Seasonal variation in the composition of aquatic organic matter in some Nova Scotian brownwaters: a nuclear magnetic resonance approach

Using a novel concentration technique (reverse osmosis and freeze-drying) as well as a standard analytical technique little used with limnological samples (solid state ¹³C nuclear magnetic resonance), we studied the chemical structure of aquatic organic matter from four closely located freshwater sites in Nova Scotia. The main conclusions drawn from the data are that: (a) the aromatic C fraction which is assumed to be refractory remains at less than 10% of the total, with a slight increase in relative importance in the fall, (b) less structurally complex aliphatic carbon decreases from winter to spring and remains at lower levels into later fall, (c) carbohydrates are at a maximum during the summer, (d) the carboxylic C fraction is at a minimum in the summer and maximum in the fall and winter. Results show roughly the same annual patterns of C composition for the two running water sites. Our data suggest that the hydrological processes which carry terrestrial and wetland DOM to streams and lakes allow a selective adsorption process of DOM to occur in soils. We compare our data to those from other freshwater studies and suggest that the importance of aromatic carbon in freshwaters has probably been overestimated in the past, with a corresponding underestimate of the more biologically labile carbohydrate and aliphatic material.Department of Chemistry, McMaster Universtiy     

Genetic Dissection of the Function of Hindbrain Axonal Commissures

In Bilateria, many axons cross the midline of the central nervous system, forming well-defined commissures. Whereas in mammals the functions of commissures in the forebrain and in the visual system are well established, functions at other axial levels are less clearly understood. Here, we have dissected the function of several hindbrain commissures using genetic methods. By taking advantage of multiple Cre transgenic lines, we have induced site-specific deletions of the Robo3 receptor. These lines developed with the disruption of specific commissures in the sensory, motor, and sensorimotor systems, resulting in severe and permanent functional deficits. We show that mice with severely reduced commissures in rhombomeres 5 and 3 have abnormal lateral eye movements and auditory brainstem responses, respectively, whereas mice with a primarily uncrossed climbing fiber/Purkinje cell projection are strongly ataxic. Surprisingly, although rerouted axons remain ipsilateral, they still project to their appropriate neuronal targets. Moreover, some Cre;Robo3 lines represent potential models that can be used to study human syndromes, including horizontal gaze palsy with progressive scoliosis (HGPPS). To our knowledge, this study is one of the first to link defects in commissural axon guidance with specific cellular and behavioral phenotypes.     

Exposure of goldsinny, rock cook and corkwing wrasse to low temperature and low salinity: survival, blood physiology and seasonal variation

Wrasse used as cleaner fish with farmed Atlantic salmon Salmo salar can be subjected to large and rapid temperature and salinity fluctuations in late autumn and early winter, when summer-warmed surface water is affected by early snowmelt episodes. Because of their containment in sea cages, wrasse which are essentially acclimated to summer temperatures may be rapidly exposed to winter conditions. Short-term tolerance of low temperature and low salinity by three species of wrasse, goldsinny Ctenolabrus rupestris rock cook Centrolabrus exoletus corkwing Crenilabrus melops caught during the summer, and winter-caught corkwing, was investigated. A 3–day period at 30 or 32‰ salinity and temperature 8, 6 or 4° C (for summer-caught fish; 4° C only for winter-caught) was followed by a decline in salinity to 24, 16 or 8‰ over c. 36 h, followed by a further 24 h at these salinities held constant, at each of the three temperatures. Controls in 30 or 32‰ were maintained at 8, 6 or 4° C. Mortality of summer-caught corkwing and rock cook was high at 4° C, whereas the influence of salinity on mortality was small. Mortality of goldsinny was low or zero in all treatments. Surviving corkwing and rock cook after 3 days at 4° C and 32‰ salinity had elevated plasma osmolality: in summer-caught corkwing, plasma [Cl°] and [Na⁺] were high, whereas in rock cook only [Na⁺] was high. Haematocrit was low in summer-caught corkwing, high in rock cook. In survivors of all three species at the end of the experiment, values of all these parameters were comparable with those of fish at the beginning of the experiment, except that survivors at low salinity (8, 16‰) had low plasma osmolality, at all temperatures, and in surviving rock cook in these treatments haematocrit was high and plasma [Cl^?] was low. Winter-caught corkwing had higher osmolality, [Na⁺] and [Cl^?] than summer-caught corkwing; there was no difference in haematocrit. Survival of wintercaught corkwing exposed to four salinities at 4° C was much higher than that of summercaught corkwing under the same conditions. Little change in blood physiology was recorded for winter-caught corkwing, with only fish subjected to 8‰ and 4° C showing signs of osmoregulatory stress. The interspecific and seasonal differences in survival and blood physiology at low temperature and low salinity are discussed in relation to wrasse survival over winter, both in the field and in salmon farms.     

Biodiversity changes in a shallow lake ecosystem: a multi-proxy palaeolimnological analysis

Biodiversity is a key measure of environmental quality in lake ecosystems. Lake biodiversity can be assessed using modern survey data, but typically these data only provide a ‘snap-shot’ measure and in most cases it is not possible to reconstruct temporal trends in biodiversity, so that human impacts can be detected. Palaeoecological techniques offer an alternative means of identifying changes in biodiversity over the period of historical records and far beyond, but there are problems associated with this approach. This is because only a select set of organisms leave a trace in the sediment record such that it is not usually possible to make reliable assessments of diversity changes within an entire taxonomic order (e.g. the algae). Moreover these organisms are typically from the lower levels of the trophic hierarchy (i.e. plants and insects). The problems of identifying changes in biodiversity from the palaeolimnological record are addressed with reference to Groby Pool, a shallow, eutrophic, medieval lake in the English Midlands, which has been subjected to eutrophication over the last 150 years. ²¹⁰Pb and ¹³⁷Cs-dated sediment cores have been used to estimate short-term alterations in the composition and diversity of three groups of indicators, representing different levels in the trophic cascade, namely diatoms, aquatic pollen and chironomids. By exploring relationships, both between these indicators and with archival macrophyte records, an assessment is made of eutrophication-related changes in overall habitat diversity at the ecosystem level. These data suggest that the lake has undergone considerable nutrient enrichment, resulting in the loss of a diverse, mesotrophic macrophyte flora from at least the turn of the century onwards and its replacement by a few highly competitive species tolerant of high nutrient concentrations. Reductions in macrophyte diversity seem to be reflected palaeoecologically by a decline in the diversity of fossil chironomid assemblages, related to the breakdown of particular host-plant relationships amongst the phytophagic species. However, diatom assemblages generally exhibit the opposite trend, which may be related to increases in macrophyte cover and increasing opportunities for the colonization of diverse epiphyte communities. The different fossil indicators have different limitations and merits, and for this reason a ‘multi-proxy’ approach is essential if meaningful inferences are to be made of changes in lake biodiversity using palaeoecological data.     

Effects of pH and salt concentration on the hydrolysis of a benzo[alpha]pyrene 7,8-diol-9,10-epoxide catalyzed by DNA and polyadenylic acid

The time-dependent absorbance change that occurs when benzo[alpha]pyrene 7,8-diol-9,10-epoxide is added to solutions of calf thymus DNA has been shown, by an unequivocal chromatographic method, to correspond to DNA-catalyzed hydrolysis of the diol-epoxide. At 25 degrees C and mu = 0.10, the kinetics of the reaction of the diol-epoxide with polyadenylic acid or DNA are consistent with preequilibrium formation of a non-covalent complex between the diol-epoxide and the polynucleotide or DNA, followed by hydrolysis of the bound epoxide by a process that is first-order in hydronium ions. Cacodylic acid also catalyzes the hydrolysis of the epoxide bound to polyadenylic acid. The rate of the DNA-catalyzed hydrolysis exhibits little or no enantiomeric selectivity for the diol-epoxide. DNA catalyzed hydrolysis of the diol-epoxide is extraordinarily sensitive to the salt concentration in the reaction medium: the rate of hydrolysis of the bound epoxide at pH 7 is retarded by a factor of approximately 45 in the presence of 0.1 M sodium chloride compared to a 1 mM buffer containing no added salt. Thus, studies of the interactions of DNA with carcinogenic diol-epoxides must take into account the ionic environment of DNA within the cell.     

Modulation of radiation-induced chromosome aberrations by DMSO, an OH radical scavenger. 1: Dose-response studies in human lymphocytes exposed to 220 kV X-rays

Human G0 lymphocytes were exposed to 220 kV X-radiation in the presence or absence of DMSO, an efficient selective scavenger of OH radicals. Our studies demonstrate that DMSO affects a concentration-dependent modulation of induced asymmetrical aberrations in human lymphocytes exposed to approximately 3.0 Gy, with maximum protectible fractions of approximately 70 percent at DMSO concentrations of greater than or equal to 1 M. The dose dependency for dicentrics in lymphocytes acutely exposed to X-ray doses of 0.51 to 4.98 Gy in the absence of DMSO is adequately described by the linear-quadratic dose-response function Y = alpha D + beta D2. Data from duplicate cultures exposed in the presence of 1 M DMSO produce an excellent fit to the regression function modified as follows: Y(+ DMSO) = alpha(delta D) + beta(delta D)2 where the 'dose modifying' factor delta = 0.501. We interpret these findings as providing evidence that OH radical-mediated lesions in DNA account for approximately 50 percent of the dose dependency for dicentrics resulting from either one-track or two-track events, following exposures of non-cycling cells to moderate-to-high doses of low LET radiation. These data may be used in additional calculations to derive an estimate of approximately 6 x 10(8) s-1 for the rate of reaction of OH radicals with DNA targets involved in aberration formation.