Typical of other oceans, 6 types of microbes (single-celled microorganisms) occur in the Barents Sea: Archaea, Bacteria, Viruses, Fungi, Protista, and Microbial Mergers. In biogeochemical cycles of the ocean, a multitude of processes are catalyzed by Bacteria and Archaea; functioning of these cycles in the Barents Sea do not differ qualitatively from those at lower latitudes. The carbon cycle serves well as an example of a biogeochemical cycle (Figure 2.4.1).
Heterotrophic prokaryotes (denoted as bacteria for simplicity) are major degraders of dissolved organic carbon (DOC) — their principle source of energy and carbon. At high latitudes, DOC accumulates in the photic zone during the productive season; concentrations then decrease in September/October due to a combination of bacterial degradation and physical mixing processes (Børsheim and Myklestad, 1997; Børsheim, 2000). Primary production is the ultimate source of DOC, but all life processes contribute to the transfer of organismal carbon from primary producers into the pool of DOC (Børsheim et al., 2005). Grazing and predation produces fecal material which may be released as DOC, or occur as pellets. Fecal pellets typically sink to the seafloor to form sediments but may also become dissolved in the water column as DOC. The Barents Sea is fairly shallow and during winter the water column mixes from surface to bottom in many parts of the shelf basin. Thus, re-suspension of sediments and leaching of DOC accumulated in the sediments provides an additional source of DOC; this occurs primarily during winter. Figure 2.4.2 shows concentrations of DOC in the northern parts of the Barents Sea during July-August, 2007. Table 1 shows the depth distribution from the same expedition (Børsheim and Drinkwater, 2014).
Total bacterial abundance in the southeastern Barents Sea varies from 1.4·105 to over 106 cells ml-1. Highest total bacterial abundance occurs in coastal areas and zones having water masses with different characteristics than open ocean waters. Profiles of total counts usually show increased abundance in the thermocline layer and bacterial biomass can vary during the year up to twice the mean value; maximal rates are observed during spring-summer, and minimal rates observed during autumn-winter (Baytaz and Baytaz, 1987, 1991; Teplinskaya, 1990; Mishustina et al., 1997).
Table 2.4.1. Depth distribution of total organic carbon (TOC), µM C ±standard deviation. Number of samples shown in parentheses.
Bacterial production rates have been measured in the Polar Front region (Table 2). Production rates were highest in warm Atlantic Water, but decreased rapidly northwards as temperatures decreased (Børsheim and Drinkwater, 2014).
Table 2.4.2. Depth distribution of bacterial production rates, mmol C m-3 day-1.
Figure 2.4.2. Distribution of TOC integrated over the upper 20 meters (mmol C m−2) at the Polar Front in the Barents Sea. Stations analyzed for TOC are labeled as: red circles = Atlantic Water; blue circles = Arctic Water; red triangles = Front with mostly Atlantic Water below a fresher layer; and blue triangles = Front but mostly Arctic Water.
Parasitism by viruses also constitutes a source of DOC. This is illustrated by the reproductive cycle of lytic bacteriophages — viruses parasitizing bacteria (Figure 2.4.3). After infecting a bacterial cell and multiplying within that cell (at the cost of the bacterial metabolism), the host cell is destroyed allowing viral particles to be released into the water. As the cell breaks up, dissolved constituents are also released. Not only bacteria, but all other organisms from phytoplankton to mammals, are susceptible to viral attacks (Brussard et al., 2007; Frada et al., 2008; Marcussen and Have, 1992). Although bacteriophages have the extreme effect of completely destroying their hosts, the subsequent release of organic substances used by bacteria is a general consequence of viral infectivity.
For viruses, the probability of finding a host to infect depends on the hosts’ concentration. For this reason, dense populations are more likely to undergo epidemic viral infections than sparse populations. This concentration effect on microbial population dynamics has been called the “killing the winner” hypothesis (Thingstad and Lignell, 1997). Populations which are successful at nutrient acquisition and fast growth increase their abundance, but with the consequence of also increasing propagation of their viral parasites. The logical inference of this hypothesis is that viruses are important to keeping high diversity.
The life-history strategy of viruses is believed to include the ability to seize genes from their hosts and from other viruses, and then incorporate them to benefit their own existence (Mann et al., 2005). In addition, genes from viruses are sometimes incorporated into genomes of their hosts. It is believed that such horizontal transfers of genes between non-related organisms are mediated by viruses, and that this is an important factor in evolution (Biers et al., 2008; Lang and Beatty, 2007). Some genes transported by viruses are associated with pathogenic properties, and have been studied extensively. The gene for toxin production in the bacterium causing cholera is carried by a virus, changing harmless cells of the common estuarine bacterium Vibrio cholera into an extremely potent pathogen in humans (Waldor and
The sheer numbers of viruses are staggering; counted under a microscope numbers of viruses normally exceed numbers of bacteria by a factor of ten, approximately. Measured as genotypes, which is a fair proxy for species, there are more than 5,000 different types of viruses in 100 liters of seawater. In a kg of sediment, the number may approximate 1 million (Breitbart et al. 2002; 2004). Even more intriguing than the high diversity of viruses is the high diversity within their individual genomes. Clearly, every genotype consists of a variety of gene sequences with a variety of ages and origins (Dinsdale, 2008).
Both bacteria and viruses are highly variable and abundant in the Barents Sea (Figure 2.4.4). A sampling transect during midsummer showed that concentrations of viruses ranged from 5·108 to 6.4·1010 particles-per-liter; while bacterial total counts varied from 4·108 to 6·109 cells·l-1 (Venger et al., 2012; Howard-Jones et al, 2002). Viral abundance co-varied to a fair degree with bacterial abundance, except for the station farthest north, which was ice-covered (Table 2.4.3). In general, the dynamics of bacteria and viruses in this northern area do not differ from other parts of the Barents Sea, but the situation in northern ice-covered areas requires further investigation.