(Paerl, 1982) and higher than the 3 weeks reported by Hutchins et al. (1993) for microautoradiograms with natural phytoplankton communities. In these previous studies,

enough silver grains for useful microautoradiograms developed after a shorter exposure time than in our study. However, the maximum of cells associated with silver grains might not have been reached, as no detailed time series are reported in these previous studies. IDH inhibitor cancer Combining our results from the different dates and incubation times reveals a linear increase in the maximum fraction of DAPI cells with silver grains and the cellular 55Fe quota up to about 1 × 10−3 dpm per cell (Fig. 3). The highest fraction of cells associated with silver grains was observed in winter at Station POLA, and it was also linked to the duration of the 55Fe incubation. The environmental conditions, overall bacterial activity, in particular the bacterial iron demand, and the bacterial community composition were most likely different between sampling dates and could have influenced the amount of 55Fe incorporated by the bacterial cells. In several experiments, the maximum percent DAPI

cells with silver grains were < 5%, suggesting Selleck SB431542 that only a subset of iron-incorporating bacteria contained the critical cellular 55Fe quota for silver grain production. Our results strongly suggest that the 55Fe quota is a critical parameter for the production of useful microautoradiograms of heterotrophic bacteria, as was already pointed out by Fuhrman & Azam (1982). For 3H, a frequently used radioisotope that emits electrons with similar energy as 55Fe, this issue was never problematic because the activity per cell is estimated in the range 7–14 × 10−3 dpm per cell,

based on data from the same study area (Laghdass et al., 2010), and therefore much higher than the per cell activity observed in the present study. We applied our protocol in combination with CARD-FISH to natural bacterial communities collected at two contrasting sites. Samples from the NW Mediterranean Sea, at Station POLA, were collected during the summer period, when concentrations of major inorganic nutrients and chlorophyll a are low (Table 2). Station E-4W sampled in the Southern Ocean has characteristic features of high-nutrient, low-chlorophyll Thiamet G waters. To compare the within-assemblage distribution of 55Fe between the bacterial communities at these contrasting sites, experiments were carried out with the same incubation time and the same concentration of 55Fe. In addition, samples for microautoradiography coupled to catalyzed reporter deposition–fluorescence in situ hybridization (MICRO-CARD-FISH) have been chosen to harbor roughly the same amount of 55Fe per cell above the minimum 55Fe quota discussed previously. The percent total DAPI cells with silver grains in these experiments were on average 5.1 ± 2.7 (n = 12) and 3.4 ± 1.