I had got the foresaid water taken out of the ditches and runnels on the 30th of August: and on coming home, while I was busy looking at the multifarious very little animalcules a-swimming in this water, I saw floating in it, and seeming to move of themselves, a great many green round particles, of the bigness of sand grains.

When I brought these little bodies before the microscope, I saw that they were not simply round, but that their outermost membrane was everywhere beset with many little projecting particles … all orderly arranged and at equal distances from one another; so that upon so small a body there did stand a full two thousand of the said projecting particles.

This was for me a pleasant sight, because the little bodies aforesaid, how oftsoever I looked upon them, never lay still; and because their progression was brought about by a rolling motion. …

Each of these little bodies had enclosed within it 5, 6, 7, nay, some even 12, very little round globules, in structure like to the body itself wherein they were contained.

While I was keeping watch, for a good time, on one of the biggest round bodies … I noticed that in its outermost part an opening appeared, out of which one of the inclosed round globules, having a fine green colour, dropt out, and took on the same motion in the water as the body out of which it came. … soon after a second globule, and presently a third, dropt out of it; and so one after another till they were all out, and each took on its proper motion. …

The Volvocales discussed in that quotation are a subset of the green algae, or Chlorophyta, the most diverse and ubiquitous group of modern eukaryotic algae. Chlorophytes are characterized by a number of shared features, such as chloroplasts that contain chlorophylls a and b and are enclosed in a double membrane, the presence of certain characteristic carotenoids and xanthophylls, the ability to store excess photosynthate as starch, and so forth (Pickett-Heaps 1975; Mattox & Stewart 1984; Bold & Wynne 1985; Melkonian 1990). Many of those features are shared, of course, by a large and conspicuous set of descendants of the green algae: the vascular plants. To put the origins of Volvox in context, it will be useful to review briefly some of the major evolutionary trends that are evident when the green algae are viewed in a broad framework.

In the late nineteenth century, “development” and “evolution” were considered to be so intimately related that many biologists used the terms interchangeably, applying them rather indiscriminately to both the process that generates a new individual resembling its parents and the process that generates a new species different from its ancestors. For most twentieth-century biologists, continuation of that practice would have been unthinkable, because evolution and development have seemed to us to be such fundamentally different phenomena. However, history has a way of repeating itself. After a century-long estrangement that began with widespread rejection of Haeckel's dogma that “ontogeny recapitulates phylogeny,” evolution and development are now undergoing a dramatic rapprochement, with genetics acting as the broker for their remarriage. Increasingly, those investigating the mechanisms by which differentiated cells and organs arise in the course of embryonic development and those seeking to understand how morphological novelties arise in the course of evolution find themselves converging on the study of related sets of genes, and talking to one another again!

Contemporary studies of Volvox, the rolling green spheroid that first fascinated Antoni van Leeuwenhoek 300 years ago, illustrate this sort of convergence. When my wife, Marilyn, and I began to study Volvox more than 20 years ago, our objective was to capitalize on its simplicity to address a central problem of development that we had found it extremely difficult to address with any clarity by studying vertebrate embryos: How do cells with very different phenotypes arise from the progeny of a single cell?

In all probability the first objects on this planet worthy of the term “living” were single cells. At present, after nearly 4 billion years of intense competition among millions of kinds of life forms, the vast majority of individuals inhabiting this planet are still unicellular. This is proof enough of the survival value of a unicellular body plan. Nevertheless, all of the conspicuous organisms on the land, in the waters, and in the air above our planet are multicellular organisms with an impressive diversity of differentiated cell types that share the labors of resource accumulation and reproduction in a highly integrated and effective manner.

These conspicuous and highly successful multicellular organisms, with their differentiated cell types and division of labor, have provided biologists some of their most intriguing and persistent puzzles: From what sorts of unicellular ancestors – and why, and when, and how – did multicellular organisms with differentiated cells arise? Such questions tend to fall into two categories: What were the ultimate (e.g., ecological) causes that fostered the evolution of multicellular organisms? What were the proximate (e.g., genetic and cytological) causes that permitted their evolution?

Shunning the intellectual division of labor that has too often characterized biology in this century, in this book we will attempt to examine both the ultimate causes and the proximate causes of the ontogeny and phylogeny of one elegantly simple and beautiful example of multicellularity and cellular differentiation: Volvox, “the fierce roller.”

SUMMARY

Seeds were collected from populations of Boehmeria cylindrica growing in naturally enriched concentrations of CO2. In the controlled environment plants grown from populations which are normally exposed to enriched atmospheres of CO2 (400 to 575 ppmv) show a greater CO2-sensitivity of growth than plants from ambient (370 ppmv) sites. Observed differences in height growth and dry weight partitioning could be explained in terms of the different rates of plant growth under the imposed treatments, rather than in terms of population differences in growth partitioning. Comparisons of stomatal density-CO2 responses for species from different climates and over the different time scales of centuries and millennia indicate a similar decline in density with increasing CO2 concentrations, with no evidence of any changes in the CO2 sensitivity. However, the clear species-specific nature of the stomatal density response suggests it is unlikely to be observed ubiquitously in plants growing by high-CO2 springs. Nevertheless, there is a close similarity between the stomatal density responses shown by plants growing under CO2 enrichment adjacent to springs, and the stomatal density changes recorded from fossil leaves, indicating one valuable use of high-CO2 springs in biological research.

INTRODUCTION

Measurements of the atmospheric concentration of CO2, typically from air locked up in permanent ice caps, provide time courses for a continuously varying CO2 concentration over the last 160,000 years (Barnola et al., 1987). Estimates of CO2 by indirect means also show variations of CO2 concentration over millions of years (Hays, Imbrie & Shackleton, 1976; Berner, 1990).

An interesting consequence of these variations is the potential for a CO2-controlled selection in higher plants.

SUMMARY

In the Burning Hills and Smoky Mountains of southern Utah (USA), coal deposits exposed to the surface have been ignited by lightning and have been burning for periods of years to over a century. We examined one of these sites, where the below-ground combustion of this low-sulfur coal releases gases to the atmosphere from vents above the burning seam. The surrounding vegetation is cold-desert shrub, typical of the region and consisted of both C3 and C4 perennial species.

Additionally, at least one weedy C4 species had invaded disturbed locations immediately adjacent to the active vent area. Atmospheric CO2 concentrations in the vicinity of the vents fluctuated significantly, however, CO2 concentrations measured approximately 500 m from the most active vents were 7 ppm elevated above ambient concentrations measured at a control site 10 km from the burning vents. CO2 concentrations at sites nearer the vents, but still with natural, undisturbed vegetation, were elevated 65 ppm above ambient background values. At vegetated sites nearest the vents, CO2 concentrations were elevated by an average of 542 ppm above ambient values.

The continuous distribution of C4 vegetation along the CO2 concentration gradient provides a means of estimating the long-term integrated CO2 concentrations at each location. Using the carbon isotope ratio of the C4 vegetation (Atriplex confertifolia and Salsola iberica) to estimate the atmospheric CO2 concentration, we observed that the ratio of intercellular to atmospheric CO2 concentrations of C3 vegetation decreased in response to elevated CO2 concentrations. This decreased ratio for Gutierrezia sarothrae (C3) was sufficient to result in a predicted doubling of water-use efficiency.

SUMMARY

The Bossoleto site in Tuscany, central Italy has been the location of a two year research programme investigating the long-term effects of enhanced CO2 concentrations on natural vegetation. Research has investigated the effects of CO2 on leaf gas exchange on a grass Phragmites australis (Cav.) Trin. ex Steudel and the tree species Quercus pubescens Willd. Preliminary results for P. australis demonstrate reductions in stomatal density, stomatal conductance, and maximum photosynthetic rates when compared with nearby control sites. In contrast, work with Q. pubescens showed no evidence of photosynthetic acclimation. The responses observed in plants growing at the Bossoleto site are consistent with results from short-term experiments, providing some confidence in this approach. The characteristics of CO2 springs can add to the challenge of designing effective experiments. It is difficult to locate control sites that have similar vegetation, soil type and environmental characteristics. When working with natural vegetation, variability in characteristics within and between populations, can exceed any response to CO2. Atmospheric concentrations can vary at time scales ranging from the order of seconds through to seasons. High CO2 concentrations at some sites may be associated with pollutants that can affect plant growth. These potential problems influence the possible types of experimental approaches. This paper uses preliminary analysis of leaf gas exchange data from the Bossoleto site, to examine how natural sources of CO2 enrichment can be used in biological research.

INTRODUCTION

How can natural sources of CO2 be used in biological research? The likely effects of enhanced atmospheric CO2 concentrations on vegetation have been the subject of intensive research for over a decade.

SUMMARY

Exhalation flux of CO2 was measured by the accumulation method within a sedimentary basin in central Italy (Siena Basin). The area is characterized by underground pressurized gas linked to low enthalpy geothermal systems. The exhalation level of CO2 appeared to be controlled by the fracturing (i.e. tectonization) degree of subsoil and was consistent with the distribution of high concentrations of CO2, Rn and He in the soil air. The CO2 flux from the whole area (about 200 km2) would be of the order of 10 Mt y-1, about one order of magnitude higher than the mean flux normally related to the soil respiration. This excess flux has clearly an endogenous component, related to underground gas domains and deep faults, acting as preferential pathways for outgassing. The results show how geologic factors influence the CO2 level at the surface and how exhalation surveys, by identifying invisible surface CO2 leakage, are useful for environmental characterization.

INTRODUCTION

High flows of CO2 from subsurface are generally associated with volcanic activity or hydrothermal circulation. In these conditions the CO2 discharge at the surface is commonly evidenced by punctual manifestations such as gas vents. In these cases it is easy to sample the gas-phase and to study its variation over time or its effects on the surrounding environment. However, in most cases CO2 leakage does not produce visible effects. Even in non-volcanic areas high CO2 leakage in soil may be continuous or intermittent and generally pervasive (e.g. Hermansson et al., 1991; Etiope & Lombardi, 1995), bearing profound effects on the level of the biological activity in large areas.

SUMMARY

Rising atmospheric CO2 levels, by increasing the pCO2/pO2 ratio at the sites of photoreduction, could reduce the basal rate of O2 activation and oxyradicals formation in plant tissues, thus alleviating the risk of oxidative stress of abiotic origin. Natural CO2 springs of geothermal origin offer the opportunity of testing this hypothesis on both crop species and natural plant communities under otherwise undisturbed field conditions. In the present work, the contents of the major antioxidant enzymes and metabolites, indexes of oxidative damage to polyunsaturated fatty acids, and the level of one of the most aggressive reactive oxygen species, namely hydroxyl radical, were measured in foliar whole extracts obtained from wheat (Triticum aestivum L. cv. Mercia) plants grown in the natural high-CO2 environment provided by a geothermal site in central Italy. The results obtained, also examined in the light of previous work on soybean (Glycine max Merrill cv. Cresir) plants grown under comparable high-CO2 conditions (Badiani et al., 1993), partly support the view that high CO2 could reduce the risk of chloroplastic oxygen toxicity. However, it appears that the alleviation of the oxidative risk could vary not only with the plant species considered but could also depend on the CO2 enrichment regime the plants are exposed to; species-specific detrimental effects on the prooxidant/antioxidant equilibrium could stem from the progressive suppression of energy-dissipating processes, such as photorespiration, under conditions of increasing CO2.

SUMMARY

The continuing rise of CO2 concentration in the atmosphere since the start of industrialisation, and the associated global warming, pose interesting questions about the response of plants to CO2. However, in the course of evolution, plants have been exposed to a much wider range of CO2 concentrations than we have seen in the last 200 years. It is likely that when photosynthesis evolved, around 3.8 billion years ago, the earth's atmosphere was CO2 rich, just as Mars and Venus are today. In order to understand the adaptability of plants to changing CO2, there are several possible experimental approaches. The presence of CO2 springs in several parts of the world provides a natural laboratory for such studies, with the important advantage that plants growing at such sites will have been exposed to elevated CO2 for many generations.

The CO2 concentration of the earth's atmosphere when photosynthesis began, some 3.8 × 109 years ago, is presumed to have been in the range 90–98%, similar to that found currently on the lifeless sister planets Mars and Venus (Emiliani, 1992; Raven, 1995). This high CO2 atmosphere originated from the outgassing of the planet's crust, which continues today albeit at a diminished rate in volcanic regions of the world.

Photosynthesis in those early days of the earth's history is believed to have been achieved by prokaryote filamentous cyanobacteria, forming crusts as seen now at coastal areas in warm parts of the world. The rates of photosynthesis achieved by these organisms may have been quite high, as a result of the large diffusion gradient from the atmosphere to the sites of carboxylation.

SUMMARY

The results of a study on the composition, the ecology and the structure of grass communities developed close around six mineral CO2-springs in central-western Italy are reported. The phytosociological sampling of the grasslands surrounding the gas vents has led to the circumscription of an azonal endemic association, the Agrostidetum caninae subsp. monteluccii, which was characterized by a strong species poorness and a high ecological specialization. The typical monospecific stands were developed on peatlike soils with pH ranging between 2.4 and 3.7 and a content of soluble aluminium well above the toxicity thresholds known within the Al-tolerant gen. Agrostis. On the basis of published data and preliminary measurements of atmospheric CO2 concentration, it is known that this association is developed in areas with elevated levels of atmospheric CO2, a factor which could contribute to enhancing the competitive capacity of its dominant species within the natural vegetation. Indeed, above-ground biomass, canopy height and other structural parameters of A. canina revealed its outstanding vegetative vigour in the monospecific grassland close around the vents. On the contrary, in the peripheral areas of the CO2 springs a decreased vigour in A. canina accounted for the different structure and density observed in the grass community and probably also for its different species composition. The syntaxonomical placing of the Agrostidetum is not immediate because of the absence in the European literature of other works on geothermal acidophilic vegetation. Therefore, despite some ecological affinities with oligotrophic species-poor communities of European acidic fens and mires of the Scheuchzerio-Caricetea fuscae, the syntaxonomical placing requires further coenological investigation of geothermal sites on a broader geographical scale.

SUMMARY

The effect of CO2 and light on NH4+ assimilation by Cyanidium caldarium, a thermophilic acidophilic unicellular alga isolated in volcanic areas in Yellowstone National Park, USA, was investigated. N-sufficient cells assimilated NH4+ at a rate of 189 μ 4.76 umol ml-1 packed cell volume (pcv) h-1. Removal of CO2 or darkening almost immediately prevented NH4+ assimilation. N-limited cells in light assimilated NH4+ at the rate of 498 ± 8.05 pmol ml-1 pcv h-1 in the presence of CO2. In darkness they assimilated NH4+ at a rate of 303 ± 1.5 μmol ml-1 pcv h-1 in the presence of CO2, which was as high as 60% of assimilation in the light, and at a similar rate in the absence of CO2. However, after 40 min under the latter conditions, assimilation underwent a time dependent inhibition and ceased after 70 min; it was resumed upon resupply of CO2. In the absence of CO2 in light, NH4+ was assimilated at a considerably lower rate than in darkness, which supports the idea that, under CO2-free conditions, a light-dependent inhibition of NH4+ assimilation occurred. These results are consistent with the contention that cells of C. caldarium, grown under excess NH+4, obtain carbon skeletons for NH+4 assimilation exclusively by photosynthetic reactions, thereby the light and CO2 dependence. Cells grown under conditions of N-limitation possess the ability to obtain a consistent amount of additional carbon skeletons from mobilization of carbon reserves.

SUMMARY

A preliminary survey for CO2-rich mineral springs around the area of Girona (NE Spain) was conducted having in mind their possible use for biological research. At present, the survey has produced only two mineral springs that create a CO2 enriched atmosphere and that have natural vegetation around them. One of them (Sant Hilari) was further studied in order to discover (1) if the CO2 degassed from a spring has a different 13C and 14C signature from that of atmospheric CO2; (2) if plants growing in the surroundings of the spring have a distinctive C isotopic composition from those growing under a normal CO2 atmosphere; and (3) if it would be possible to back-estimate the mean CO2 concentration that plants have experienced during their lifetime, from considering plant C isotopic content as a mixture of two end-members, air CO2 and spring-derived CO2.

Results showed that: - Dissolved inorganic carbon of springwater was slightly enriched in 13C (-1.6%∘ PDB) and very depleted in 14C (3.15% of modern carbon) compared to atmospheric CO2 (-8.0%∘ PDB 13C and 100% 14C).

- The isotopic signal of 14C of CO2 spring was detected in plant materials but not that of 13C. This result corresponds to the much higher difference between the two end-members (spring and atmospheric CO2) compositions for 14C (near 100%) than for 13C(only 6.4%∘).

- Consequently, it is possible to estimate from 14C, but not from 13C, analysis that the mean CO2 concentration that Angelica sylvestris (Apiaceae) plants growing around the spring have experienced during their lifetime was 405 ppmv.

SUMMARY

The CO2-emitting mineral springs of Iceland have been surveyed and assessed for their use in understanding the potential effects of elevated atmospheric CO2 on northern ecosystems. One spring near Olafsvik is described in detail here. This CO2 spring emits CO2 in such a way that the surrounding vegetation is exposed to mean CO2 concentrations which are consistent with those predicted for the next century. Results from carbon isotope analyses show that Nardus stricta plants growing near this CO2 spring are exposed to mean CO2 concentrations of approximately 880, 650 and 430 ppm, depending on their location relative to the CO2 vent. Maps and tables are provided to show how the concentration of CO2 measured in the vegetation varies over time on calm and windy days. A list of the species growing around the Olafsvik CO2 spring is included and the research potential of CO2 springs is discussed at length. The use of natural CO2 springs as surrogates for elevated CO2 experiments appears highly promising.

INTRODUCTION

The concentration of carbon dioxide in the atmosphere is increasing due to the burning of fossil fuels and deforestation. How plants evolve in response to elevated CO2 and global climate change will ultimately affect many of the important responses of the biosphere including primary productivity, trace gas flux, and vegetation boundaries.

SUMMARY

The objects of the present study were:

1. 1) to investigate the stomatal morphology of leaves of holm oak trees grown in a naturally CO2-enriched environment;

2. 2) to compare it with the stomatal density of leaves of holm oak seedlings grown in an artificially CO2-enriched environment.

Among the stomatal morphology parameters we analysed, the only significant alteration we observed in trees which had grown by the CO2 spring was a reduction in stomatal density.

The rather special response of one tree might be related to the rock on which it stands, which may be causing the tree severe water stress. However, the reduction did not increase significantly as the concentrations of CO2 increased.

The experiment in artificially CO2-enriched atmospheres indicated that the stomatal density of holm oak leaves was reduced by an increase in CO2. The results are discussed with an analysis of herbarium holm oak leaves.

INTRODUCTION

The effect on stomatal density of the increase of atmospheric CO2 is not yet fully clarified, although some experiments exposing plants to elevated CO2 regimes in small scale chambers and field enclosures indicate that stomatal density decreases with increasing CO2 concentrations (Madsen, 1973; O'Leary & Knetch, 1981; Thomas & Harvey, 1983; Imai, Coleman & Yanagisawa, 1984; Woodward, 1987), despite contradictory responses which were sometimes observed.

Criticisms have been levelled at this kind of experiment because of poor balance between energy supply and water loss from leaves (Morison, 1987). Conversely, natural vegetation is well coupled with the atmosphere, in terms of both its energy budget and its environmental feedback mechanisms.

SUMMARY

Gas vent areas, where CO2 of deep origin is naturally released into the atmosphere, provide a valuable opportunity to study long-term effects of CO2 enrichment on natural vegetation. A study of adaptive traits in relation to elevated CO2 was carried out on genetically isolated populations of Scirpus lacustris, a rhizomatous emergent wetland sedge growing at several CO2 springs in Italy. Plants were grown in the laboratory from rhizomes collected in CO2 springs and control sites and their photosynthetic capacities were compared using gas-exchange techniques. Carbon isotope discrimination, nitrogen content and stomatal density were then measured in the field on plants of the same species growing along a transect traced from gas vents outwards. Photosynthetic rates of plants grown in the laboratory from CO2 springs were not significantly different from those of control plants. The absence of decline in photosynthetic capacity with increasing external CO2 concentration was supported by results from plants grown along the CO2 gradient in the field. They showed no change in nitrogen content and in Ci/Ca ratio, and exhibited a downward regulation of stomatal density. The possible role of wetland ecosystems as sinks for atmospheric CO2 under greenhouse climate conditions is discussed.

INTRODUCTION

Short-term exposure to elevated CO2 concentration is known to cause an increase in the photosynthetic rate in most C3 species, whereas long-term growth under elevated CO2 concentration often results in a downward regulation of photosynthetic capacity (Sage, Sharkey & Seemann, 1989; Arp, 1991).

SUMMARY

The impacts of elevated atmospheric CO2 concentration on decomposition processes are reviewed, with consideration given to both the direct and indirect effects which CO2 may exert on soil processes.

Present work indicates that elevated concentrations of CO2 affect litter decomposition through changes in litter quality.

In particular, elevated CO2 decreases litter N concentrations with a resulting increase in C/N and lignin/N ratios, leading to a slow down in litter decomposition rates.

Such observations are true for both leaf and root litters, and for laboratory as well as field incubations. CO2 treatment does not appear to exert any effect on litter decay rates when there are no measurable changes in leaf chemical composition.

Furthermore, it has been identified that elevated CO2 is unlikely to exert any direct effects on soil biological activity, and that only effects mediated via changes in plants are likely to occur.

Although there is currently debate about the effect which increased C inputs to soils resulting from elevated CO2 may have on the turnover rates of existing soil organic matter, the overall conclusion is that soil C stores are increasing, and will continue to increase, under higher CO2 concentrations.

INTRODUCTION

If accurate predictions of the influence of elevated CO2 on terrestrial carbon (C) balance and on the global C cycle are to be made, then more information is needed about the development of above- and below-ground C stores under different atmospheric CO2 concentrations. The extent to which terrestrial ecosystems are able to ‘buffer’ against rising atmospheric CO2 concentrations by storing more or less C needs to be assessed, together with the changes which are currently occurring.