Gas treatment in trickle-bed biofilters: biomass, how much is enough?

The objective of this article is to define and validate a mathematical model that desribes the physical and biological processes occurring in a trickle-bed air biofilter for waste gas treatment. This model considers a two-phase system, quasi-steady-state processes, uniform bacterial population, and one limiting substrate. The variation of the specific surface area with bacterial growth is included in the model, and its effect on the biofilter performance is analyzed. This analysis leads to the conclusion that excessive accumulation of biomass in the reactor has a negative effect on contaminant removal efficiency. To solve this problem, excess biomass is removed via full media fluidization and backwashing of the biofilter. The backwashing technique is also incorporated in the model as a process variable. Experimental data from the biodegradation of toluene in a pilot system with four packed-bed reactors are used to validate the model. Once the model is calibrated with the estimation of the unknown parameters of the system, it is used to simulate the biofilter performance for different operating conditions. Model predictions are found to be in agreement with experimental data. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 583-594, 1997.     

Hsp70 and larval thermotolerance in Drosophila melanogaster: how much is enough and when is more too much?

Heat shock proteins (Hsps) and other molecular chaperones perform diverse cellular roles (e.g., inducible thermotolerance) whose functional consequences are concentration dependent. We manipulated Hsp70 concentration quantitatively in intact larvae of Drosophila melanogaster to examine its effect on survival, developmental time and tissue damage after heat shock. Larvae of an extra-copy strain, which has 22 hsp70 copies, produced Hsp70 more rapidly and to higher concentrations than larvae of a control strain, which has the wild-type 10 copies of the gene. Increasing the magnitude and duration of pretreatment increased Hsp70 concentrations, improved tolerance of more severe stress, and reduced delays in development. Pretreatment, however, did not protect against acute tissue damage. For larvae provided a brief or mild intensity pretreatment, faster expression of Hsp70 in the extra-copy strain improved survival to adult and reduced tissue damage 21h after heat shock. Negative effects on survival ensued in extra-copy larvae pretreated most intensely, but their overexpression of Hsp70 did not increase tissue damage. Because rapid expression to yield a low Hsp70 concentration benefits larvae but overexpression harms them, natural selection may balance benefits and costs of high and low expression levels in natural populations.     

Phylogenetic inference: how much evolutionary history is knowable?

In order to reconstruct phylogenetic trees from extremely dissimilar sequences it is necessary to estimate accurately the extent of sequence divergence. In this paper a new method of sequence analysis, Markov triple analysis, is developed for determining the relative frequencies of nucleotide substitutions within the three branches of a three-taxon dendrogram. Assuming that nucleotide sites are independently and identically distributed and assuming a Markov model for nucleotide (or protein) evolution, it is shown that the unique Markov matrices can be reconstructed given only the joint probability distribution relating three taxa. (In the much simpler case involving only two taxa and two character states, Markov matrices can also be reconstructed, provided symmetry assumptions are placed on the elements of the matrices.) The method is illustrated using sequence data from the combined first and second codon positions derived from complete human, mouse, and cow mitochondrial sequences.      

Collagen-induced arthritis and related animal models: how much of their pathogenesis is auto-immune, how much is auto-inflammatory?

In this review, we discuss our studies on the pathogenesis of collagen-induced arthritis (CIA) and related mouse models for rheumatoid arthritis. Of note, these models invariably rely on the use of complete Freund's adjuvant (CFA). Our analysis has focused on explaining the dichotomous - either protective or disease-promoting - role of endogenous IFN-γ. Induction of a myelopoietic burst by CFA was identified as an important and underestimated factor in mediating the role of IFN-γ and other cytokines (IL-6, IL-17, GCP-2, RANK-L). Myelopoiesis provides an excess in precursors for joint-infiltrating neutrophils and osteoclasts. We postulate that classical CIA is primarily an auto-inflammatory disease, in part because of a strong innate immune response to the adjuvant. Superimposed on this, collagen-specific auto-immunity reinforces inflammatory reactivity in joints.     

Vitamin D: a hormone for all seasons--how much is enough?

Further understanding of its endocrine mechanisms and increased evidence for autocrine/paracrine actions has recently enhanced our knowledge of the biological activities of the vitamin D metabolite 1,25-dihydroxyvitamin D (1,25(OH)2D). The recognition of the contribution of vitamin D depletion to increased risk of osteoporosis, and most importantly the risk of hip fracture in the elderly, has increased the clinical significance of clinical laboratory testing for vitamin D status. Research has revealed that at least three genes contribute to vitamin D activity within tissues. These are the vitamin D receptor as well as two major vitamin D metabolising enzymes, CYP27B1, responsible for synthesis of 1,25(OH)2D and CYP24, responsible for catabolism of vitamin D metabolites. Current research focuses on the contribution of vitamin D metabolism to increasing vitamin D activity. This is of particular interest in bone forming cells where increased 1,25(OH)2D activity has been proposed to contribute to strengthening the skeleton. As well, solid tumours such as prostate, breast and colon cancers are another increasing area of vitamin D research. The major issues for the clinical laboratory in vitamin D testing include defining clinical decision limits for the interpretation of serum 25-hydroxyvitamin D (25OHD) levels and improving the precision and accuracy of this assay.     

Too much teaching, not enough learning: what is the solution?

The curriculum is packed with so much content that teachers resort to telling students what they know and students simply commit facts to memory. The packed curriculum leaves little time for students to acquire a deep understanding of the subject or to develop life-long skills such as critical thinking, problem solving, and communication. However, learning is not committing a set of facts to memory, but the ability to use resources to find, evaluate, and apply information. This paper addresses these concerns by discussing "how we learn" and reviewing the literature on what works to improve learning. It is clear that active processing of information, not passive reception of information, leads to learning. That is, students must construct their own understanding of concepts, relationships, and procedures. Teachers can encourage this process by carefully considering the type and organization of information as well as instructional strategies. Specifically, teachers should reduce the total amount of factual information students are expected to memorize, reduce our use of the passive lecture format, and devote much more effort to helping students become active, independent learners and problem solvers. Collaborative learning activities, interactive models, educational games, and establishing a culture of inquiry/scholarship are critical for achieving these goals.     

The shape of life: how much is written in stone?

Considering the enormous diversity of living organisms, representing mostly untapped resources for studying ecological, ontogenetic and phylogenetic patterns and processes, why should evolutionary biologists concern themselves with the remains of animals and plants that died out tens or even hundreds of millions of years ago? The reason is that important new insights into some of the most vexing evolutionary questions are being revealed at the interfaces of palaeontology, developmental biology and molecular biology. Attempts to synthesise information from these disciplines, however, often encounter their greatest hurdles in considerations of the radiation of the Metazoa. Ongoing challenges relate to the origins of body plans, the relationships of the metazoan phyla and the timing of major evolutionary radiations. Palaeontology not only has its own unique contributions to the study of evolutionary processes, but provides a lynchpin for many of the emerging techniques.     

The evolution of noncoding DNA: how much junk, how much func?

Comparative sequence analysis on a genomic scale has opened the door for the systematic analysis of cis-acting regulatory DNA. It is now possible to begin to answer basic questions such as, how much meaningful noncoding sequence is in the genome? How strong is natural selection on functional noncoding sequences in different species? Two recent articles have capitalized on the comparative genomic approach in an attempt to answer these questions with surprising results.     

Variation in hominid brain size: how much is due to method?

Brain size represented by cranial capacity (CC) is one of the most frequently analysed characters of hominids. Accuracy of individual CC estimates depends on completeness of specimens and methods used for reconstruction and measurement. A file of published estimates of CC of hominids dated from 3.2 Ma (million years) to 10 Ka (thousand years) including 606 estimates for 243 specimens was compiled. In the file, 75 specimens are available with multiple values (3 to 15) obtained by various methods and/or by various authors. Using individuals as classes in ANOVA, intraclass variation, which represents "error" of estimates, was calculated. For the total sample of multiple estimates (N = 382) the error variance is 5315 ml2. The error standard deviation is 73 ml (coefficient of variation (CV = 7.3%), quite large in comparison to the actual variation in CC in modern humans, SD = 157 ml (CV = 11.6%). This large error makes us wonder whether any fossil can be reliably placed with respect to a particular "cerebral Rubicon" between palaeospecies. Recent discussions concerning cranial capacity of Stw505 are a case in point regarding errors in CC estimation. In actual repeated 30 time measurements on a research quality cast we obtained with various methods (water, seeds, plasticine) CC estimates ranging from 484 to 586 ml. The range of estimates in the literature is from 515 to 625 ml. When hominid CC by taxon with date as a covariate is subjected to ANOVA, taxon is responsible for 5% of the variance while date is responsible for the main portion, (89%). The relationship between CC and date is best characterised as a gradual time trend. It is proven by the ANOVA test for linearity, by gamma test for trend and by ASReml fitting of a linear function. The line of best fit to this time trend is a double exponential curve which explains 90% of the total variance in CC: CC = 306.63 (4.83(0.9995)DATE) Essentially the same curve fits subsamples of CC dated at less than 1 Ma and at 3.2-1.0 Ma. This has several implications for the nature of the Darwinian process to be reconstructed.     

Return of splenic function after splenectomy: how much tissue is needed?

Ninety patients whose spleen had been removed either because of trauma (41 cases) or as an elective procedure (49) were investigated for return of splenic function by counting pitted red cells and examining spleen scans made after injection of heat damaged 99mTc labelled red cells. There was no significant difference in the proportion of pitted red cells between the two groups of patients. Evidence of splenic tissue in scintiscans was not invariably associated with low pitted red cell values, suggesting that the presence of splenic tissue did not necessarily mean return of splenic function. In every patient whose proportion of pitted red cells was less than 16.2% the scintiscan showed splenic uptake. The proportion of patients with pitted red cell values below 16.2% was significantly higher in the group operated on for trauma, and it is concluded that this was due to splenosis. A high inverse correlation between pitted red cell counts and computed splenic volumes was found. Patients with pitted red cell values of less than 16.2% had computed volumes of 22-133 cm3; below this range the proportion of pitted red cells rose very sharply. These results confirm that splenosis occurs in adults, though less often than in children, and suggest that when splenic tissue is to be implanted a graft of at least 20-30 cm3 is needed to ensure satisfactory return of splenic function.