Lucas J. Stal

Bio: Lucas J. Stal is an academic researcher from University of Amsterdam. The author has contributed to research in topics: Cyanobacteria & Nitrogenase. The author has an hindex of 61, co-authored 189 publications receiving 12708 citations. Previous affiliations of Lucas J. Stal include Utrecht University & Royal Netherlands Academy of Arts and Sciences.

Dinitrogen fixation in the world's oceans

Abstract: The surface water of the marine environment has traditionally been viewed as a nitrogen (N) limited habitat, and this has guided the development of conceptual biogeochemical models focusing largely on the reservoir of nitrate as the critical source of N to sustain primary productivity. However, selected groups of Bacteria, including cyanobacteria, and Archaea can utilize dinitrogen (N2) as an alternative N source. In the marine environment, these microorganisms can have profound effects on net community production processes and can impact the coupling of C-N-P cycles as well as the net oceanic sequestration of atmospheric carbon dioxide. As one component of an integrated ‘Nitrogen Transport and Transformations’ project, we have begun to re-assess our understanding of (1) the biotic sources and rates of N2 fixation in the world’s oceans, (2) the major controls on rates of oceanic N2 fixation, (3) the significance of this N2 fixation for the global carbon cycle and (4) the role of human activities in the alteration of oceanic N2 fixation. Preliminary results indicate that rates of N2 fixation, especially in subtropical and tropical open ocean habitats, have a major role in the global marine N budget. Iron (Fe) bioavailability appears to be an important control and is, therefore, critical in extrapolation to global rates of N2 fixation. Anthropogenic perturbations may alter N2 fixation in coastal environments through habitat destruction and eutrophication, and open ocean N2 fixation may be enhanced by warming and increased stratification of the upper water column. Global anthropogenic and climatic changes may also affect N2 fixation rates, for example by altering dust inputs (i.e. Fe) or by expansion of subtropical boundaries. Some recent estimates of global ocean N2 fixation are in the range of 100−200 Tg N (1−2 × 1014 g N) yr −1, but have large uncertainties. These estimates are nearly an order of magnitude greater than historical, pre-1980 estimates, but approach modern estimates of oceanic denitrification.

N2 Fixation by non-heterocystous cyanobacteria

Abstract: Many, though not all, non-heterocystous cyanobacteria can fix N2. However, very few strains can fix N2 aerobically. Nevertheless, these organisms may make a substantial contribution to the global nitrogen cycle. In this general review, N2 fixation by laboratory cultures and natural populations of non-heterocystous cyanobacteria is considered. The properties and subcellular location of nitrogenase in these organisms is described, as is the response of N2 fixation to environmental factors such as fixed nitrogen, O2 and the pattern of illumination. The integration of N2 fixation with other aspects of cell metabolism (in particular photosynthesis) is also discussed. Similarities and differences between different individual strains of non-heterocystous cyanobacteria are highlighted.

Physiological ecology of cyanobacteria in microbial mats and other communities

Abstract: In this review some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example. The majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats. These micro-scale ecosystems are characterized by steep and fluctuating physico-chemical gradients of which those of light, oxygen and sulphide are the most conspicuous. Light is strongly attenuated in the sediment, and owing to constant sedimentation, the mat-forming cyanobacteria have to move upwards towards the light. However, at the sediment surface, light intensity, particularly in the u.v. part of the spectrum, is often deleterious. The gliding movement of the cyanobacteria, with photo- and chemotaxis, allows the organism to position itself in a thin layer at optimal conditions. The organic matter produced by cyanobacterial photosynthesis is decomposed by the ruicrobial community. Sulphate-reducing bacteria are important in the end-oxidation of the organic matter. These organisms are obligate anaerobes and produce sulphide. Gradients of sulphide and oxygen move up and down in the sediment as a response to diurnal variations of light intensity. Cyanobacteria, therefore, are sometimes exposed to large concentrations of the extremely toxic sulphide. Some species are capable of sulphide-dependent anoxygenic photosynthesis. Other cyanobacteria show increased rates of oxygenic photosynthesis in the presence of sulphide and have mechanisms to oxidize sulphide while avoiding sulphide toxicity. Iron might play an important role in this process. Under anoxic conditions in the dark, mat-forming cyanobacteria switch to fermentative metabolism. Many species are also capable of fermentative reduction of elemental sulphur to sulphide. The gradients of sulphide and oxygen are of particular importance for nitrogen fixation. Very few microbial mats are formed by heterocystous cyanobacteria, which are best adapted to diazntrophic growth. However, these organisms probably cannot tolerate greater concentrations of sulphide or anoxic conditions or both. Under such conditions non-heterocystous cyanobacteria become dominant as diazotrophs. These organisms avoid conditions of oxygen supersaturation. In the ecosystem, nitrogen fixation and photosynthesis might be separated temporally as well as spatially. In addition, non-heterocystous diazotrophic cyanobacteria have mechanisms at the subcellular level to protect the oxygen-sensitive nitrogenase from inaction. CONTENTS Summary 1 I. Introduction 2 II. Microbial mats 3 III. Cyanobacteria in light gradients 7 IV. Dark metabolism 10 V. Interactions with sulphide 13 VI. Nitrogen fixation 16 VII. References 28.

Cyanobacterial Mats and Stromatolites

Abstract: Cyanobacteria are often the key organisms comprising microbial mats. They form dense micrometer-scale communities in which the full plethora of microbial metabolism can be present. Such mats are therefore excellent model systems and because of their analogy with Precambrian stromatolites they are also attractive subjects for evolutionary studies. Growth and metabolism of the oxygenic phototrophic cyanobacteria enrich the sediment with organic matter. However, in mature mats net growth of cyanobacteria appears to be of less importance. Most of the organic matter produced from photosynthetic CO2 fixation is liberated in the sediment by one of the following: fermentation, photorespiration, pouring out of solutes or secretion of mucus although grazing may also be important. This organic matter is degraded by chemotrophic microorganisms, among which sulphate-reducing bacteria are particularly prominent. The combined activities of the cyanobacteria and sulphate-reducing bacteria result in steep and fluctuating gradients of sulphide and oxygen. Cyanobacteria therefore have to cope with high concentrations of sulphide, oxygen supersaturated – and anoxic conditions. These physicochemical gradients force different functional groups of microorganisms to particular vertical stratified positions in the mat. This, and the fact that accretion of sediment fluctuates, gives rise to one of the most conspicuous properties of microbial mats namely their laminated structure. Modern microbial mats have this laminated structure in common with Precambrian stromatolites. Most modern mats do not lithify but this may also have been the case for Archean microbial mats. Only a few examples of modern calcifying stromatolithic microbial mats are known. A hypothesis has been developed which conceives a role for extracellular polysaccharides in calcification. Extracellular polysaccharides in cyanobacterial mats are often produced as the result of unbalanced growth caused by nitrogen deficiency. The mat organisms are embedded in the extensive polysaccharide matrix that inhibits calcification. All cyanobacterial mats can fix atmospheric dinitrogen, which covers part of their nitrogen demand, but the fluctuating physicochemical gradients limits the efficiency of this process.

The biofilm matrix

Abstract: The microorganisms in biofilms live in a self-produced matrix of hydrated extracellular polymeric substances (EPS) that form their immediate environment. EPS are mainly polysaccharides, proteins, nucleic acids and lipids; they provide the mechanical stability of biofilms, mediate their adhesion to surfaces and form a cohesive, three-dimensional polymer network that interconnects and transiently immobilizes biofilm cells. In addition, the biofilm matrix acts as an external digestive system by keeping extracellular enzymes close to the cells, enabling them to metabolize dissolved, colloidal and solid biopolymers. Here we describe the functions, properties and constituents of the EPS matrix that make biofilms the most successful forms of life on earth.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Abstract: The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Nitrogen cycles: past, present, and future

Abstract: This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially dis- connected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N2 by denitrification.