Spatial and temporal bacterioplankton dynamics during destratification of the James River estuary, Virginia, USA

Bacterioplankton abundance and production were examined over the course of a destratification event in the lower James River, Virginia, USA, Goals of the study were to determine if destratification would influence temporal patterns of bacterioplankton parameters and relationships between bacterioplankton and other biological and abiological parameters. Mean bacterial abundance grouped over stations did not change over the course of the study, and were characterized by much smaller coefficients of variation than all other planktonic constituents. However, bacterial production measured by H-3-thymidine (H-3-Tdr) incorporation decreased significantly from a stratified (324 mu g Cl-1 d(-1)) to a destratified (187 mu g Cl-1 d(-1)) hydrography. The importance of bacterial-autotrophic coupling was also suggested from oxygen metabolism experiments, which indicated substrate limitation of bacteria, and the existence of a rapidly utilized photosynthetically produced substrate. Correlative relationships between bacterial parameters with chlorophyll a were significant during stratified hydrography, but diminished or became non-significant during destratified hydrography. Estimates of microzooplankton grazing rates upon bacteria decreased significantly during the onset of destratification. During the stratified hydrography, bacterial parameters displayed highly significant negative correlations to ammonium, however these relationships disappeared during the destratified hydrography. Results of this study indicate that destratification changes the trophic interactions of bacteria within the microbial loop, however these changes are not necessarily reflected by temporal patterns of bacterial abundance

Evolution of Mycobacterium ulcerans and other mycolactone-producing mycobacteria from a common Mycobacterium marinum progenitor

It had been assumed that production of the cytotoxic polyketide mycolactone was strictly associated with Mycobacterium ulcerans, the causative agent of Buruli ulcer. However, a recent study has uncovered a broader distribution of mycolactone-producing mycobacteria (MPM) that includes mycobacteria cultured from diseased fish and frogs in the United States and from diseased fish in the Red and Mediterranean Seas. All of these mycobacteria contain versions of the M. ulcerans pMUM plasmid, produce mycolactones, and show a high degree of genetic relatedness to both M. ulcerans and Mycobacterium marinum. Here, we show by multiple genetic methods, including multilocus sequence analysis and DNA-DNA hybridization, that all MPM have evolved from a common M. marinum progenitor to form a genetically cohesive group among a more diverse assemblage of M. marinum strains. Like M. ulcerans, the fish and frog MPM show multiple copies of the insertion sequence IS2404. Comparisons of pMUM and chromosomal gene sequences demonstrate that plasmid acquisition and the subsequent ability to produce mycolactone were probably the key drivers of speciation. Ongoing evolution among MPM has since produced at least two genetically distinct ecotypes that can be broadly divided into those typically causing disease in ectotherms (but also having a high zoonotic potential) and those causing disease in endotherms, such as humans

Internal versus external drivers of periodic hypoxia in a coastal plain tributary estuary: the York River, Virginia

The formation of periodic hypoxia within tributary estuaries, and its relationship to the spring-neap tidal cycle, has been well documented in several systems along the US east coast. However, the importance and scale of other physical and biological processes, which ultimately control the frequency and spatial extent of hypoxia, are less well understood. This study synthesized in situ measurements, metabolic incubations, and high-resolution water quality monitoring into a spatially explicit, temporally integrated mass balance to examine the significance of multiple organic matter sources and oxygen sinks in relation to hypoxia in the York River estuary (YRE), Virginia, USA. Results highlight episodic peaks in gross primary production (GPP) mostly unrelated to the spring-neap cycle, with phytoplankton accounting for the bulk of total GPP. Despite extensive shoals, microphytobenthos contributed under 20% and typically under 10% of total GPP. Respiration rates were sufficient to drive bottom waters to hypoxia during transitions from spring to neap tides. While GPP generally appeared to be relatively balanced with respiration, results indicated an area at the mesohaline-polyhaline boundary that was net heterotrophic. Phytoplankton production dominated the estimated inputs of organic carbon from the tributaries and surrounding watersheds, and was 1.5 times greater than inputs from the Chesapeake Bay, which roughly balanced exports. Management efforts to alleviate hypoxia should focus on reducing internal phytoplankton production, although inputs of labile organic matter from the Chesapeake Bay represent an important source that can only be controlled by more regional efforts

Characterization of photochromogenic Mycobacterium spp. from Chesapeake Bay striped bass Morone saxatilis

A large diversity of Mycobacterium spp. has been isolated from striped bass Morone saxatilis in Chesapeake Bay, USA. The new species M. shottsii and M. pseudoshottsii are the dominant isolates, while the classical fish pathogen M. marinum is found much less frequently. M. fortuitum and M. chelonae, other Mycobacterium spp. known to commonly infect fishes, have not yet been aseptically isolated from striped bass within Chesapeake Bay. While M. pseudoshottsii and M. shottsii have been phenotypically and genotypically characterized, other less common mycobacterial isolates have not. In the present study, we describe 17 photochromogenic isolates from Chesapeake Bay striped bass using phenotypic characterization and multilocus sequencing of 16S rRNA, hsp65 and rpoB genes. Genetic characterization reveals that these isolates are related to widely divergent portions of the mycobacterial phylogeny; however, some interesting trends are observed, such as a majority of isolates (10/17) belonging to the M. simiae-related grouping. Five additional isolates were assigned to the slow-growing mycobacteria (including 2 identified as M. marinum), while 2 are clearly shown to belong genetically to the fast-growing mycobacteria