Viral and microbial loop dynamics in the water column and sea-ice of Prydz Bay

Brief description

Metadata record for data from ASAC Project 2751
See the link below for public details on this project.

Public
Viruses are tiny particles that cannot reproduce by themselves. To reproduce they have to parasitise a bacterial cell, or another organism. In the sea viruses infect bacteria and phytoplankton cells and can cause those cells to die and break open, thereby liberating more virus particles into the environment to re-infect more host cells. They effectively short-circuit the carbon cycle - recycling carbon to the pool of dissolved and particulate organic carbon before it can be eaten by organisms higher in the food chain. Our research will elucidate the role of viruses in the water column and sea-ice over a year.

Taken from the 2008-2009 Progress Report:
Project objectives:
BACKGROUND
Since the microbial loop was first described, a wealth of data has appeared on the species composition and interactions among bacterioplankton and Protozoa, both heterotrophic and mixotrophic, and their role in biogeochemical cycling in marine and lacustrine environments. An additional dimension to the microbial loop was discovered when high concentrations of viruses (bacteriophage) were first described from marine samples. The supposition was that infected bacteria might be lysed, and their carbon returned to the pool before it could be grazed by Protozoa, short-circuiting the microbial loop. Both heterotrophic bacteria and cyanobacteria were found to be infected by viruses and later work revealed that viruses may also attack algae and protists, but the database on the viruses of these groups is far less detailed than for bacteriophages.

Viruses are now the focus of considerable attention in aquatic environments. The role of viruses is more complex than simply causing the mortality of bacteria and phytoplankters. Viruses also play a role in maintaining the clonal diversity of host communities through gene transmission (transduction), and indirectly by causing the mortality of dominant host species. Moreover, viruses can act as a potential source of food for heterotrophic and mixotrophic flagellates. Based on decay rates an ingestion of 3.3 viruses per flagellate h-1 was calculated, and experiments with fluorescently labelled microspheres demonstrated that nanoflagellates may gain significant carbon through ingesting viruses.

Early studies suggested that the majority of viruses in marine waters were lytic. More recently lysogeny has been found in both marine and freshwater systems ranging up to 71% in both marine and freshwaters. Thus aquatic viruses may exist in a lysogenic condition within their hosts where they replicate and are passed on in the host's progeny during division. This condition may continue until a factor, or a combination of factors, initiates the lytic cycle. Clearly it is disadvantageous to embark on a lytic cycle when the concentration of potential hosts is low.

Long term seasonal studies of viruses and their potential hosts are relatively few, and have focussed on a specific aspects, for example the abundance of lysogenic bacteria in an estuary and Lake Superior and viral control of bacterial production in the River Danube. A recent study of annual patterns of viral abundance and seasonal microbial plankton dynamics in two lakes in the French Massif Central, suggested that a weakened correlation between viruses and bacteria in the more productive of the two lakes was indicative of an increase in non-bacterial hosts as trophic status increased.

We have conducted a year long study of virus dynamics in three of the saline lakes in the Vestfold Hills, our hypothesis being that they may be regarded as a proxy for the marine environment, but with the difference that top-down control is lacking in food webs that are microbially dominated. Our results revealed that virus numbers showed no clear seasonal pattern and were high in winter and summer (range 0.89 x 107 plus or minus 0.038 mL-1 to 12.017 x 107 plus or minus 1.28 mL-1). However, the lysogenic cycle was predominant in winter (up to 73% of the bacteriophage were lysogenic), while in summer the lytic cycle dominated. There was a strong negative correlation between virus numbers and photosynthetically active radiation. Viruses are subject to destruction or decay when subjected to full sunlight, even when UV- B radiation is excluded. During summer in Antarctica there is 24 hour daylight as well as significant UV-B radiation in spring and early summer when one might expect high levels of viral decay. UV-B radiation penetrates lake ice and the water column, though attenuation is rapid. PAR and UV-B penetration to the water column increases as the ice thins. It is likely that low decay rates in winter allowed the survival and build up of VLP numbers, while in summer when the lytic cycle predominated, decay rates were high. Seasonal variation in decay rates may in part account for the poor correlation between bacterial numbers and VLP in our study. High virus to bacteria ratios in the saline lakes (reaching 115 in Pendant Lake) and viral production rates comparable to those seen in temperate lakes suggest that viruses may play an important role in these microbially dominated extreme environments.

Data from Antarctic marine waters are limited. Bacteria to virus ratios ranged between 15 - 40 in the sea-ice region, but were lower (3-15) in the open ocean. Higher ratios under ice may indicate that ice and its impact on light climate, reduces viral decay rates and enhances the ratios between bacteria and viruses. The sea-ice itself provides another habitat for bacteria and their viral parasites with abundances of viruses reaching 109 mL-1.

OVERALL AIM
We wish to undertake a year long study in the inshore marine environment in Prydz Bay focussing on viral dynamics in relation to microbial loop functioning. We will investigate the water column and the communities within the sea-ice. Within the context of the International Polar Year it is important that we further knowledge of microbial processes in the Southern Ocean. Changes in the length and thickness of ice-cover in response to climate warming and the impact on the sea-ice community, may have knock on impacts on water column microbial community and carbon cycling.

SPECIFIC OBJECTIVES:
1. The quantify viral dynamics (numbers, production and levels of lysogeny) within the context of the microbial loop processes in the water column and sea-ice of Prydz Bay over an annual cycle.
2. To link viral/bacterial dynamics to physical and chemical parameters such as temperature, UV radiation, Photosynthetically Active Radiation, dissolved organic carbon (DOC) and total organic carbon (TOC) and inorganic nutrients. (N and P).
3. To ascertain linkages between microbial processes iin the sea-ice and water column, particularly during the melt phase.
4. To ascertain the effects of UV-B on viral decay rates below ice and in the open water phase.

Progress against objectives:
Detailed time series sampling of the sea ice in Prydz Bay has been completed. Bacterial production and viral production, along with the level of lysogeny were conducted. Abundances of viruses, bacterial and nanoflagellates have been completed. Chlorophyll, DOC and TOC, inorganic nutrients also completed. Ciliate samples are still to analysed as are frozen preparations from viral production and lysogeny experiments.

Lineage

Progress Code: completed

Statement: The Dates provided in temporal coverage are approximate only, and represent the beginning and end of the 2007 - 2010 Antarctic seasons.

The latitudes and longitudes provided in spatial coverage are approximate only.

See the word document in the file download for further information.

Taken from the 2008-2009 Progress Report:
Field work:
Three sites in Prydz Bay were sampled at two week intervals following the formation of the sea ice until its breakout.

Laboratory activity/analysis:
A significant amount of analysis was undertaken at Davis during the course of the project, however the workload was significant for a single postdoctoral scientist. Consequently a portion of material is being returned to the University of Tasmania to complete the sample analysis. This will occur in the next 6 months.