Mitchell Sogin, a senior scientist at the Marine Biological Laboratory in Woods Hole, Mass., discussed the role of the “rare biosphere” at the microbiology and immunology seminar this Tuesday at Dartmouth Medical School.
The rare biosphere refers to the immense diversity of bacterial genetics. Sogin seeks to uncover the full spectrum of bacteria residing in the Earth’s oceans. His findings suggest that the number of different microbes in the ocean, barring viruses and phages, could be as high as 3.6×1031.
Sogin used 454 parallel tag sequencing to show that bacterial communities in the North Atlantic are substantially more diverse than previous research has shown.
454 parallel tag sequencing is a fast, accurate, and cost efficient method of DNA sequencing. It utilizes a method known as pyrosequencing, which detects pyrophosphate release upon nucleotide incorporation. The system synthesizes DNA complementary to the desired sequence and then reports which base pairs are used, with greater speed and accuracy than previous methods.
Past estimates placed oceanic diversity at roughly 106 organisms. This vast underestimation was caused by extrapolation from ocean samples in which only majority populations of bacteria were detected, Sogin said.
Operational taxonomic units (OTUs) are used to identify different bacterial organisms. OTUs that represent minor groups of bacteria require substantially larger surveys, which are now possible due to the increased efficiency of the 454 sequencing system.
Sogin investigated the reasons for the rarity of certain bacterial species within sampled ocean water. He discussed the bacteria’s quorum-sensing, which is a population-based bacterial growth regulation mechanism.
Sogin asked if certain bacterial populations are perpetually rare, and if so, whether or not this is an evolved trait.
Evidence to answer these questions comes from research on rats treated with antibiotics. Normal intestinal bacterial populations almost completely disappeared in treated rats. However, without the natural competition of surrounding populations of bacteria, certain rare species of microorganisms grew to extreme levels.
Elucidating mechanisms of bacterial population equilibrium holds promise not only for further understanding microorganism evolution, but also for development of more complete strategies of sterilization and disease fighting.
Sogin’s research reveals a seemingly unlimited amount of bacterial diversity that could offer key insights about the Earth’s ecological past and provide new genetic tools for the future.