Incorpooration of microheterotrophic processes into the classical paradigm of the planktonic food web

The paper examines the conventional notion of the food chain of the oceans in the light of field studies of microheterotrophic processes. In common with the earlier observations of POMEROY (1974), the conclusion is that the classical paradigm is not compatible with contemporary observations and appears to need extending in order to take them into account. The evidence would seem to point towards at least half of primary production passing through the planktonic microheterotrophs before it is mineralized. The possible routes of flow of organic material from the classical food chain (phytoplankton excretion, losses during grazing and zooplankton excretion) were examined in some detail. The conclusions were that the cumulative production of dissolved organic material from the above sources, estimated to amount to about 60% of primary production, are sufficient to sustain the anticipated rates of microheterotrophic activity. These considerations, by themelves, give no reason to seriously doubt the accuracy of contemporary measurements of primary production. It was calculated, given the present day estimates of microbial growth yields, that secondary production at the microbial level may be comparable to or greater than that of herbivorous zooplankton. When considering the sources of supply of organic material for the microheterotrophs, the events occurring prior to herbivore ingestion were found to be more important than those subsequent to ingestion. As a consequence, the overall accuracy of the estimates of the supply of organic material to the microheterotrophs was very much dependent upon the assessment of total phytoplankton exudation of organic material, i.e. the measured excretion plus that taken up by heterotrophic micro-organisms during the measurement period. The review also highlighted the need for a better understanding of the fate(s) of microbial production: to what extent it is utilized directly by metazoan hervibores as opposed to passing through a protozoan food chain

The Fate of Nutrients in Estuarine Plumes

Estuaries are highly biologically active zones lying between freshwater and marine systems. The classical view is that materials such as nitrates and phosphates which run into rivers as a result of man’s activity are used by the planktonic algae, or phytoplankton, for growth – in some cases causing nuisance blooms of these organisms. The management of the reduction of these blooms is based on the classical assumption that the materials stimulating them are brought into the estuary by the river, and that effective control of the blooms can be achieved by setting limits on the initial discharge of these materials into rivers. Funded under the EU INTERREG II (Ireland-Wales) programme, two groups of marine scientists from the University of Wales, Bangor and the National University of Ireland, Galway made a co-operative study of the Waterford (Ireland) and Conwy (Wales) estuaries. It was found that whereas the source of nitrogen for the estuarine phytoplankton was from the rivers, the main source of phosphate was from the sea. Phytoplankton blooms were being encouraged within the plume zone near the mouth of the estuaries, a region poised between a nitrate-rich freshwater and, relatively, phosphate-rich seawater. The management consequences of the findings are profound. Phosphates contribute significantly to the pollution of rivers and lakes, systems where there is usually an abundance of nitrogen and algal growth is governed by the availability of phosphorus. Management of these freshwater systems is thus achieved through control of the input of phosphates. Results achieved during the present study show that this criterion does not apply to estuaries and estuarine blooms, as material (phosphate) supporting them comes from the seawater end of the system and is therefore obviously unmanageable. The requirement to control nitrogen (nitrate) levels in estuaries is therefore all the more important in order to properly manage phytoplankton blooms, and thus water quality, in estuaries.Funder: European Unio

Ammonium regeneration: Its contribution to phytoplankton nitrogen requirements in a eutrophic environment

Ammonium regeneration, nutrient uptake, bacterial activity and primary production were measured from March to August 1980 in Bedford Basin, Nova Scotia, Canada, a eutrophic environment. Rates of regeneration and nutrient uptake were determined using 15N isotope dilution and tracer methodology. Although primary production, nutrient uptake and ammonium regeneration were significantly intercorrelated, no relationship was detected between these parameters and heterotrophic activity. The average contribution of ammonium to total nitrogen (ammonium+nitrate) uptake was similar in the spring and in the summer (approximately 60%). On a seasonal average basis, 36% of the phytoplankton ammonium uptake could be supplied by rapid remineralization processes. In spite of the high average contribution of NH4 regeneration to phytoplankton ammonia uptake, there is indirect evidence suggesting that other NH4 sources may occasionally be important

Krill Excretion Boosts Microbial Activity in the Southern Ocean

Antarctic krill are known to release large amounts of inorganic and organic nutrients to the water column. Here we test the role of krill excretion of dissolved products in stimulating heterotrophic bacteria on the basis of three experiments where ammonium and organic excretory products released by krill were added to bacterial assemblages, free of grazers. Our results demonstrate that the addition of krill excretion products (but not of ammonium alone), at levels expected in krill swarms, greatly stimulates bacteria resulting in an order-of-magnitude increase in growth and production. Furthermore, they suggest that bacterial growth rate in the Southern Ocean is suppressed well below their potential by resource limitation. Enhanced bacterial activity in the presence of krill, which are major sources of DOC in the Southern Ocean, would further increase recycling processes associated with krill activity, resulting in highly efficient krill-bacterial recycling that should be conducive to stimulating periods of high primary productivity in the Southern Ocean.This research is a contribution to projects ICEPOS (REN2002-04165-CO3-O2) and ATOS (POL2006-00550/CTM), funded by the Spanish Ministry of Science and Innovation

Climate change, phenological shifts, eco-evolutionary responses and population viability: toward a unifying predictive approach

The debate on emission targets of greenhouse gasses designed to limit global climate change has to take into account the ecological consequences. One of the clearest ecological consequences is shifts in phenology. Linking these shifts to changes in population viability under various greenhouse gasses emission scenarios requires a unifying framework. We propose a box-in-a-box modeling approach that couples population models to phenological change. This approach unifies population modeling with both ecological responses to climate change as well as evolutionary processes. We advocate a mechanistic embedded correlative approach, where the link from genes to population is established using a periodic matrix population model. This periodic model has several major advantages: (1) it can include complex seasonal behaviors allowing an easy link with phenological shifts; (2) it provides the structure of the population at each phase, including the distribution of genotypes and phenotypes, allowing a link with evolutionary processes; and (3) it can incorporate the effect of climate at different time periods. We believe that the way climatologists have approached the problem, using atmosphere–ocean coupled circulation models in which components are gradually included and linked to each other, can provide a valuable example to ecologists. We hope that ecologists will take up this challenge and that our preliminary modeling framework will stimulate research toward a unifying predictive model of the ecological consequences of climate change

Bacterial metabolism of algal extracellular carbon

Measurements of microbial utilization of extracellular organic carbon (EOC) released by phytoplankton commonly consider only EOC fractions subject to rapid uptake. Questions remain whether other EOC fractions are metabolized, what portion is labile, and with what assimilation efficiency this carbon substrate is utilized. 14 C-EOC was prepared by incubation of the natural mixed planktonic community from an oligotrophic lake with H 14 CO 3 in the light. 14 C-EOC which was not rapidly removed by heterotrophs remained in solution and was isolated by filtration. This residual EOC was inoculated with lake microheterotrophs in laboratory microcosms, and utilization kinetics were determined through long-term assays of cumulative 14 CO 2 production. Time-courses for 14 CO 2 production were consistent for all assays and were well described by a deterministic mixed-order degradation model. On twelve sampling occasions, from 29% to 76% of residual 14 C-EOC was labile to further metabolism by lake heterotrophs. First-order rate constants for EOC utilization showed a mode of 0.05 to 0.15 per day. From 33% to 78% of gross 14 C-EOC uptake was respired (mean 50%), indicating appreciable return of algal EOC to the pelagic food web as microbial biomass.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42876/1/10750_2004_Article_BF00015524.pd

Polygenic Risk Scores for Prediction of Breast Cancer and Breast Cancer Subtypes.

Stratification of women according to their risk of breast cancer based on polygenic risk scores (PRSs) could improve screening and prevention strategies. Our aim was to develop PRSs, optimized for prediction of estrogen receptor (ER)-specific disease, from the largest available genome-wide association dataset and to empirically validate the PRSs in prospective studies. The development dataset comprised 94,075 case subjects and 75,017 control subjects of European ancestry from 69 studies, divided into training and validation sets. Samples were genotyped using genome-wide arrays, and single-nucleotide polymorphisms (SNPs) were selected by stepwise regression or lasso penalized regression. The best performing PRSs were validated in an independent test set comprising 11,428 case subjects and 18,323 control subjects from 10 prospective studies and 190,040 women from UK Biobank (3,215 incident breast cancers). For the best PRSs (313 SNPs), the odds ratio for overall disease per 1 standard deviation in ten prospective studies was 1.61 (95%CI: 1.57-1.65) with area under receiver-operator curve (AUC) = 0.630 (95%CI: 0.628-0.651). The lifetime risk of overall breast cancer in the top centile of the PRSs was 32.6%. Compared with women in the middle quintile, those in the highest 1% of risk had 4.37- and 2.78-fold risks, and those in the lowest 1% of risk had 0.16- and 0.27-fold risks, of developing ER-positive and ER-negative disease, respectively. Goodness-of-fit tests indicated that this PRS was well calibrated and predicts disease risk accurately in the tails of the distribution. This PRS is a powerful and reliable predictor of breast cancer risk that may improve breast cancer prevention programs