sea ice microbial production to Antarctic benthic communities is driven by sea ice dynamics and composition of functional guilds

Organic matter produced by the sea ice microbial community (SIMCo) is an important link between sea ice dynamics and secondary production in near-shore food webs of Antarctica. Sea ice conditions in McMurdo Sound were quantified from time series of MODIS satellite images for Sept. 1 through Feb. 28 of 2007-2015. A predictable sea ice persistence gradient along the length of the Sound and evidence for a distinct change in sea ice dynamics in 2011 were observed. We used stable isotope analysis (δ¹³ C and δ¹⁵ N) of SIMCo, suspended particulate organic matter (SPOM) and shallow water (10-20 m) macroinvertebrates to reveal patterns in trophic structure of, and incorporation of organic matter from SIMCo into, benthic communities at eight sites distributed along the sea ice persistence gradient. Mass-balance analysis revealed distinct trophic architecture among communities and large fluxes of SIMCo into the near-shore food web, with the estimates ranging from 2 to 84% of organic matter derived from SIMCo for individual species. Analysis of patterns in density, and biomass of macroinvertebrate communities among sites allowed us to model net incorporation of organic matter from SIMCo, in terms of biomass per unit area (g/m² ), into benthic communities. Here, organic matter derived from SIMCo supported 39 to 71 per cent of total biomass. Furthermore, for six species, we observed declines in contribution of SIMCo between years with persistent sea ice (2008-2009) and years with extensive sea ice breakout (2012-2015). Our data demonstrate the vital role of SIMCo in ecosystem function in Antarctica and strong linkages between sea ice dynamics and near-shore secondary productivity. These results have important implications for our understanding of how benthic communities will respond to changes in sea ice dynamics associated with climate change and highlight the important role of shallow water macroinvertebrate communities as sentinels of change for the Antarctic marine ecosystem.

How much is too much? Identifying benchmarks of adverse effects of macroalgae on the macrofauna in intertidal flats

Eutrophication, defined as the accumulation of organic matter typically in response to anthropogenically enhanced nutrient inputs, often takes the form of macroalgal blooms in shallow estuaries and causes a cascade of adverse ecosystem effects. Confidence in the use of macroalgae as an indicator of eutrophication in estuaries is limited by the lack of quantitative data on thresholds of adverse effects. Field experiments can provide "benchmarks" of no effect or adverse effects that can be used to validate thresholds derived statistically from field data. To determine a benchmark of adverse effects of macroalgal abundance on macrobenthic faunal communities in intertidal flats, experiments were conducted in two sites in Bodega Harbor (BOD) and two sites in Upper Newport Bay (UNB), California, USA. At each site, 24 cages maintained six treatments of macroalgae for eight weeks, with mat depths of 0, 1.0, 1.5, 2.5, 3.5, and 5.0 cm composed mostly of bloom-forming green macroalgae in the genus Ulva. Every two weeks, cores of sediment (10 cm deep) were collected, and macrofauna were quantified. Mats 1 cm deep, equivalent to a biomass of 110-120 g dry mass (dm)/m2 or 840-930 g wet mass/m2, resulted in the reduction of macrofaunal abundance by at least 67% and species richness by at least 19% within two weeks at three of four sites. Loss was attributed to the decline of key functional groups. Surface-deposit feeders were eliminated from one site at BOD within four weeks and at one site in UNB within six weeks, while 1-cm mats negatively affected suspension feeders and herbivores in the second site at BOD. In contrast, the other site at UNB was not affected by macroalgal treatment, likely due to an initial community composed of a high proportion of subsurface-deposit feeders tolerant of stressful environments. Macroalgal abundances as low as 110-120 g dm/m2 had significant and rapid negative effects on macrobenthic invertebrates, providing a clear benchmark of adverse effects of macroalgal blooms on macrofaunal abundance and community structure, two indicators of ecosystem health. This information can inform the establishment of appropriate metrics for macroalgal abundance in eutrophic estuaries.

The Genus Cladophora Kützing (Ulvophyceae) as a Globally Distributed Ecological Engineer

The green algal genus Cladophora forms conspicuous nearshore populations in marine and freshwaters worldwide, commonly dominating peri-phyton communities. As the result of human activities, including the nutrient pollution of nearshore waters, Cladophora-dominated periphyton can form nuisance blooms. On the other hand, Cladophora has ecological functions that are beneficial, but less well appreciated. For example, Cladophora has previously been characterized as an ecological engineer because its complex structure fosters functional and taxonomic diversity of benthic microfauna. Here, we review classic and recent literature concerning taxonomy, cell biology, morphology, reproductive biology, and ecology of the genus Cladophora, to examine how this alga functions to modify habitats and influence littoral biogeochemistry. We review the evidence that Cladophora supports large, diverse populations of microalgal and bacterial epiphytes that influence the cycling of carbon and other key elements, and that the high production of cellulose and hydrocarbons by Cladophora-dominated periphyton has the potential for diverse technological applications, including wastewater remediation coupled to renewable biofuel production. We postulate that well-known aspects of Cladophora morphology, hydrodynamically stable and perennial holdfasts, distinctively branched architecture, unusually large cell and sporangial size and robust cell wall construction, are major factors contributing to the multiple roles of this organism as an ecological engineer.

Ecophysiology of photosynthesis in macroalgae

Macroalgae occur in the marine benthos from the upper intertidal to depths of more than 200 m, contributing up to 1 Pg C per year to global primary productivity. Freshwater macroalgae are mainly green (Chlorophyta) with some red (Rhodophyta) and a small contribution of brown (Phaeophyceae) algae, while in the ocean all three higher taxa are important. Attempts to relate the depth distribution of three higher taxa of marine macroalgae to their photosynthetic light use through their pigmentation in relation to variations in spectral quality of photosynthetically active radiation (PAR) with depth (complementary chromatic adaptation) and optical thickness (package effect) have been relatively unsuccessful. The presence (Chlorophyta, Phaeophyceae) or absence (Rhodophyta) of a xanthophyll cycle is also not well correlated with depth distribution of marine algae. The relative absence of freshwater brown algae does not seem to be related to their photosynthetic light use. Photosynthetic inorganic carbon acquisition in some red and a few green macroalgae involves entry of CO(2) by diffusion. Other red and green macroalgae, and brown macroalgae, have CO(2) concentrating mechanisms; these frequently involve acid and alkaline zones on the surface of the alga with CO(2) (produced from HCO(3) (-)) entering in the acid zones, while some macroalgae have CCMs based on active influx of HCO(3) (-). These various mechanisms of carbon acquisition have different responses to the thickness of the diffusion boundary layer, which is determined by macroalgal morphology and water velocity. Energetic predictions that macroalgae growing at or near the lower limit of PAR for growth should rely on diffusive CO(2) entry without acid and alkaline zones, and on NH(4) (+) rather than NO(3) (-) as nitrogen source, are only partially borne out by observation. The impact of global environmental change on marine macroalgae mainly relates to ocean acidification and warming with shoaling of the thermocline and decreased nutrient flux to the upper mixed layer. Predictions of the impact on macroalgae requires further experiments on interactions among increased inorganic carbon, increased temperature and decreased nitrogen and phosphorus supply, and, when possible, studies of genetic adaptation to environmental change.

Sea ice microbial production supports Ross Sea benthic communities: influence of a small but stable subsidy

Diversity in guilds of primary producers enhances temporal stability in provision of organic matter to consumers. In the Antarctic ecosystem, where temporal variability in phytoplankton production is high, sea ice contains a diatom and microbial community (SIMCO) that represents a pool of organic matter that is seasonally more consistent, although of relatively small magnitude. The fate of organic material produced by SIMCO in Antarctica is largely unknown but may represent an important link between sea ice dynamics and secondary production in nearshore food webs. We used whole tissue and compound-specific stable isotope analysis of consumers to test whether the sea ice microbial community is an important source of organic matter supporting nearshore communities in the Ross Sea. We found distinct gradients in delta13C and delta15N of SIMCO corresponding to differences in inorganic carbon and nitrogen acquisition among sites with different sea ice extent and persistence. Mass balance analysis of a suite of consumers demonstrated large fluxes of SIMCO into the nearshore food web, ranging from 5% to 100% of organic matter supplied to benthic species, and 0-10% of organic matter to upper water column or pelagic inhabitants. A delta13C analysis of nine fatty acids including two key biomarkers for diatoms, eicosapentaenoic acid (EPA, 20:5omega3), and docosahexaenoic acid (DHA, 22:6omega3), confirmed these patterns. We observed clear patterns in delta13C of fatty acids that are enriched in 13C for species that acquire a large fraction of their nutrition from SIMCO. These data demonstrate the key role of SIMCO in ecosystem functioning in Antarctica and strong linkages between sea ice extent and nearshore secondary productivity. While SIMCO provides a stabilizing subsidy of organic matter, changes to sea ice coverage associated with climate change would directly affect secondary production and stability of benthic food webs in Antarctica.

IMPACT OF TAXONOMY, GEOGRAPHY, AND DEPTH ON δ(13) C AND δ(15) N VARIATION IN A LARGE COLLECTION OF MACROALGAE(1)

The natural abundance of carbon stable isotopes (δ(13) C) of marine macrophytes has been measured in previous studies and used to analyze differences in Ci assimilation among the three macroalgal phyla, Chlorophyta, Ochrophyta, and Rhodophyta, and seagrasses, distinguishing diffusive CO2 entry from the operation of a CO2 -concentrating mechanisms (CCM). The work reported here further resolves the patterns of δ(13) C variation in aquatic macrophytes related to their taxonomy, geographic location (and consequently climatic conditions), and vertical zonation. Analyses of δ(13) C for 87 species are reported, including eight that have not been previously examined, belonging to taxa in the three macroalgal phyla, plus two species of seagrasses, collected at different latitudes. For one species of each phylum, analyses were also conducted through a vertical depth gradient. Representative species were used in a pH drift experiment, in order to compare the mechanism of Ci acquisition for photosynthesis with the δ(13) C subsequently determined on the same specimen. Our results suggest that the δ(13) C values were mostly determined by taxonomy. Depth effects on C stable isotope composition differed among taxa. The parallel measurements of δ(15) N are more difficult to interpret mechanistically; there are no robust phylogenetic and large-scale biogeographic correlations; local factors of natural (e.g., upwellings) and anthropogenic (e.g., sewage outfall) inputs predominate in determining the macrophyte δ(15) N.

SHORT-TERM MEASUREMENT OF CARBON STABLE ISOTOPE DISCRIMINATION IN PHOTOSYNTHESIS AND RESPIRATION BY AQUATIC MACROPHYTES, WITH MARINE MACROALGAL EXAMPLES(1)

Progress in the study of stable isotope discrimination in carbon assimilation by aquatic macrophytes has been slower than for other groups of primary producers, such as phytoplankton and terrestrial plants. A probable reason has been the methodologies employed for such a study: field collections or long-term incubations, both relying on the observation of changes in carbon isotope composition of plant tissue. Here, we present a short-term incubation method based on the change in carbon stable isotope composition in water. Its fundamental advantage over the other approaches is that the change in stable isotope composition in water in a closed system is much faster than in the plant tissue. We applied the method to investigate the relationship between carbon assimilation intensity and isotope discrimination. The results included a relatively small discrimination in respiration, a significant influence of carbon assimilation rate on discrimination, and the suggestion of HCO3 (-) or CO2 uptake in photosynthesis. The information gathered using this method would be difficult to obtain in other ways, and so we believe that it should contribute to a better understanding of the physiology and ecology of aquatic macrophytes.

AN ECOLOGICAL REVIEW OF CLADOPHORA GLOMERATA (CHLOROPHYTA) IN THE LAURENTIAN GREAT LAKES(1)

Cladophora glomerata (L.) Kütz. is, potentially, the most widely distributed macroalga throughout the world's freshwater ecosystems. C. glomerata has been described throughout North America, Europe, the Atlantic Islands, the Caribbean Islands, Asia, Africa, Australia and New Zealand, and the Pacific Islands. Cladophora blooms were a common feature of the lower North American Great Lakes (Erie, Michigan, Ontario) from the 1950s through the early 1980s and were largely eradicated through the implementation of a multibillion-dollar phosphorus (P) abatement program. The return of widespread blooms in these lakes since the mid-1990s, however, was not associated with increases in P loading. Instead, current evidence indicates that the resurgence in blooms was directly related to ecosystem level changes in substratum availability, water clarity, and P recycling associated with the establishment of dense colonies of invasive dreissenid mussels. These results support the hypothesis that dreissenid mussel invasions may induce dramatic shifts in energy and nutrient flow from pelagic zones to the benthic zone.

Algae lacking carbon-concentrating mechanisms

Most of the algae and cyanobacteria that have been critically examined express a carbon-concentrating mechanism (CCM) when grown at, or below, the current atmospheric CO2 concentration. This paper considers algae that appear to lack a CCM. Critical examination of the evidence on which the presence or absence of a CCM is decided shows that more information is frequently needed before the criteria can be fully applied. Examples are the pathways of glycolate metabolism in nongreen algae, and the 13C/12C discrimination shown by form ID Rubisco in vitro. The available evidence suggests that the algae lacking CCMs are some terrestrial green microalgae, some florideophyte freshwater red macroalgae, and a number of florideophyte red macroalgae from the supralittoral, littoral, and sublittoral, and almost all of the freshwater chrysophytes and synurophytes examined. Certain environmental, biochemical, and biophysical factors may permit the occurrence of algae lacking CCMs. The absence of CCMs is presumably the plesiomorphic (i.e., ancestral) condition in cyanobacteria (and algae?).Key words: CO2 diffusion, chrysophyte algae, ecology, evolution, green algae, photosynthesis, red algae.

CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution

The evolution of organisms capable of oxygenic photosynthesis paralleled a long-term reduction in atmospheric CO2 and the increase in O2. Consequently, the competition between O2 and CO2 for the active sites of RUBISCO became more and more restrictive to the rate of photosynthesis. In coping with this situation, many algae and some higher plants acquired mechanisms that use energy to increase the CO2 concentrations (CO2 concentrating mechanisms, CCMs) in the proximity of RUBISCO. A number of CCM variants are now found among the different groups of algae. Modulating the CCMs may be crucial in the energetic and nutritional budgets of a cell, and a multitude of environmental factors can exert regulatory effects on the expression of the CCM components. We discuss the diversity of CCMs, their evolutionary origins, and the role of the environment in CCM modulation.

Comparative physiology of plant and arthropod land adaptation

Plants related to aquatic Charophycean green algae were probably terrestrial by the early to mid Silurian; these plants were the ancestors of the vascular plants that have dominated the Earth’s flora since the Devonian. The arthropods have been the major herbivores and carnivores in many terrestrial communities since the Devonian: they arose from a number of aquatic arthropod stocks which invaded the land from the Silurian onwards. The vascular plants and arthropods conduct their basic metabolism in the same way as their aquatic counterparts, but in the aerial environment which differs greatly from the aquatic in the exchange of materials, momentum and heat between organisms and their environment. Terrestrial organisms differ from their aquatic relatives in ( inter alia ) the water vapour loss attendant on the exchange of gases in photosynthesis and respiration; the potential for large and rapid changes in body temperature; and differences in the structural requirements for maintenance of posture and, in animals, locomotion. The (putatively) adaptive responses to these problems of terrestrial life show a number of im portant parallels between the vascular plants and arthropods, including internalization of gas-exchange surfaces, regulation of gas diffusion between the gas-exchange surfaces and the outside air, a wax layer over the general body surface which restricts non-respiratory and non-photosynthetic water loss, and the importance of rigid skeletal members (present in the ancestral aquatic arthropods, but not in algae). At the biochemical level many of the prerequisites for the special structures and functions found in terrestrial organisms can be traced in their algal and aquatic arthropod relatives. The seductive argument that increasing O 2 levels in the atmosphere in the Siluro-Devonian were of great significance in permitting larger phototrophs (absence of restriction of plants to shaded habitats to avoid ultraviolet, and increased bulk of non-photosynthetic parts permitted by greater O 2 availability) and larger and more active phagotrophs (as a result of greater O 2 availability) is, alas, very difficult to test quantitatively.

Inorganic carbon acquisition by aquatic photolithoatrophs of the Dighty Burn, Angus, U.K.: uses and limitations of natural abundance measurements of carbon isotopes

The ¹³ C/¹² C ratio (expressed as δ¹³ C) of benrhic photolithotrophs. in the Dighn Water (= Burn) were measured fur comparison with that of the potential inorganic carhun sources. CO₂ and HCO₃ ^- , in the Burn. The Burn water contains an average of 65.7 mmol m^-3 CO₂ with δ¹³ C of -14.7% and 1600 mmol m^-3 HCO₃ ^- with δ¹³ C of -4.%. δ¹³ C values of riparian vegetation were also measured as contributors, after respiration in the soil or the Burn, to the δ¹³ C of inorganic carbon in the Burn. The potential range of differences in ¹³ C/1^2C between dissolved CO² and plant organic C is set by the intrinsic ¹³ c/¹² C discrimination (α value) in CO₂ fixation by Rubisco. Main results and conclusions are. as follows, (i) A literature survey suggests that there is no convincing evidence that the α, Values (rate constant for ¹² CO₂ fixation relative to that for ¹³ CO₂ fixation by Rubisco in the absence of CO₂ transport limitation) for the'lower plants'in the Burn (diatoms, green and red algae, mosses) are significantly different from the well-established α_p values for the flowering plum enzyme. (ii) In confirmation of earlier work, the semi-erect 'streamer'gametophytes of the red alga Lemanea mamillosa and the moss Fontinalis antipyetica have δ¹³ C values which can only be interpreted in terms of diffusive CO₂ entry with minimal limitation of photosynthesis by CO- diffusion, (iii) The serui-erect grren alga Cladophora glomerata and the flowering plant Ranunculus penicillatus ssp. pseudofluitons (formerly var. calcareus) are- both able to use HCO₃ ^- . Their δ¹³ C values indicate that, if the HCO₃ ^- -use system does not (as is likely) discriminate significantly between ¹³ C and ¹² C, then a substantial fraction of the inorganic C made available to Rubisco must return to the medium, carrying ¹³ C-inorganic C not fixed by Rubisco. (iv) Two sets of δ¹³ C data from different hydrodynamic regimes distance from leading edge of a flat stone; different size of thalli) show that the attainable differences in situ in thickness of the diffusion boundary layer do not alter the fractional limitation of photosynthesis of Cladophora by external diffusion of inorganic C, considered with HCO₃ use. (vi) The entrusting red alga Hildenbrandia rivularis has a δ¹³ C value suggestive of CO₂ as the inorganic C source, but not entirely ruling nut HCO₃ ^- . Marine species of both Hildenbrundia and Cladophora have δ¹³ C values which, even when corrected for source inorganic C δ¹³ C values, are 10%, more positive than the freshwater species. (vii) Mats of pennate diatoms were shown by pH-drift to by able to use HCO₃ ^- ; the relatively high (i.e. not very negative) δ¹² C value of these mats could relate to a relatively'non-leaky'HCO₃ ^- aequisition mechanism and/or to limitation by external diffusion (e.g. through the mat).

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

This paper relates the ¹³ C/¹² C ratio of C₃ plant material relative to that of source CO₂ to the N source for growth, the organic N content of the plant, and the extent of organic acid synthesis. The ¹³ C/¹² C ratio is quantified as Δ, defined as (δ¹³ C substrate -δ¹³ C product)/(1+δ¹³ C product), where δ¹³ C values of substrate or product (i.e. the samples) are defined as [¹³ C/¹² C]_sample ]/[(¹³ C/¹² C)_standard ]-1. The computation is performed by relating differences in plant composition as a function of N nutrition and acid synthesis to the fraction of plant C which is acquired via Rubisco and via other carboxylases. The fractional contribution of the different carboxylases to C gain is then related, using the known isotopic fractionations exhibited by these carboxylases, in a model to predict the final Δ of the plant (relative to atmospheric CO₂ ). Application of this approach to a 'typical' C₃ land plant yields predictions of the decrease of Δ relative to a hypothetical case in which all C is fixed via Rubisco. The predicted decreases range from 0-24 %, for NH₄ ⁺ assimilation (which always occurs in the roots) to 2-80%, for NO₃ ^- assimilation in shoots with the organic acid salt which results from acid-base balance, plus any additional organic acid salts plus free acids for a plant with a basal C:N molar ratio in organic material of 15. Intermediate values are predicted for symbiotic growth with N₂ , or where NO₃ ^- assimilation in root or shoot is accompanied by some acid-base regulation via OH- loss to the root medium. Comparison with published data on the difference in Δ of Ricinus communis cultured with NH₄ ⁺ or NO₃ ^- shows that the measured influence of nitrogen source is in the right direction (NO₃ ^- grown plants with a smaller Δ, i.e. a larger deviation from the value predicted for the absence of non-Rubisco carboxylations) to be explained by the observed difference in composition and hence fractional C contribution by the various carboxylases. However, the effect of N source on Δ is greater than that predicted by the model, i.e. a 2.1 % decrease as opposed to a 0.10 % decrease. It is likely that the major cause of the difference in δ¹³ C of the plants grown on the two N sources is a change in the ratio of transport and biochemical conductances of leaf photosynthesis. Such a change is quantitatively consistent with the lower water use efficiency of NH₄ ⁺ -grown plants. The predicted, and observed, changes in Δ as a function of N source are of the same magnitude as those found for C₃ terrestrial species grown at different temperatures or photon flux densities, or in environments yielding different water use efficiencies by changing root water supply relative to shoot evaporation potential. Variations in N source should be added to the factors which might alter δ of plants growing in the field.

Transport and assimilation of inorganic carbon by Lichina pygmaea under emersed and submersed conditions

Photosynthetic O₂ evolution by the upper littoral lichen, Lichina pygmaea (Lightf.) C.Ag., under light-saturated conditions at 5 °C is saturated by the 2 mol m^-3 inorganic C found in seawater at pH 8.0. Photosynthesis is not reduced when pH is increased to pH 9.4, and is slightly reduced at pH 10.0, when submersed in seawater with 2 mol m^-3 inorganic C. The rate of photosynthesis at pH 10 greatly exceeds the rate of uncatalysed conversion of HCO₃ ^- . It is concluded that HCO₃ ^- is used in photosynthesis. Since extracellular carbonic anhydrase is present, it is possible that CO₂ enters the photobiont (Calothrix) cells even during HCO₃ use. pH drift experiments support the notion of HCO₃ ^- use. Emersed photosynthesis at 5 °C is more than half-saturated by 35 Pa (normal atmospheric) CO₂ ; the light- and CO₂ -saturated emersed photosynthetic rate is not significantly different from the light and inorganic C-saturated photosynthetic rate when submersed. Inorganic C diffusion from the thallus surface to the photobiont needs, at least under some conditions, carbonic anhydrase activity which permits HCO₃ ^- fluxes to supplement CO₂ movement. The CO₂ compensation partial pressure at 5 °C is 0.83 Pa, i.e. at the low range of values found for terrestrial cyanobacterial lichens. Dark ¹⁴ C-inorganic C assimilation when submersed is a small fraction of the dark respiratory rate, consistent with the observed absence of diel CAM-like variation in intracellular titratable acidity. The high value (-11.5%) of δ¹³ C, the low CO₂ compensation partial pressure, and the relatively high affinity for inorganic C., are consistent with the operation of an inorganic C concentrating mechanism such as occurs in free-living cyanobacteria and probably occurs in terrestrial cyanobacterial lichens and in most intertidal algae.

The role of CO2 uptake by roots and CAM in acquisition of inorganic C by plants of the isoetid life-form: a review, with new data on Eriocaulon decangulare L

The isoetid life-form was originally defined on morphological grounds; subsequent physiological investigations showed that all of the isoetids examined took up a large fraction of the inorganic C fixed in their leaves from the root medium under natural conditions, and that some of them carried out much of their assimilation of inorganic C via a CAM-like mechanism. Root-dominated uptake of inorganic C appeared to be unique to, and ubiquitous in, the isoetids. I However, a large capacity for CAM-like metabolism in submerged vascular plants is not universal in isoetids, nor is it restricted to this life-form, being also found in Crassulaa aquatica. The work described here shows that submerged specimens of the North American Eriocaulon decangulare have a high fraction of their dry weight in the root system, a trait characteristic of isoetids but uncommon in other submerged vascular plants. E. decangulare has vesicular-arbuscular mycorrhizas, as do other flowering plant isoetids hut not, generally, submerged Isoetes spp. Under conditions of natural supply of inorganic C, E. decangulare, like other isoetids, takes up most of its inorganic C through its roots. Uptake of inorganic C by both roots and shoots involves CO₂ rather than HCO₃ : photosynthesis at high external pH values does not exceed the rate of uncatalysed HCO₃ ^- to CO₂ conversion in the medium and there is no detectable extracellular carbonic anhydrase activity. Measurements of titratable acidity and of malate content of leaves sampled at dawn and at dusk showed that E. decangulare, growing and tested under either emersed or submersed conditions, did not exhibit CAM-like behaviour. CAM was also absent from three non-isoetid aquatic macrophytes (Amphibolic antarctica, Eeklonia radiata and Vallisneria spiralis) which were examined. E. decangulare thus resembles all other isoetids tested in acquiring much of its inorganic C via the root system. E. decangulare also resembles most of the isoetids which are not members of the Isoetaceae (e.g.) E. septangulare, Lobelia dortmanna and Subularia aquatica) but differs from submerged Isoetaceae and Littorella uniflora in lacking CAM. The ecological significance of uptake of CO₂ via the roots and, where it occurs, of CAM in isoetids may be related to either inorganic C or, via improved N use efficiency, inorganic C as a limiting resource. The isoetid life-forms has evolved independently in at last five different families of vascular plants; it probably derived fairly immediately from terrestrial or amphibious ancestors with a similar rosette form. Emergent Isoetaceae with acquisition of CO₂ via roots and CAM probably evolved from submerged isoetids. CONTENTS Summary 123 I. Introduction 126 II. Material and Methods 127 III. Results and Discussion 129 IV. Conclusions 142 Acknowledgements 142 References 143.

Photosynthetic pathways in the Bromeliaceae of Trinidad: relations between life-forms, habitat preference and the occurrence of CAM

An investigation was carried out into the photosynthetic pathways of the complete bromeliad flora of Trinidad (West Indies). Carbon-isotope ratios (δ¹³C values) were used to distinguish obligate C₃ and crassulacean acid metabolism (CAM) species. Measurements were also carried out on some species in the field to test for day-night changes in leaf titratable acidity.A wide range of δ¹³C values was found. The obligate CAM species had values of -10 to -20‰ and the obligate C₃ species of -23 to -35‰ CAM was found (a) in the majority of Tillandsia spp. (Tillandsioideae) and (b) in all species of Bromelioideae. The other genera of the Tillandsioideae appeared to be at least predominantly C₃. One species, Guzmania monostachia var. monostachia, was identified as a C₃-CAM intermediate, and others may well exist in the Trinidad flora. The influence of factors such as source CO₂, photosynthetic photon flux density and ambient humidity in determining the δ¹³C values is discussed.The taxonomic distribution of C₃ and CAM species within the Bromeliaceae is analyzed in terms of the life-forms and ecological types recognized by Pittendrigh (1948). The most xerophytic species (the light-demanding "atmospherics") all show CAM and are restricted to the drier parts of the island. Most of the species with waterstoring "tanks" have a wide geographic distribution: these include light-demanding C₃ plants and less light-demanding CAM plants. The shade-tolerant bromeliads, which show a requirement for high ambient humidity, are all C₃ plants. We discuss the phylogenetic origins of CAM and the epiphytic habit in the Bromeliaceae.