Strong, weak, and missing links in a microbial community of the N.W. Mediterranean Sea

Abstract

Planktonic microbial communities often appear stable over periods of days and thus tight links are assumed to exist between different functional groups (i.e. producers and consumers). We examined these links by characterizing short-term temporal correspondences in the concentrations and activities of microbial groups sampled from 1 m depth, at a coastal site of the N.W. Mediterranean Sea, in September 2001 every 3 h for 3 days. We estimated the abundance and activity rates of the autotrophic prokaryote Synechococcus, heterotrophic bacteria, viruses, heterotrophic nanoflagellates, as well as dissolved organic carbon concentrations. We found that Synechococcus, heterotrophic bacteria, and viruses displayed distinct patterns. Synechococcus abundance was greatest at midnight and lowest at 21:00 and showed the common pattern of an early evening maximum in dividing cells. In contrast, viral concentrations were minimal at midnight and maximal at 18:00. Viral infection of heterotrophic bacteria was rare (0.5-2.5%) and appeared to peak at 03:00. Heterotrophic bacteria, as % eubacteria-positive cells, peaked at midday, appearing loosely related to relative changes in dissolved organic carbon concentration. Bacterial production as assessed by leucine incorporation showed no consistent temporal pattern but could be related to shifts in the grazing rates of heterotrophic nanoflagellates and viral infection rates. Estimates of virus-induced mortality of heterotrophic bacteria, based on infection frequencies, were only about 10% of cell production. Overall, the dynamics of viruses appeared more closely related to Synechococcus than to heterotrophic bacteria. Thus, we found weak links between dissolved organic carbon concentration, or grazing, and bacterial activity, a possibly strong link between Synechococcus and viruses, and a missing link between light and viruses.

Dynamics and diversity of newly produced virioplankton in the North Sea

“…Diel dynamics in VP were detectable although not pronounced (Table 1), probably due to the low diel dynamics in bacterial activity in the North Sea (Winter et al, 2004). More pronounced diel patterns in bacterioplankton production have been reported for oligotrophic systems such as the Mediterranean Sea (Bettarel et al, 2002). The short-term variability observed in the viral infection cycle in the coastal North Sea as the presence and absence of distinct bands in the newly produced viral populations was evident on a time scale of days ( Figure 5).…”

“…Considering that the richness of bacteriophages in coastal surface waters is relatively high (Wommack et al, 1999b) and that the decay of viruses in surface waters is in the range of 1-2 days (Heldal and Bratbak, 1991;Wilhelm et al, 1998), a rapid succession of infection targets can be assumed and consequently, a tight control of bacterioplankton evenness by bacteriophages. These temporal dynamics in viral infection and phage production have been addressed only on short-time scales (Bettarel et al, 2002;Winter et al, 2004) but viral diversity was not analyzed thus far and is subject of this study.…”

The diversity of cyanomyovirus populations along a North–South Atlantic Ocean transect

“…Crucially, our study assessed host and viral diversity simultaneously-samples that were discrete in both time and space. A further factor that may help to explain the lack of correlation is that in surface waters, viral turnover rates are variable, in the order of hours (Bettarel et al, 2002;Clokie et al, 2006) to a few days (Parada et al, 2008). For these reasons, it may be unreasonable to expect that a single snapshot will reveal infection dynamics.…”

Virus isolation studies suggest short-term variations in abundance in natural cyanophage populations of the Indian Ocean

“…The studies disagree with each other as to when viral abundance reaches a maximum (Bettarel et al, 2002). One of the most comprehensive surveys of marine microbial communities was carried out by Bettarel who showed that natural viral abundance peaked at around 1800 h (Bettarel et al, 2002).…”