Posted by ycpmicro
An interesting object lesson on the scientific process here (Thanks to John Frizalone, 11 AM BIO230 for the link!) So what’s going on with arsenic today? Last week’s NASA press conference generated a lot of buzz around the Internet. I summarized the findings of the scientific paper, and the potential impact of the paper here on the YCP micro blog. The upshot of it was that a group of NASA researchers found that a terrestrial halophilic bacterium was potentially able to incorporate arsenic into biological macromolecules, a biological activity that was heretofore unheard of. This finding was met with a range of responses, some of which were very negative. There’s an old adage that “extraordinary claims require extraordinary proof,” and this is no exception.
The original scientific paper used a several techniques to demonstrate the presence of arsenic in DNA. The main principle behind their experiments is the concept of radiolabeling, where radioactive arsenic is provided to the organism while it is growing under conditions where there is extremely low levels of available phosphorus. The levels of phosphorus present in the growth media preclude sufficient levels to promote cellular growth. One then recovers bacteria after they have had a chance to grow with the radioactive arsenic, and then one looks to see if any radioactivity can be detected in molecules such as DNA. If radioactive DNA is obtained, that is used as evidence that the arsenic has become part of the DNA. The main criticism to this approach however is that unless the radioactivity is physically demonstrated to be a part of the DNA, the radioactive arsenic may only be ‘hanging on’ to the DNA and carrying through the purification process, kind of like the candy coating on an M&M. The candy coating is not part of the chocolate, though it is not easily removed from the final product.
Living cells require phosphorus to make nucleic acids and phospholipids, and prokaryotes are VERY good at getting the things that they need to grow. The NASA scientists have created a situation where a microbe is put into an environment where there is approximately 10,000 fold more arsenic than phosphorus present, a growth condition that is toxic for all other living cells, however phosphorus is not absent. The scientists then examined what happened to the arsenic, and found it inside the cells, leading them to the conclusion that arsenic had become a part of the cell. However, the growth rate of the bacteria under these conditions was not very rapid, with a doubling time of 1.1 day (27 hrs) in comparison to a growth rate of 0.7 d (17 hrs) under phosphorus containing conditions. This is not a hugely significant difference. The biomass obtained by the cultures in arsenic containing media was 10-fold less than with phosphorus-containing media, that is, there were ten times more bacteria in stationary phase when phosphorus was present in comparison to arsenic. This is a pretty big difference, and has led to the alternative hypothesis that the small amounts of phosphorus available to the bacteria are being grabbed up and stored as effectively as possible to promote growth. Arsenic found inside the bacteria would therefore be due to it being hugely more abundant in the environment in comparison to phosphorus, and being pulled along for the ride in the experimental analysis.
So where does this leave arsenic in living cells? I must admit that I found the initial announcement last very exciting, and personally would find it academically interesting if the findings are supported as future experiments are carried out. The data and conclusions published by the NASA researchers will need to be confirmed by other laboratories, and the possibility that arsenic is an experimental contaminant needs to be eliminated. Here’s an experiment I would do: I would put a defined piece of DNA (a plasmid) into the arsenic-loving bacterium and grow it under conditions that would promote the inclusion of arsenic into DNA. I would do the same experiment under regular growth conditions, and collect the plasmid DNA from both. Because arsenic is a heavier atom than phosphorus, the plasmid DNA (which will be identical in DNA sequence in both conditions) would be heavier if it contains arsenic. An experiment like this was performed by Meselson and Stahl to demonstrate the nature of DNA replication in all cells.
About ycpmicroMy name is David Singleton, and I am an Associate Professor of Microbiology at York College of Pennsylvania. My main course is BIO230, a course taken by allied-health students at YCP. Views on this site are my own.
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