Viruses of Hydrothermal Vents. Abundance and Roles?

As mentioned in the previous blog I posted about virus-host interactions, it is only recently that the role marine viruses play within marine systems is becoming understood. However there are still a lot yet unknown and the authors of this current paper identify the distribution and abundance of viruses at deep-sea hydrothermal vents as an area lacking in knowledge. In this paper the authors examined the abundance and distribution of viruses within an active-vent field and associated plume with the aim of determining whether virus-mediated mortality is affecting microbial dynamics.

Virus mediated mortality of prokaryotes would affect carbon cycling, cellular components are converted in DOM by viral lysis, this makes it unavailable for grazing zooplankton but could potentially stimulate growth of heterotrophs. Previous studies have indicated that prokaryotic communities change as hydrothermal plumes age.

The authors point to previous studies investigating abundances of deep sea viruses looking at viral lysis such as (Cochlanetal.,1993;Haraetal., 1996) that found deep sea virus abundance to be 10-1000 fold lower than in surface waters and suggest that low temperatures and low availability of organic matter combined with a lower growth rate of prokaryotes will mean virus production will generally be lower in deep sea environments. They also suggest however that due to the relatively high microbial biomass and production hydrothermal vents may lead to increased amounts of lytic viral production and so may be an exception.

Hydrothermal vents often form along tectonically active mid-ocean ridges. They create unique conditions, and through the formation of plumes can influence a large area of the surrounding water. Chemically altered seawater is vented into the ocean at temperatures up to 400°C. Also unique are the organisms living there. Whereas the majority of deep ocean life relies on external energy, depending on small amounts of low-quality photosynthetic production rained down from higher in the water column, organisms of hydrothermal vents are supported mainly by in-situ production from chemosynthetic prokaryotes (both symbionts and free livers).

The study found a similar abundance of prokaryotes adjacent to black smokers and white smokers for diffuse-flow areas in the Galapagos Vent Field as a previous study but a 10-fold increase in abundance for diffuse-flow from the Main Endeavour Field. They also report that their estimates of prokaryotic abundance were approximately 2- and 100- fold higher than two previous studies of the same plume and 10-100-fold higher than a previous study at a different plume. Also abundance estimates from outside the plume were approximately 2-fold higher than most other deep-sea prokaryote abundances. The authors attribute the differences in abundance to the Yo-Pro method of enumeration that they used. They state that previous studies have estimated abundance by fixing samples in glutaraldehyde or formaldehyde where both prokaryotes and viruses decay quickly. The Yo-Pro method used in this study however is carried out on unfixed samples and decay of virus particles is not an issue. The authors backed this up by referencing another study that also used Y-Pro techniques and found similar results as this study, therefore suggesting that other studies that used preserved samples had underestimated abundances.

The study suggests that the distribution of prokaryotes and viruses around active hydrothermal vents is strongly influenced by temperature rather than the type of field sampled. Smokers have a higher temperature and were found to have a lower abundance of prokaryotes and viruses compared to diffuse flow samples. High temperatures of smoker faces prevent a lot of animal colonization. The higher animal biomass at diffuse-flow sites provides more surfaces and substrates for microbial growth. In relation to hydrothermal vent abundances compared to abundances of similar depths the authors report a 3.5-fold increase in prokaryotes and a 6-fold increase in virus abundance, showing that virus production and prokaryotic growth occurred at the vents.

The main conclusion made in this paper was that virus-mediated mortality of prokaryotes within hydrothermal-vent environments was significant and that the role viruses play in the deep ocean is a significant one. They may for example reduce energy flow for higher trophic levels.

Ortmann and Suttle (2005) High abundances of viruses in a deep-sea hydrothermal vent system indicates viral mediated microbial mortality. Deep-Sea Research I 52 (2005) 1515-1527