Bioavailable DOC : reactive macronutrient ratios control heterotrophic nutrient assimilation - An experimental proof of the macronutrient-balance hypothesis

<p>We assess the “macronutrient-balance hypothesis,” which we define as: “Aquatic heterotrophic nutrient assimilation is controlled by the balance between the bioavailable DOC : reactive macronutrient stoichiometry and the microbial stoichiometric macronutrient demand.” Here we define the reactive macronutrients as the sum of dissolved inorganic nitrogen, soluble-reactive phosphorus (SRP), and dissolved bioavailable organic N (bDON) & P (bDOP). A global meta-analysis of monitoring data from various freshwaters suggests this hypothesis, yet clear experimental support is missing.</p><p>We assessed this hypothesis in a proof-of-concept experiment for waters from four different small agricultural streams. We used seven different bioavailable DOC (bDOC) : reactive N and bDOC : reactive P ratios, induced by seven different levels of alder leaf leachate addition. With these treatments and a stream-water specific bacterial inoculum, we conducted a separate 3-day experiment, with three independent replicates per combination of stream water, treatment and sampling occasion. Here, we extracted dissolved organic matter (DOM) fluorophores by measuring excitation-emission matrices with subsequent parallel factor decomposition (EEM-PARAFAC). We assessed the true bioavailability of DOC, DON, and the DOM fluorophores as solute concentration difference between the beginning and end of each experiment. Separately, we predicted bDOC and bDON concentrations based on the bioavailable fluorophores, which we compared to their true bioavailability measured before. Due to very low DOP concentrations, the DOP determination uncertainty was high, and we had to neglect DOP as part of the reactive P.</p><p>For bDOC and bDON, the bioavailability measurements agreed with the same fractions calculated indirectly from bioavailable DOM fluorophores (bDOC r² = 0.96, p < 0.001; bDON r² = 0.77, p < 0.001), hence we could predict bDOC and bDON concentrations based on the molecular composition of DOM. Moreover, we found that bDOC : reactive nutrient ratios at specific ranges (molar bDOC : reactive N = 2 − 17; molar bDOC : reactive P = 50 − 300) control microbial heterotrophic nutrient uptake.</p><p>In summary, the results of our simple laboratory experiment provide first proof that the bDOC : reactive macronutrient ratio strongly controls heterotrophic reactive macronutrient uptake. Combined with the previous large-scale monitoring evidence, our study implies that the “macronutrient-balance hypothesis” holds in many aquatic ecosystems. However, this hypothesis needs to be corroborated by further experiments with different DOC sources and assessments of changes in bDOC : reactive macronutrient ratios on freshwater carbon and nutrient cycles.</p>

Challenging the charge balance hypothesis: reconsidering buffer effect and reuptake of previously excreted organic acids by Penicillium ochrochloron

ABSTRACT Penicillium ochrochloron was used in the past for the leaching of zinc from a zinc oxide containing filter dust via excreted organic acids. Organic acid excretion by P. ochrochloron was stimulated by the addition of an extracellular buffer (2-(N-Morpholino)ethanesulfonic acid, MES; or zinc oxide, ZnO: ZnO + 2 H+ → Zn2+ + H2O). It was tested if the buffer stimulated excretion of organic acid anions is due to the necessity of an anion efflux across the plasma membrane to maintain electroneutrality by balancing the excretion of protons by the H+-ATPase. This charge balance hypothesis was previously postulated for P. ochrochloron. Two strains of P. ochrochloron were studied, which differed in growth parameters and amount of excreted organic acids. From the results, it was concluded that charge balance at the plasma membrane is not the main reason for organic acid excretion in these two strains of P. ochrochloron. Furthermore, the phenomenon of reuptake of excreted organic acids in the presence of about 100 mM of glucose is confirmed. It is suggested that the equilibrium between extracellular and intracellular organic acid anions may be maintained passively by a facilitated diffusion transporter.

No support for the adaptive hypothesis of lagging-strand encoding in bacterial genomes

ABSTRACTGenes are preferentially encoded on the leading instead of the lagging strand of DNA replication in most bacterial genomes1. This bias likely results from selection against lagging-strand encoding, which can cause head-on collisions between DNA polymerases and RNA polymerases that induce transcriptional abortion, replication delay, and possibly mutagenesis1. But there are still genes encoded on the lagging strand, an observation that has been explained by a balance between deleterious mutations bringing genes from the leading to the lagging strand and purifying selection purging such mutations2. This mutation-selection balance hypothesis predicts that the probability that a gene is encoded on the lagging strand decreases with the detriment of its lagging-strand encoding relative to leading-strand encoding, explaining why highly expressed genes and essential genes are underrepresented on the lagging strand3,4. In a recent study, Merrikh and Merrikh proposed that the observed lagging-strand encoding is adaptive instead of detrimental, due to beneficial mutations brought by the potentially increased mutagenesis resulting from head-on collisions5. They reported empirical observations from comparative genomics that were purported to support their hypothesis5. Here we point out methodological flaws and errors in their analyses and logical problems of their interpretation. Our reanalysis of their data finds no evidence for the adaptive hypothesis.

The Balance Hypothesis for the Avian Lumbosacral Organ and an Exploration of Its Morphological Variation

Synopsis Birds (Aves) exhibit exceptional and diverse locomotor behaviors, including the exquisite ability to balance on two feet. How birds so precisely control their movements may be partly explained by a set of intriguing modifications in their lower spine. These modifications are collectively known as the lumbosacral organ (LSO) and are found in the fused lumbosacral vertebrae called the synsacrum. They include a set of transverse canal-like recesses in the synsacrum that align with lateral lobes of the spinal cord, as well as a dorsal groove in the spinal cord that houses an egg-shaped glycogen body. Based on compelling but primarily observational data, the most recent functional hypotheses for the LSO consider it to be a secondary balance organ, in which the transverse canals are analogous to the semicircular canals of the inner ear. If correct, this hypothesis would reshape our understanding of avian locomotion, yet the LSO has been largely overlooked in the recent literature. Here, we review the current evidence for this hypothesis and then explore a possible relationship between the LSO and balance-intensive locomotor ecologies. Our comparative morphological dataset consists of micro-computed tomography (μ-CT) scans of synsacra from ecologically diverse species. We find that birds that perch tend to have more prominent transverse canals, suggesting that the LSO is useful for balance-intensive behaviors. We then identify the crucial outstanding questions about LSO structure and function. The LSO may be a key innovation that allows independent but coordinated motion of the head and the body, and a full understanding of its function and evolution will require multiple interdisciplinary research efforts.

Upsetting the excitatory-inhibitory balance hypothesis of autism

Increased excitatory-inhibitory ratio of neuronal activity may be protective rather than pathological in mouse model of autism.

Attack rates ofSirex noctilioand patterns of pine tree defenses and mortality in northern Patagonia

Accidental and intentional global movement of species has increased the frequency of novel plant–insect interactions. In Patagonia, the European woodwasp,Sirex noctilio, has invaded commercial plantations of North American pines. We compared the patterns of resin defenses andS. noctilio-caused mortality at two mixed-species forests near San Carlos de Bariloche, Argentina. We observed lower levels of resin flow and higher levels of mortality inPinus contortacompared withPinus ponderosa. In general,S. noctilioattacked trees with lower resin compared with neighboring trees. Resin production inP. ponderosawas not related to growth rates, but forP. contorta, slower growing trees produced less resin than faster growing conspecifics. For all infested trees, attack density and number of drills (ovipositor probes) per attack did not vary with resin production. Most attacks resulted in one or two drills. Attack rates and drills/attack were basically uniform across the bole of the tree except for a decrease in both drills/attack and attack density in the upper portion of the crown, and an increase in the attack density for the bottom 10% of the tree. Planted pines in Patagonia grow faster than their counterparts in North America, and produce less resin, consistent with the growth-differentiation balance hypothesis. Limited resin defenses may help to explain the high susceptibility ofP. contortato woodwasps in Patagonia.