BIOGEOCHEMICAL CYCLE OF ORGANIC MATTER IN COASTAL MARINE ENVIRONMENT: THE ROLE OF VIRUSES IN CONTROLLING BACTERIAL PROLIFERATION

KARUZA, ANA

BIOGEOCHEMICAL CYCLE OF ORGANIC MATTER IN COASTAL MARINE ENVIRONMENT: THE ROLE OF VIRUSES IN CONTROLLING BACTERIAL PROLIFERATION

KARUZA, ANA

2006-04-20

http://thesis2.sba.units.it/store/handle/item/13296

http://hdl.handle.net/10077/11513

Doctoral Thesis

Contributor(s)

FONDA, SERENA

•

DEL NEGRO, PAOLA

Abstract

The main goal of the present study was to extend the knowledge relatively to the role of virioplankton in microbial food webs and biogeochemical fluxes. The first section of the study describes spatial and temporal virioplankton distribution in the Northern Adriatic Sea. Samplings were carried in a coastal station of the Gulf of Trieste at different depths. In the first period samplings were carried out on a monthly basis and were subsequently intensified to a twice a month sampling frequency. The obtained data implemented the time series of virioplankton abundances present in data base of Marine Biology Laboratory (Trieste), which were monitored from 1998 at the same sampling station. The obtaining of the time series was necessary to define the 'normality' state of the system regarding viral component of marine plankton in order to detect eventual 'alterations', such as mucilage formation, known to sporadically interest in its particularly massive form the northern basin of the Adriatic Sea, seriously affecting turism and fishery. The analysis of spatial virioplankton distribution on extended sampling area, conducted over 3 seasonal oceanographic cruises during 2004 in the Northern Adriatic basin, in collaboration with Veneto Interreg Project, provided the opportunity to explore the functioning of 'microbial circuit' and significance of viruses under contrasting environmental conditions, since the Northern Adriatic Sea displays highly evident trophic gradient due to the Po River inflow. In fact, the study area played a role of natural laboratory since the introduction of hydrological parameters strongly affected the distribution of microbial components and their interactions, highlighting the adaptability of the basin in 'cause-effect' studies. The results obtained by spatial sampling strategy did not overlap with those obtained by long-term temporal study in a coastal station of the Northern Adriatic, but extended the information amount: different methodological strategies allowed us to acquire precious findings regarding viral population's dynamics, since viral interaction with their hosts is particularly difficult to define because of the variety of interaction types. The investigation of most common viral ecology indexes, largely used to evaluate virus-host interaction, such as VBR (Virus-to-Bacterium Ratio) and phage-host density product ( analogically named VBP from Virus-to-Bacterium Product ), evidenced the alteration of virus and bacteria interactions before, during and after mucilage formation relatively to spring-summer period of 2000. The obtained results evidenced the presence of particularly enhanced viral lysis that strongly affected bacterial community, which, moreover, in that period was characterized by low species diversity. The experimental approach allowed the evaluation of the entity of viral lysis, which not only provoke bacterial mortality but also strongly affects carbon flux in the pelagic system and influences the energy transfer in the food web. Several experiments were performed in order to evaluate the depending of viral activity upon host cell metabolism and organic substrate availability of microbial community. Moreover, mesocosm experiments allowed evaluating the role of viruses in the presence of different availability of inorganic phosphorous, particularly important since representing the limiting factor for plankton community in the Adriatic Sea. The results obtained both by field studies and experimental approach confirmed the temporal stability of virioplankton abundance in marine system and highlighted the need of informations relatively to virioplankton balance between its production and decay rates. Since there is no standard method for the estimate of viral production, three different experimental protocols were set up in order to compare the accuracy of the obtained results and to establish the usefulness of a single technique. The first technique provides an estimate of virus production rates by radiotracer incorporation method that measures production of DNA-containing viral progeny over incubation period. Another method provides the rates of virus production of the bacteria infected prior to the beginning of the experiment, while the serial dilution technique designed originally for the estimate of grazing activity was adopted and modified to determine virus production in natural phage-host assemblage. This experimental framework, together with the experiment set up to evaluate virus-mediated mortality of different bacterial groups ( distinguished according to their metabolism), provided new findings relatively to viral impact on bacterioplankton and allowed the estimate of viral production in different environmental conditions. Molecular biology techniques, such as PCR (Polymerase Chain Reaction) e DGGE (Denaturant Gradient Gel Electrophoresis) were used to verify the influence of virus-mediated mortality on the bacterial community structure.