Peribacteroid space acidification: a marker of mature bacteroid functioning in Medicago truncatula nodules

Legumes form a symbiotic interaction with Rhizobiaceae bacteria, which differentiate into nitrogen‐fixing bacteroids within nodules. Here, we investigated in vivo the pH of the peribacteroid space (PBS) surrounding the bacteroid and pH variation throughout symbiosis. In vivo confocal microscopy investigations, using acidotropic probes, demonstrated the acidic state of the PBS. In planta analysis of nodule senescence induced by distinct biological processes drastically increased PBS pH in the N₂‐fixing zone (zone III). Therefore, the PBS acidification observed in mature bacteroids can be considered as a marker of bacteroid N₂ fixation. Using a pH‐sensitive ratiometric probe, PBS pH was measured in vivo during the whole symbiotic process. We showed a progressive acidification of the PBS from the bacteroid release up to the onset of N₂ fixation. Genetic and pharmacological approaches were conducted and led to disruption of the PBS acidification. Altogether, our findings shed light on the role of PBS pH of mature bacteroids in nodule functioning, providing new tools to monitor in vivo bacteroid physiology.     

The role of canopy gaps in the regeneration of coastal dune forest

In regenerating coastal dune forest, the canopy consists almost exclusively of a single species, Acacia karroo. When these trees die, they create large canopy gaps. If this promotes the persistence of pioneer species to the detriment of other forest species, then the end goal of a restored coastal dune forest may be unobtainable. We wished to ascertain whether tree species composition and richness differed significantly between canopy gaps and intact canopy, and across a gradient of gap sizes. In three known‐age regenerating coastal dune forest sites, we measured 146 gaps, the species responsible for gap creation, the species most likely to reach the canopy and the composition of adults, seedlings and saplings. We paired each gap with an adjacent plot of the same area that was entirely under intact canopy and sampled in the same way. Most species (15 of 23) had higher abundance in canopy gaps. The probability of self‐replacement was low for A. karroo even in the largest gaps. Despite this predominance of shade‐intolerant species, regenerating dune forest appears to be in the first phase of succession with ‘forest pioneers’ replacing the dominant canopy species. The nature of these species should lead to successful regeneration of dune forest.     

A study of wall shear stress in 12 aneurysms with respect to different viscosity models and flow conditions

Recent computational fluid dynamics (CFD) studies relate abnormal blood flow to rupture of cerebral aneurysms. However, it is still debated how to model blood flow with sufficient accuracy. Common assumptions made include Newtonian behaviour of blood, traction free outlet boundary conditions and inlet boundary conditions based on available literature. These assumptions are often required since the available patient specific data is usually restricted to the geometry of the aneurysm and the surrounding vasculature. However, the consequences of these assumptions have so far been inadequately addressed.     

Species‐specific ontogenetic diet shifts among Neotropical Crenicichla: using stable isotopes and tissue stoichiometry

Ontogenetic diet shifts were compared among five sympatric pike cichlids Crenicichla in a subtropical South American stream using stable C and N isotopes and tissue stoichiometry (C:N). Within species, stable N isotopes were positively related to body size while C:N showed negative relationships. Stable C isotopes, however, were not related to body size in any species. By modelling the switch to piscivory using gut content‐isotope‐body size relationships, diet shifts were shown to be species‐specific with regard to both rate and degree of piscivory. Compared to other piscivorous lineages, Crenicichla appear to be unusually small‐bodied (based on maximum body size). Because of their diversity, abundance and dynamic size‐structured functional roles, Crenicichla may exert broad and complex predation pressures on the aquatic community.     

Identifying reproductive events using archival tags: egg‐laying behaviour of the small spotted catshark Scyliorhinus canicula

The use of archival depth telemetry as a means of remotely assessing the reproductive rates of free‐ranging fishes is explored. This is achieved by electronically tracking the vertical movements of individual female small spotted catsharks Scyliorhinus canicula in the natural environment, whilst simultaneously evaluating the temporal and vertical distributions of egg‐laying in this species. Distinctive patterns of short‐term (0·3–3·7 h), shallow‐water activity are documented in the time–depth profiles of female S. canicula that occur at an appropriate depth (1·0–2·3 m) and periodicity (every 10–12 days) to represent egg‐laying behaviour. Putative egg‐laying behaviour was exhibited simultaneously by two individually tracked female S. canicula during late‐spring and early‐summer. The results highlight that, provided species behaviour is suitable and complementary methods such as previous data, laboratory experiments and field surveys can be used to validate the patterns observed, archival depth telemetry offers an unobtrusive means by which egg production and egg‐laying behaviour of free‐living fishes can be estimated. As precise information regarding life‐history parameters is difficult to obtain for free‐ranging fish species, this technique could be used to improve the parameterization of species demographic models that are relevant to the management of wild fish populations.     

Changes in nasal airflow and heat transfer correlate with symptom improvement after surgery for nasal obstruction

Surgeries to correct nasal airway obstruction (NAO) often have less than desirable outcomes, partly due to the absence of an objective tool to select the most appropriate surgical approach for each patient. Computational fluid dynamics (CFD) models can be used to investigate nasal airflow, but variables need to be identified that can detect surgical changes and correlate with patient symptoms. CFD models were constructed from pre- and post-surgery computed tomography scans for 10 NAO patients showing no evidence of nasal cycling. Steady-state inspiratory airflow, nasal resistance, wall shear stress, and heat flux were computed for the main nasal cavity from nostrils to posterior nasal septum both bilaterally and unilaterally. Paired t-tests indicated that all CFD variables were significantly changed by surgery when calculated on the most obstructed side, and that airflow, nasal resistance, and heat flux were significantly changed bilaterally as well. Moderate linear correlations with patient-reported symptoms were found for airflow, heat flux, unilateral allocation of airflow, and unilateral nasal resistance as a fraction of bilateral nasal resistance when calculated on the most obstructed nasal side, suggesting that these variables may be useful for evaluating the efficacy of nasal surgery objectively. Similarity in the strengths of these correlations suggests that patient-reported symptoms may represent a constellation of effects and that these variables should be tracked concurrently during future virtual surgery planning.     

Temporal patterns in growth and survival of the round goby Neogobius melanostomus

Monthly, overwinter and annual instantaneous growth rates for round goby Neogobius melanostomus were calculated with maximal growth occurring in July and August and almost no growth observed between ice appearance (October) and melt (March). Annual absolute growth rates averaged 27·3 ± 1·9 mm for males and 19·8 ± 2·4 mm for females. The most parsimonious Cormack–Jolly–Seber model indicated that both the survival and recapture probabilities were dependent on sampling date, but not sex. Survival estimates remained high throughout the 13 month study with a median weekly survival probability of 0·920 (25 and 75% quartiles: 0·767 and 0·991), an overwinter survival probability of 99% and an annual survival rate of 67%. Survival probabilities were lowest for both sexes near the completion of the N. melanostomus reproductive season in July and August which supports existing evidence of higher mortality after reproduction, while challenging the paradigm that male N. melanostomus suffer comparatively higher mortality as a result of reproduction than females. Evidence indicating that growth and mortality rates are highest at the end of the reproductive season not only highlights seasonal variability in N. melanostomus natural history, but may also guide the control of this invasive species to periods when they are most vulnerable.     

Carbon allocation under light and nitrogen resource gradients in two model marine phytoplankton1

Marine phytoplankton have conserved elemental stoichiometry, but there can be significant deviations from this Redfield ratio. Moreover, phytoplankton allocate reduced carbon (C) to different biochemical pools based on nutritional status and light availability, adding complexity to this relationship. This allocation influences physiology, ecology, and biogeochemistry. Here, we present results on the physiological and biochemical properties of two evolutionarily distinct model marine phytoplankton, a diatom (cf. Staurosira sp. Ehrenberg) and a chlorophyte (Chlorella sp. M. Beijerinck) grown under light and nitrogen resource gradients to characterize how carbon is allocated under different energy and substrate conditions. We found that nitrogen (N)‐replete growth rate increased monotonically with light until it reached a threshold intensity (~200 μmol photons · m^?2 · s^?1). For Chlorella sp., the nitrogen quota (pg · μm^?3) was greatest below this threshold, beyond which it was reduced by the effect of N‐stress, while for Staurosira sp. there was no trend. Both species maintained constant maximum quantum yield of photosynthesis (mol C · mol photons^?1) over the range of light and N‐gradients studied (although each species used different photophysiological strategies). In both species, C:chl a (g · g^?1) increased as a function of light and N‐stress, while C:N (mol · mol^?1) and relative neutral lipid:C (rel. lipid · g^?1) were most strongly influenced by N‐stress above the threshold light intensity. These results demonstrated that the interaction of substrate (N‐availability) and energy gradients influenced C‐allocation, and that general patterns of biochemical responses may be conserved among phytoplankton; they provided a framework for predicting phytoplankton biochemical composition in ecological, biogeochemical, or biotechnological applications.     

Cellular interactions: lessons from the nitrogen‐fixing cyanobacteria

Marine nitrogen‐fixing cyanobacteria play a central role in the open‐ocean microbial community by providing fixed nitrogen (N) to the ocean from atmospheric dinitrogen (N₂) gas. Once thought to be dominated by one genus of cyanobacteria, Trichodesmium, it is now clear that marine N₂‐fixing cyanobacteria in the open ocean are more diverse, include several previously unknown symbionts, and are geographically more widespread than expected. The next challenge is to understand the ecological implications of this genetic and phenotypic diversity for global oceanic N cycling. One intriguing aspect of the cyanobacterial N₂ fixers ecology is the range of cellular interactions they engage in, either with cells of their own species or with photosynthetic protists. From organelle‐like integration with the host cell to a free‐living existence, N₂‐fixing cyanobacteria represent the range of types of interactions that occur among microbes in the open ocean. Here, we review what is known about the cellular interactions carried out by marine N₂‐fixing cyanobacteria and where future work can help. Discoveries related to the functional roles of these specialized cells in food webs and the microbial community will improve how we interpret their distribution and abundance patterns and contributions to global N and carbon (C) cycles.