Growth is one of the basic attributes of any living organism. Surprisingly, the growth rates of marine bacterioplankton are only poorly known. Current data suggest that marine bacteria grow relatively slowly, having generation times of several days. However, some bacterial groups, such as the aerobic anoxygenic phototrophic (AAP) bacteria, have been shown to grow much faster. Two manipulation experiments, in which grazing, viruses, and resource competition were reduced, were conducted in the coastal Mediterranean Sea (Blanes Bay Microbial Observatory). The growth rates of AAP bacteria and of several important phylogenetic groups (the Bacteroidetes, the alphaproteobacterial groups Roseobacter and SAR11, and the Gammaproteobacteria group and its subgroups the Alteromonadaceae and the NOR5/OM60 clade) were calculated from changes in cell numbers in the manipulation treatments. In addition, we examined the role that top-down (mortality due to grazers and viruses) and bottom-up (resource availability) factors play in determining the growth rates of these groups. Manipulations resulted in an increase of the growth rates of all groups studied, but its extent differed largely among the individual treatments and among the different groups. Interestingly, higher growth rates were found for the AAP bacteria (up to 3.71 day ؊1 ) and for the Alteromonadaceae (up to 5.44 day ؊1 ), in spite of the fact that these bacterial groups represented only a very low percentage of the total prokaryotic community. In contrast, the SAR11 clade, which was the most abundant group, was the slower grower in all treatments. Our results show that, in general, the least abundant groups exhibited the highest rates, whereas the most abundant groups were those growing more slowly, indicating that some minor groups, such the AAP bacteria, very likely contribute much more to the recycling of organic matter in the ocean than what their abundances alone would predict.The structure of bacterioplankton communities is defined by the type of organisms and by their relative proportions. Marine surface waters are typically composed of a few abundant groups, generally members of the Alphaproteobacteria and Gammaproteobacteria and the phylum Bacteroidetes (17), and many low-abundant taxa (32). The sizes of the different populations depend to a large extent on their growth rates, which can range from organisms that are almost inactive or dormant to cells growing very rapidly (12). In addition to the differences in activity between individual cells (40), recent evidence suggests that variability among different bacterioplankton groups also occurs (42,44,45). Since actively growing bacteria are responsible for major carbon and nutrient transformations in the ocean, determining the growth rates of individual groups is critical to understand their ecological roles and specific contributions to marine biogeochemical cycles.One of the groups that has been reported to grow at high rates in the ocean is the aerobic anoxygenic phototrophic (AAP) bacteria, which are phot...

The distribution of aerobic anoxygenic phototrophs (AAPs) was surveyed in various regions of the Mediterranean Sea in spring and summer. These phototrophic bacteria were present within the euphotic layer at all sampled stations. The AAP abundances increased with increasing trophic status ranging from 2.5 × 10(3) cells per ml in oligotrophic Eastern Mediterranean up to 90 × 10(3) cells per ml in the Bay of Villefranche. Aerobic anoxygenic phototrophs made up on average 1-4% of total prokaryotes in low nutrient areas, whereas in coastal and more productive stations these organisms represented 3-11% of total prokaryotes. Diel bacteriochlorophyll a decay measurements showed that AAP community in the Western Mediterranean grew rapidly, at rates from 1.13 to 1.42 day(-1). The lower AAP abundances registered in the most oligotrophic waters suggest that they are relatively poor competitors under nutrient limiting conditions. Instead, AAPs appear to be metabolically active organisms, which thrive better in more eutrophic environments providing the necessary substrates to maintain high growth rates.

Glacial lakes cover large areas of northern Europe and North America. These freshwater systems represent important habitats accommodating diverse microbial communities. Here, 27 lakes in northern Germany, Poland and Finland were surveyed for the presence of aerobic anoxygenic phototrophic (AAP) bacteria during the summer seasons of 2008 and 2009. Using infrared epifluorescence microscopy, AAP bacteria were found to represent from 2 to 12% of total bacteria in surface layers of the studied lakes. The AAP abundance was correlated with total bacterial numbers, total phosphorus and chlorophyll concentration. The size fractionation analyses indicated that a significant portion of AAP cells was attached to particles. Our findings suggest that AAP bacteria represent a significant component of the microbial community in the studied lakes. KEY WORDS: Lakes · Aerobic photosynthetic bacteria · AAP bacteria · Bacteriochlorophyll a · PhotoheterotrophyResale or republication not permitted without written consent of the publisher