Heterotrophic bacterial turnover along the 20°W meridian between 59°N and 37°N in July 1996

Heterotrophic bacteria were enumerated by flow cytometry in samples from 200 m depth profiles along the 20°W meridian between 59°N and 37°N during June and July 1996. Bacterial volume estimates made by size fractionation were used in the determination of heterotrophic bacterial biomass. Concentrations of heterotrophic bacteria in the surface mixed layer were close to 106 ml?1 along most of the transect, but decreased to about 0.2×106 ml?1 at the southern end; this corresponded with a biomass range between about 2 and 15 mg C m?3. Concentrations also decreased to 0.1–0.3×106 ml?1 below the top mixed layer. Production of heterotrophic bacteria was measured in samples from eight profiles by following the simultaneous incorporation of isotopically labelled thymidine and leucine; in the mixed layer it ranged between a maximum of 1.5–2 mg C m?3 d?1 in the region of a frontal system near 50°N and 0.2–0.25 mg C m?3 d?1 at the southern end of the transect, with bacterial growth rates generally about 0.1 d?1. The daily production of heterotrophic bacteria integrated for the euphotic layer ranged between about 5 and 15% of the daily 14C primary production of phytoplankton. Heterotrophic nanoplankton biomass reflected the bacterial biomass and was about half as large; it showed a strong correlation with the production of heterotrophic bacteria. At the station with the highest bacterial biomass, the rate of grazing of bacteria by heterotrophic nanoplankton was 5–6% d?1, but at all other stations it was too low (&lt;5% d?1) to be measured reliably by the techniques used. The data suggest that the nanoplankton grazed at least half of the bacterial production.<br/

Microbial community structure and standing stocks in the NE Atlantic in June and July of 1996

The standing stocks of nanophytoplankton and picoplankton in the northeast Atlantic Ocean in June and July 1996 were quantified using flow cytometry and microscopy. Diatoms and dinoflagellates were analysed by microscopy and coccolithophores, other nanophytoplankton, picoeukaryotic phytoplankton, cyanobacteria (Synechococcus spp.), prochlorophytes (Prochlorococcus spp.) and heterotrophic bacteria by flow cytometry. The research was divided into three components: a lagrangian study of a nutrient replete cold-core eddy centred around 59° 12?N 20° 12?W; a transect close to the 20°W meridian from 59° 18?N to 37°N, which passed through contrasting water masses; and a lagrangian study in oligotrophic waters, centred around 36° 42?N 19° 12?W. The eddy was characterised by a bloom of the coccolithophore Coccolithus pelagicus whose standing stocks averaged 4.26 g C m?2 over the upper 50 m. C. pelagicus and other nanophytoplankton (excluding diatoms and dinoflagellates) dominated the standing stocks of the microbial community, averaging approx. 70% of the total microbial standing stocks of the groups quantified. The majority of the remaining biomass was accounted for by the picoeukaryotic phytoplankton and heterotrophic bacteria. The microbial community immediately outside the eddy was significantly different in both composition and standing stocks. There were no C. pelagicus outside the eddy and fewer nanophytoplankton, resulting in microbial standing stocks of approx. one-third that found in the eddy. The transect was characterised by a frontal region at approx. 52° 30?N. There was a general decrease in the standing stocks of all components of the microbial community from the start of the transect to the front. Just to the south of the front, nanophytoplankton, Synechococcus spp. and heterotrophic bacteria showed marked increases in standing stocks, especially the nanophytoplankton, which increased from 3.43 to 7.90 g C m?2. The nanophytoplankton dominated the microbial standing stocks throughout the transect, even in the oligotrophic waters where the integrated carbon biomass was 4.58 g C m?2, representing 69% of the total microbial standing stocks. During the lagrangian study around 37°N the picoplanktonic community was dominated by heterotrophic bacteria. However, heterotrophic bacteria standing stocks decreased with time, along with Synechococcus spp. and picoeukaryotic phytoplankton. Peak biomass for these three groups shifted deeper down in the water column with time. Prochlorococcus spp. were only present towards the end of the transect and at the oligotrophic site. At the oligotrophic site their standing stocks increased, unlike other groups, so that they became the dominant picophytoplanktonic group.<br/

Morphological variability is greater at developing than mature mouse neuromuscular junctions

The neuromuscular junction (NMJ) is the highly specialised peripheral synapse formed between lower motor neuron terminals and muscle fibres. Post‐synaptic acetylcholine receptors (AChRs), which are found in high density in the muscle membrane, bind to acetylcholine released into the synaptic cleft of the NMJ, thereby enabling the conversion of motor action potentials to muscle contractions. NMJs have been studied for many years as a general model for synapse formation, development and function, and are known to be early sites of pathological changes in many neuromuscular diseases. However, information is limited on the diversity of NMJs in different muscles, how synaptic morphology changes during development, and the relevance of these parameters to neuropathology. Here, this crucial gap was addressed using a robust and standardised semi‐automated workflow called NMJ‐morph to quantify features of pre‐ and post‐synaptic NMJ architecture in an unbiased manner. Five wholemount muscles from wild‐type mice were dissected and compared at immature (post‐natal day, P7) and early adult (P31−32) timepoints. The inter‐muscular variability was greater in mature post‐synaptic AChR morphology than that of the pre‐synaptic motor neuron terminal. Moreover, the developing NMJ showed greater differences across muscles than the mature synapse, perhaps due to the observed distinctions in synaptic growth between muscles. Nevertheless, the amount of nerve to muscle contact was consistent, suggesting that pathological denervation can be reliably compared across different muscles in mouse models of neurodegeneration. Additionally, mature post‐synaptic endplate diameters correlated with fibre type, independently of muscle fibre diameter. Altogether, this work provides detailed information on healthy pre‐ and post‐synaptic NMJ morphology from five anatomically and functionally distinct mouse muscles, delivering useful reference data for future comparison with neuromuscular disease models