It’s not too late to register for the EMBO (European Molecular Biology Organization) training course in Metagenomics. The course will be held at EMBL‘s German Laboratory which is located in one of my favorite cities: Heidelberg, Germany. Here is the link for more information about the course, and an overview of the discussion topics and instructors, and how to register for the course.
It’s been a busy summer, but I’m back to focusing on some recent research. In fact, there’s been a flurry of recent papers which I plan to highlight here. I’m exploring fungal and bacterial abundance in forest soils using pyrosequencing techniques with my own research, so I was interested to read this paper on bacterial activity in oceans off the Delaware coast.
In a study from the July 18th early online edition of the journal PNAS, researchers from the University of Delaware and University of Southern California sequenced the bacteria in seawater off the Delaware coast every month over the course of three years. The research, authored by Barbara Campbell and her colleagues, measured both 16s rDNA and rRNA using next generation pyrosequencing techniques. By measuring both the presence of DNA (a marker for species presence and overall abundance) and RNA (a marker for relative activity or, more accurately, ribosome activity) in this constantly shifting ecosystem, the authors hoped to explore and understand abundance of both rare and frequently found bacteria in a coastal ocean environment. I already told you about an article featuring the Rappemonad bacteria, some of which were studied in this paper.
It has been hypothesized in ocean ecosystems that abundant bacteria are found frequently because they have high growth rates and are better at competing against slower growing bacterial. Conversely, rare bacteria have long been considered to have slower growth rates, just be poor competitors to the more abundant bacteria, or have more streamlined genomes which are better suited to wait in dormancy until the right factor, most likely a specific nutrient, comes into play.
More than 600 OTUs (Operational Taxonomic Units – a term for individuals observed from the environment) were observed and these organisms formed a typical rank abundance curve that we have come to expect from environmental sampling, so there were no surprises in that finding.
What was more surprising, or should I say interesting, was what the authors found by comparing both DNA and RNA from their samples. After the quality control of their 454 pyrosequencing reads, the authors included more than 500,000 nucleotide samples in their analysis. More than half of the individual bacteria cycled between abundant and rare during the three years of sampling. Interestingly, almost half of the bacteria were always considered rare, and close to 12 percent remained rare and inactive, and less than 5 percent were considered to be always abundant throughout the sampling. The researchers used quantitative RT-PCR to validate specific DNA and RNA concentrations for five separate OTUs to verify the findings from the pyrosequencing portion of the study.
Also quite interesting was that the authors did not observe a pronounced seasonally affected microbial component or an environmental factor that could explain the abundance or scarcity in this ocean environment. It appears by all accounts that the microbial community observed in this study is constantly changing and may not be regulated by many other factors except the community itself. See here for a press release from the University of Delaware on this study.
Horizontal Gene Transfer (HGT) goes against what we typically consider the normal transfer of genetic material from parent to offspring. HGT involves the transfer of genetic material from one organism to another. Within the bacteria, whose mode of survival typically depends on phagocytosis, there is a fairly amount of HGT. Events of HGT have been rarely observed in Eukaryotes because numerous barriers exist to prevent foreign nucleotides from entering a cell’s nucleus. Some of these barriers in the Fungi include a substantial cell wall made of chitin, multiple cell and nuclear membranes to cross, and the secretion of metabolic enzymes to the outside of the cells and subsequent uptake of the nutrients. Despite these barriers, there is now evidence of multiple occurrences of HGT in the fungi.
In a recent article published in the journal Current Biology, Jason Slot and Antonis Rokas, both of Vanderbilt University, provided evidence of HGT in two Ascomycete clades. In this study, the authors identified a 23-gene cluster from the genus Aspergillus which relocated to the genus Podospora. Genes that are in this cluster synthesize the toxic compound, Sterigmatocystin, which is a precursor to aflatoxins, noted for their production in Aspergillus. Both genera are located in the subphylum Pezizomycotina, so each clade is not distantly related, but HGT was observed using different methods.
While it’s easy to observe genetic material passed from generation to generation, recognizing HGT is a little more difficult. The main way the researchers have identified HGT is using phylogenetic methods to identify gene clusters whose homology cannot be explained by lineage alone.
Thomas Richards points out in his commentary on the Slot & Rokas paper (also in Current Biology), that because fungi do not phagotrophically consume their food they are less likely to incur HGT event. There are two notable hypotheses to why we do see HGT in the fungi. First, many secondary pathway genes in Eukaryotes are encoded in gene clusters, and the fungi have a fair amount of these clusters. Gene clusters, which are more functional in a natural selection sense, are therefore more likely to persist upon transmission, as opposed to individual genes. Data from HGT studies in fungi support this hypothesis. Second, fungi are naturally, from the basis of their biology and natural history, intimately tied to other organisms, and fulfill roles as saprobes, pathogens, or symbionts. This close intimacy increases the opportunity for genes to transfer from one organism to another. Data suggests that this hypothesis is true also, as many of the recorded instances of HGT in fungi have been observed in organisms with overlapping environments.