Pluses and minuses of ammonium and nitrate uptake and assimilation by phytoplankton and implications for productivity and community composition, with emphasis on nitrogen-enriched conditions

© 2016 Association for the Sciences of Limnology and Oceanography. Anthropogenic activities are altering total nutrient loads to many estuaries and freshwaters, resulting in high loads not only of total nitrogen (N), but in some cases, of chemically reduced forms, notably NH4+. Long thought to be the preferred form of N for phytoplankton uptake, NH4+ may actually suppress overall growth when concentrations are sufficiently high. NH4+ has been well known to be inhibitory or repressive for NO3- uptake and assimilation, but the concentrations of NH4+ that promote vs. repress NO3- uptake, assimilation, and growth in different phytoplankton groups and under different growth conditions are not well understood. Here, we review N metabolism first in a "generic" eukaryotic cell, and the contrasting metabolic pathways and regulation of NH4+ and NO3- when these substrates are provided individually under equivalent growth conditions. Then the metabolic interactions of these substrates are described when both are provided together, emphasizing the cellular challenge of balancing nutrient acquisition with photosynthetic energy balance in dynamic environments. Conditions under which dissipatory pathways such as dissimilatory NO3-/ NO2- reduction to NH4+ and photorespiration that may lead to growth suppression are highlighted. While more is known about diatoms, taxon-specific differences in NH4+ and NO3- metabolism that may contribute to changes in phytoplankton community composition when the composition of the N pool changes are presented. These relationships have important implications for harmful algal blooms, development of nutrient criteria for management, and modeling of nutrient uptake by phytoplankton, particularly in conditions where eutrophication is increasing and the redox state of N loads is changing

Silica burial enhanced by iron limitation in oceanic upwelling margins

In large swaths of the ocean, primary production by diatoms may be limited by the availability of silica, which in turn limits the biological uptake of carbon dioxide. The burial of biogenic silica in the form of opal is the main sink of marine silicon. Opal burial occurs in equal parts in iron-limited open-ocean provinces and upwelling margins, especially the eastern Pacific upwelling zone. However, it is unclear why opal burial is so efficient in this margin. Here we measure fluxes of biogenic material, concentrations of diatom-bound iron and silicon isotope ratios using sediment traps and a sediment core from the Gulf of California upwelling margin. In the sediment trap material, we find that periods of intense upwelling are associated with transient iron limitation that results in a high export of silica relative to organic carbon. A similar correlation between enhanced silica burial and iron limitation is evident in the sediment core, which spans the past 26,000 years. A global compilation also indicates that hotspots of silicon burial in the ocean are all characterized by high silica to organic carbon export ratios, a diagnostic trait for diatoms growing under iron stress. We therefore propose that prevailing conditions of silica limitation in the ocean are largely caused by iron deficiency imposing an indirect constraint on oceanic carbon uptake

Nitrogen-limited mangrove ecosystems conserve N through dissimilatory nitrate reduction to ammonium

Earlier observations in mangrove sediments of Goa, India have shown denitrification to be a major pathway for N loss1. However, percentage of total nitrate transformed through complete denitrification accounted for <0–72% of the pore water nitrate reduced. Here, we show that up to 99% of nitrate removal in mangrove sediments is routed through dissimilatory nitrate reduction to ammonium (DNRA). The DNRA process was 2x higher at the relatively pristine site Tuvem compared to the anthropogenically-influenced Divar mangrove ecosystem. In systems receiving low extraneous nutrient inputs, this mechanism effectively conserves and re-circulates N minimizing nutrient loss that would otherwise occur through denitrification. In a global context, the occurrence of DNRA in mangroves has important implications for maintaining N levels and sustaining ecosystem productivity. For the first time, this study also highlights the significance of DNRA in buffering the climate by modulating the production of the greenhouse gas nitrous oxide

Metabolic compartmentalization in the human cortex and hippocampus: evidence for a cell- and region-specific localization of lactate dehydrogenase 5 and pyruvate dehydrogenase

BACKGROUND: For a long time now, glucose has been thought to be the main, if not the sole substrate for brain energy metabolism. Recent data nevertheless suggest that other molecules, such as monocarboxylates (lactate and pyruvate mainly) could be suitable substrates. Although monocarboxylates poorly cross the blood brain barrier (BBB), such substrates could replace glucose if produced locally.The two key enzymatiques systems required for the production of these monocarboxylates are lactate dehydrogenase (LDH; EC1.1.1.27) that catalyses the interconversion of lactate and pyruvate and the pyruvate dehydrogenase complex that irreversibly funnels pyruvate towards the mitochondrial TCA and oxydative phosphorylation. RESULTS: In this article, we show, with monoclonal antibodies applied to post-mortem human brain tissues, that the typically glycolytic isoenzyme of lactate dehydrogenase (LDH-5; also called LDHA or LDHM) is selectively present in astrocytes, and not in neurons, whereas pyruvate dehydrogenase (PDH) is mainly detected in neurons and barely in astrocytes. At the regional level, the distribution of the LDH-5 immunoreactive astrocytes is laminar and corresponds to regions of maximal 2-deoxyglucose uptake in the occipital cortex and hippocampus. In hippocampus, we observed that the distribution of the oxidative enzyme PDH was enriched in the neurons of the stratum pyramidale and stratum granulosum of CA1 through CA4, whereas the glycolytic enzyme LDH-5 was enriched in astrocytes of the stratum moleculare, the alveus and the white matter, revealing not only cellular, but also regional, selective distributions. The fact that LDH-5 immunoreactivity was high in astrocytes and occurred in regions where the highest uptake of 2-deoxyglucose was observed suggests that glucose uptake followed by lactate production may principally occur in these regions. CONCLUSION: These observations reveal a metabolic segregation, not only at the cellular but also at the regional level, that support the notion of metabolic compartmentalization between astrocytes and neurons, whereby lactate produced by astrocytes could be oxidized by neurons

Novel Crystalline SiO2 Nanoparticles via Annelids Bioprocessing of Agro-Industrial Wastes

The synthesis of nanoparticles silica oxide from rice husk, sugar cane bagasse and coffee husk, by employing vermicompost with annelids (Eisenia foetida) is reported. The product (humus) is calcinated and extracted to recover the crystalline nanoparticles. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and dynamic light scattering (DLS) show that the biotransformation allows creating specific crystalline phases, since equivalent particles synthesized without biotransformation are bigger and with different crystalline structure

Culling-Induced Changes in Badger (Meles meles) Behaviour, Social Organisation and the Epidemiology of Bovine Tuberculosis

In the UK, attempts since the 1970s to control the incidence of bovine tuberculosis (bTB) in cattle by culling a wildlife host, the European badger (Meles meles), have produced equivocal results. Culling-induced social perturbation of badger populations may lead to unexpected outcomes. We test predictions from the ‘perturbation hypothesis’, determining the impact of culling operations on badger populations, movement of surviving individuals and the influence on the epidemiology of bTB in badgers using data dervied from two study areas within the UK Government's Randomised Badger Culling Trial (RBCT). Culling operations did not remove all individuals from setts, with between 34–43% of badgers removed from targeted social groups. After culling, bTB prevalence increased in badger social groups neighbouring removals, particularly amongst cubs. Seventy individual adult badgers were fitted with radio-collars, yielding 8,311 locational fixes from both sites between November 2001 and December 2003. Home range areas of animals surviving within removed groups increased by 43.5% in response to culling. Overlap between summer ranges of individuals from Neighbouring social groups in the treatment population increased by 73.3% in response to culling. The movement rate of individuals between social groups was low, but increased after culling, in Removed and Neighbouring social groups. Increased bTB prevalence in Neighbouring groups was associated with badger movements both into and out of these groups, although none of the moving individuals themselves tested positive for bTB. Significant increases in both the frequency of individual badger movements between groups and the emergence of bTB were observed in response to culling. However, no direct evidence was found to link the two phenomena. We hypothesise that the social disruption caused by culling may not only increase direct contact and thus disease transmission between surviving badgers, but may also increase social stress within the surviving population, causing immunosuppression and enhancing the expression of disease

Estimates of new and total productivity in central Long Island Sound from in situ measurements of nitrate and dissolved oxygen

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Estuaries and Coasts 36 (2013): 74-97, doi:10.1007/s12237-012-9560-5.Biogeochemical cycles in estuaries are regulated by a diverse set of physical and biological variables that operate over a variety of time scales. Using in situ optical sensors, we conducted a high-frequency time-series study of several biogeochemical parameters at a mooring in central Long Island Sound from May to August 2010. During this period, we documented well-defined diel cycles in nitrate concentration that were correlated to dissolved oxygen, wind stress, tidal mixing, and irradiance. By filtering the data to separate the nitrate time series into various signal components, we estimated the amount of variation that could be ascribed to each process. Primary production and surface wind stress explained 59% and 19%, respectively, of the variation in nitrate concentrations. Less frequent physical forcings, including large-magnitude wind events and spring tides, served to decouple the relationship between oxygen, nitrate, and sunlight on about one-quarter of study days. Daytime nitrate minima and dissolved oxygen maxima occurred nearly simultaneously on the majority (> 80%) of days during the study period; both were strongly correlated with the daily peak in irradiance. Nighttime nitrate maxima reflected a pattern in which surface-layer stocks were depleted each afternoon and recharged the following night. Changes in nitrate concentrations were used to generate daily estimates of new primary production (182 ± 37 mg C m-2 d-1) and the f-ratio (0.25), i.e., the ratio of production based on nitrate to total production. These estimates, the first of their kind in Long Island Sound, were compared to values of community respiration, primary productivity, and net ecosystem metabolism, which were derived from in situ measurements of oxygen concentration. Daily averages of the three metabolic parameters were 1660 ± 431, 2080 ± 419, and 429 ± 203 mg C m-2 d-1, respectively. While the system remained weakly autotrophic over the duration of the study period, we observed very large day-to-day differences in the f-ratio and in the various metabolic parameters.This work was supported by the Yale Institute for Biospheric Studies, the Sounds Conservancy of the Quebec-Labrador Foundation, and the Yale School of Forestry and Environmental Studies Carpenter-Sperry Fund.2014-01-0

Abundances of Iron-Binding Photosynthetic and Nitrogen-Fixing Proteins of Trichodesmium Both in Culture and In Situ from the North Atlantic

Marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans, where they can dominate the diazotrophic community in regions with high inputs of the trace metal iron (Fe). Iron is necessary for the functionality of enzymes involved in the processes of both photosynthesis and nitrogen fixation. We combined laboratory and field-based quantifications of the absolute concentrations of key enzymes involved in both photosynthesis and nitrogen fixation to determine how Trichodesmium allocates resources to these processes. We determined that protein level responses of Trichodesmium to iron-starvation involve down-regulation of the nitrogen fixation apparatus. In contrast, the photosynthetic apparatus is largely maintained, although re-arrangements do occur, including accumulation of the iron-stress-induced chlorophyll-binding protein IsiA. Data from natural populations of Trichodesmium spp. collected in the North Atlantic demonstrated a protein profile similar to iron-starved Trichodesmium in culture, suggestive of acclimation towards a minimal iron requirement even within an oceanic region receiving a high iron-flux. Estimates of cellular metabolic iron requirements are consistent with the availability of this trace metal playing a major role in restricting the biomass and activity of Trichodesmium throughout much of the subtropical ocean

Enhanced carbon pump inferred from relaxation of nutrient limitation in the glacial ocean

The modern Eastern Equatorial Pacific (EEP) Ocean is a large oceanic source of carbon to the atmosphere. Primary productivity over large areas of the EEP is limited by silicic acid and iron availability, and because of this constraint the organic carbon export to the deep ocean is unable to compensate for the outgassing of carbon dioxide that occurs through upwelling of deep waters. It has been suggested that the delivery of dust-borne iron to the glacial ocean, could have increased primary productivity and enhanced deep-sea carbon export in this region, lowering atmospheric carbon dioxide concentrations during glacial periods. Such a role for the EEP is supported by higher organic carbon burial rates documented in underlying glacial sediments but lower opal accumulation rates cast doubts on the importance of the EEP as an oceanic region for significant glacial carbon dioxide drawdown. Here we present a new silicon isotope record that suggests the paradoxical decline in opal accumulation rate in the glacial EEP results from a decrease in the silicon to carbon uptake ratio of diatoms under conditions of increased iron availability from enhanced dust input. Consequently, our study supports the idea of an invigorated biological pump in this region during the last glacial period that could have contributed to glacial carbon dioxide drawdown. Additionally, using evidence from silicon and nitrogen isotope changes, we infer that, in contrast to the modern situation, the biological productivity in this region is not constrained by the availability of iron, silicon and nitrogen during the glacial period. We hypothesize that an invigorated biological carbon dioxide pump constrained perhaps only by phosphorus limitation was a more common occurrence in low-latitude areas of the glacial ocean