Quantifying nutrient throughput and DOM production by algae in continuous culture.

Freshwater and marine algae can balance nutrient demand and availability by regulating uptake, accumulation and exudation. To obtain insight into these processes under nitrogen (N) and phosphorus (P) limitation, we reanalyze published data from continuous cultures of the chlorophyte Selenastrum minutum. Based on mass budgets, we argue that much of the non-limiting N and P had passed through the organisms and was present as dissolved organic phosphorus or nitrogen (DOP or DON). We construct a model that describes the production of biomass and dissolved organic matter (DOM) as a function of the growth rate. A fit of this model against the chemostat data suggests a high turnover of the non-limiting N and P: at the highest growth rates, N and P atoms spent on average only about 3 h inside an organism, before they were exuded as DON and DOP, respectively. This DOM exudation can explain the observed trends in the algal stoichiometric ratios as a function of the dilution rate. We discuss independent evidence from isotope experiments for this apparently wasteful behavior and we suggest experiments to quantify and characterize DON and DOP exudation further.

Genotypic and phenotypic variation in diatom silicification under paleo-oceanographic conditions.

Diatoms have co-evolved with the silicon cycle and are largely responsible for reducing surface concentrations of silicate in the ocean to their present levels. We quantify silicification in marine diatoms at a range of high silicate concentrations representative of environments found over their geological history. The species examined include Stephanopyxis turris, an ancient centric species found throughout the Cenozoic, Thalassiosira pseudonana and Thalassiosira weissflogii, two younger centric species, and two pennate ecotypes of Staurosirella pinnata isolated from different nutrient regimes. Frustule thickness and micromorphological structure are strongly affected by silicate concentration. All species become increasingly silicified with silicate concentrations at concentrations vastly in excess of surface ocean concentrations today. In contrast, the half-saturation constant for silicate uptake for most modern diatoms is below 2 µM. Based on the results, we hypothesize that silicate uptake is multiphasic in diatoms and that multiple silicate transport systems may have evolved in response to decreases in surface silicate concentration over geological time. The oldest species examined is more heavily silicified than the more modern species, presumably reflecting the conditions under which it originated. Yet diversification in silicification can be rapid, as illustrated by greater silicification in onshore versus the offshore ecotype of the same modern species. This work suggests that silicification of fossil frustules may eventually provide a paleoproxy for surface silicate concentrations over the Cenozoic, although development of species-specific calibrations will be necessary and the effects of a range of environmental conditions must be investigated.

A universal driver of macroevolutionary change in the size of marine phytoplankton over the Cenozoic.

The size structure of phytoplankton assemblages strongly influences energy transfer through the food web and carbon cycling in the ocean. We determined the macroevolutionary trajectory in the median size of dinoflagellate cysts to compare with the macroevolutionary size change in other plankton groups. We found the median size of the dinoflagellate cysts generally decreases through the Cenozoic. Diatoms exhibit an extremely similar pattern in their median size over time, even though species diversity of the two groups has opposing trends, indicating that the macroevolutionary size change is an active response to selection pressure rather than a passive response to changes in diversity. The changes in the median size of dinoflagellate cysts are highly correlated with both deep ocean temperatures and the thermal gradient between the surface and deep waters, indicating the magnitude and frequency of nutrient availability may have acted as a selective factor in the macroevolution of cell size in the plankton. Our results suggest that climate, because it affects stratification in the ocean, is a universal abiotic driver that has been responsible for macroevolutionary changes in the size structure of marine planktonic communities over the past 65 million years of Earth's history.

A multiyear estimate of the effective pollen donor pool for Albizia julibrissin.

Studies of pollen movement in plant populations are often limited to a single reproductive event, despite concerns about the adequacy of single-year measures for perennial organisms. In this study, we estimate the effective number of pollen donors per tree from a multiyear study of Albizia julibrissin Durazz (mimosa, Fabaceae), an outcrossing, insect-pollinated tree. We determined 40 seedling genotypes for each of 15 seed trees during 4 successive years. A molecular analysis of variance of the pollen gametes fertilizing the sampled seeds was used to partition variation in pollen pools among seed trees, among years, and within single tree-year collections. Using these variance components, we demonstrate significant male gametic variability among years for individual trees. However, results indicate that yearly variation in the 'global pollen pool', averaged over all 15 seed trees for these 4 years, is effectively zero. We estimate the effective number of pollen donors for a single mimosa tree (N(ep)) to be 2.87. Single season analyses yield N(ep) approximately 2.05, which is 40% less than the value of N(ep) estimated from 4 years of data. We discuss optimal sampling for future studies designed to estimate N(ep). Studies should include more trees, each sampled over at least a few years, with fewer seeds per tree per year than are needed for a traditional parentage study.

Light absorption by phytoplankton and the filter amplification correction: cell size and species effects.

Filter amplification corrections are presented for eight marine centric diatoms. These functions are required to correct the amplified optical path-length associated with the glass-fiber filter used in the measurement of phytoplankton absorption. Correction factors constructed from phytoplankton cultures in the laboratory are often applied to phytoplankton assemblages in the field. This study demonstrates significant differences in the filter amplification correction correlated to species and cell volume. This variation in the filter amplification correction can result in significant error in estimated absorption coefficients, compromising subsequent estimates of quantum yield and primary production.

Evolution of altruism in stepping-stone populations with overlapping generations.

We study the evolution of altruism in one- and two-dimensional stepping-stone populations with discrete overlapping generations. We find that increasing survival probability facilitates the evolution of altruism, in agreement with recent results for a patch-structured population. We allow the altruistic behaviour to affect either fecundity or survival probability. In the first case, altruism is favoured compared to a randomly interacting population, but in the second case, altruism is less likely to evolve. We consider the iterated prisoner's dilemma as a description of an altruistic interaction and compare our results with recent simulations of lattice populations.

Overlapping generations can promote altruistic behavior.

We use an inclusive fitness model to study the evolution of altruism in a patch-structured population in which there is positive probability of breeder survival from one generation to the next. We find first that breeder survival promotes altruism and second that there is a marked difference between benefits of fecundity and benefits of survival. Under the first altruism is more strongly favored, and under the second altruism is less strongly favored than in a randomly mixing population.

Modeling size-dependent photosynthesis: light absorption and the allometric rule.

Microalgal photosynthesis can be predicted using empirical allometric or mechanistic bio-optic models. These two descriptions are usually considered independently. We compare the size scaling of photosynthesis predicted by these two models. Size scaling exponents for phytoplankton often deviate from the allometric 3/4 rule. This may be because the allometric model does not account for the size dependence of light absorption and its effect on the size scaling of photosynthesis. In contrast to the allometric model and experimental data, the bio-optic model predicts photosynthesis should be independent of cell size when intracellular pigment concentrations are low or inversely related to cell diameter. A composite of the allometric and bio-optic models is described and compared to laboratory data of light-limited nutrient-saturated diatom photosynthesis. The allo-bio-optic model provides a mechanistic explanation for the anomalous size scaling found in laboratory and field studies of microalgal photosynthesis and growth.

Evolution of dispersal in a stepping-stone population with overlapping generations.

We use Hamilton's inclusive fitness method to calculate the evolutionarily stable dispersal rate in 1- and 2-dimensional stepping-stone populations. This extends previous results by introducing a positive probability for adults to survive into the next generation and breed again. Relatedness between nearby individuals generally decreases with increasing survival, decreasing competition with kin and favouring greater dispersal rates.

Interpretation of the in vitro proliferation response of mcf-7 cells to potential oestrogens and non-oestrogenic substances.

Chemicals may interact with the oestrogen receptor (ER) in mammalian cells and thereby exhibit oestrogenic activity which may result in effects on development and reproduction in both wildlife and humans. It has been suggested that in vitro assays provide the best way forward for investigating the oestrogenic potential of large numbers of chemicals. We compared the effects of a series of chemicals in an MCF-7 cell proliferation assay with results from similar assays reported in the literature in order to investigate the practical use of such assays. Alkylphenols and benzylbutylphthalate were found to cause proliferation of MCF-7 cells, which could be antagonized by tamoxifen. However, all the substances tested, including ethanol, progesterone and epidermal growth factor, were found to have some effect on cell proliferation, suggesting that cell proliferation may result from mechanisms other than via the ER. This is in contrast to some reports that MCF-7 cells only respond to oestrogens and highlights MCF-7 cell strain differences. Such differences and other reports in the literature of the effects of growth modulators on MCF-7 cells indicate that interpretation of results from MCF-7 cell proliferation assays needs to be considered carefully. Comparison between the results of in vitro assays and appropriate in vivo assays is also required in order to evaluate the significance of in vitro oestrogenicity assay results and an understanding of the mechanisms responsible for the biological effects is necessary.