Diel Variation of Viral Production in a Coastal Subtropical Marine System

Viral production (VP) and bacterial mortality by viral lysis critically influence the production and mortality of aquatic bacteria. Although bacterial production, mortality by viral lysis, and viral density have been found to exhibit diel variations, the diel change in viral production has rarely been investigated. In this study, we conducted two diel dilution incubation experiments in a semi-enclosed, nutrient-rich coastal region in northeastern Taiwan to estimate the diel viral production and the mortality by viral lysis. We also compared two methods (linear regression between viral density and time versus arithmetic mean of VP during incubation) of estimating viral production. We found that viral production estimated by linear regression and bacterial mortality by viral lysis were higher during the daytime than during the nighttime. A possible explanation for the high viral production at daytime is that the bacterial community was composed of cell types with higher burst sizes at daytime. We further argued that the classical linear regression method can be used only when viral density significantly linearly increases with time, which does not always occur in dilution incubations. This study offered observations of diel variation in viral dynamics and discussed the methods estimating viral production in a marine environment.

Evaluation of the Mechanical and Biocidal Properties of Lapacho from Tabebuia Plant as a Biocomposite Material

The aim of this article is to discuss in detail the physicochemical properties of polylactide (PLA) reinforced by cortex fibers, which may cause bacterial mortality and increased biodegradation rates. PLA biocomposites containing cortex Lapacho fibers from Tabebuia (1–10 wt %) were prepared by extrusion and injection moulding processes. The effects of Lapacho on the mechanical and biocidal properties of the biocomposites were studied using tensile and impact tests, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), thermogravimetry (TG), and the method of evaluating the antibacterial activity of antibacterial treated according to the standard ISO 22196:2011. It also presented the effects of Lapacho on the structural properties and biodegradation rates of biocomposites. This research study provides very important results complementing the current state of knowledge about the biocidal properties of Lapacho from Tabebuia plants and about cortex-reinforced biocomposites.

Bacterial CRISPR abundance jumps up at around 45°C

Although functioning for adaptive immunity, CRISPR-Cas systems are present in only 40% of bacterial genomes. In this study, we observed an abrupt transition of bacterial CRISPR abundance at around 45°C. Phylogenetic comparative analysis showed that the CRISPR abundance correlates with growth temperature only in temperature ranges around 45°C. The existence of a saltation point indicates that temperature unlikely determines the thermal distribution of CRISPR-Cas systems directly. We put forward a novel hypothesis referring to the predator effect on immune function previously observed in birds. At around 45°C, an abrupt decrease of cellular predators rapidly decreases bacterial mortality resulting from cellular predator grazing. Consequently, the relative contribution of viral lysis in bacterial mortality increases rapidly with temperature; mutants allocating more resources to the adaptive immunity would be favored. In temperature ranges where the abundance of cellular predators does not change with temperature, the temperature effect on CRISPR abundance would be negligible.

Relative Contributions ofHalobacteriovoraxand Bacteriophage to Bacterial Cell Death under Various Environmental Conditions

ABSTRACTThe role of protists and bacteriophages in bacterial predation in the microbial food web has been well studied. There is mounting evidence thatBdellovibrioand like organisms (BALOs) also contribute to bacterial mortality and, in some cases, more so than bacteriophages. A full understanding of the ecologic function of the microbial food web requires recognition of all major predators and the magnitude of each predator’s contribution. Here we investigated the contribution ofHalobacteriovorax, one of the BALOs, and bacteriophages when incubated with their common prey,Vibrio vulnificus, in a seawater microcosm. We observed thatHalobacteriovoraxwas the greatest responder to the prey, increasing 18-fold with a simultaneous 4.4-log-unit reduction ofV. vulnificusat 40 h, whereas the bacteriophage population showed no significant increase. In subsequent experiments to formulate a medium that would support the predatory activities and replication of both predators, low-nutrient media favored the predation and replication of theHalobacteriovorax, whereas higher-nutrient media enhanced phage growth. The greatest prey reduction and replication of bothHalobacteriovoraxand phage were observed in media with moderate nutrient levels. Additional experiments show that the predatory activities of both predators were influenced by environmental conditions, specifically, temperature and salinity. The two predators combined exerted greater control onV. vulnificus, a synergism that may be exploited for practical applications to reduce bacterial populations. These findings suggest that along with bacteriophage and protists,Halobacteriovoraxhas the potential to have a prominent role in bacterial mortality and cycling of nutrients, two vital ecologic functions.IMPORTANCEAlthough much has been reported about the marine microbial food web and the role of micropredators, specifically viruses and protists, the contribution ofBdellovibrio-like predators has largely been ignored, posing a major gap in understanding food web processes. A complete scenario of the microbial food web cannot be developed until the roles of all major micropredators and the magnitude of their contributions to bacterial mortality, structuring of microbial communities, and cycling of nutrients are assessed. Here we show compelling evidence thatHalobacteriovorax, a predatory bacterium, is a significant contributor to bacterial death and, in some cases, may rival viruses as agents of bacterial mortality. These results advance current understanding of the microbial loop and top-down control on the bacterial community.

Importance of predation and viral lysis for bacterial mortality in a tropical western Indian coral-reef ecosystem (Toliara, Madagascar)

Size fractionation was performed using water from the Great Reef of Toliara (Madagascar) taken from two different habitats (ocean and lagoon) during the dry and wet seasons, to study the growth and mortality rates of bacterioplankton. Experiments were conducted with 1 and 100% of heterotrophic nanoflagellate (HNF) concentrations and virus-free water was obtained by tangential filtration (10kDa). During the dry season, in both environments, bacterial abundance and production were significantly lower than values recorded during the wet season. Bacterial growth rates without grazers were 0.88 day–1 in the lagoon and 0.58 day–1 in the ocean. However, growth rates were statistically higher without grazers and viruses (1.58 day–1 and 1.27 day–1). An estimate of virus-induced bacterial mortality revealed the important role played by viruses in the lagoon (0.70 day–1) and the ocean (0.69 day–1). During the wet season, bacterial growth rates without grazers were significantly higher in both environments than were values obtained in the dry season. However, the bacterial growth rates were paradoxally lower in the absence of viruses than with viruses in both environments. Our results suggest that changes in nutrient concentrations can play an important role in the balance between viral lysis and HNF grazing in the bacterial mortality. However, virus-mediated bacterial mortality is likely to act simultaneously with nanoflagellates pressure in their effects on bacterial communities.

Dynamics of auto- and heterotrophic picoplankton and associated viruses in Lake Geneva

Microbial dynamics have rarely been investigated in Lake Geneva, known as the largest lake in western Europe. From a 5-month survey, we report dynamic patterns of free-living virus, bacteria and small phytoplankton abundances in response to a variety of environmental parameters. For the first time, we fractionated the primary production to separate the contribution of different size-related biological compartments and measured both bacterial and viral production in addition to experiments conducted to quantify the virus-induced bacterial mortality. We observed marked seasonal and vertical variations in picocyanobacteria, bacteria and virus abundances and production. The contribution of picoplankton and nanoplankton production to the total primary production was high (reaching up to 76% of total primary production) in November and the spring–summer transition period, respectively. The impact of viral lysis on both bacteria and picocyanobacteria was significantly higher than grazing activities. Virus-induced picocyanobacterial mortality reached up to 66% of cell removal compared to virus induced (heterotrophic) bacterial mortality, which reached a maximum of 34% in July. Statistical analyzes revealed that temperature and top-down control by viruses are among important factors regulating the picocyanobacterial dynamics in this lake. More generally speaking, our results add to the growing evidence and accepted view nowadays that viruses are an important actor of freshwater microbial dynamics and more globally of the functioning of the microbial food webs.