The estuarine copepod Acartia tonsa naturally carried diverse strains of bacteria on its body. The bacterial community composition (BCC) remained very conservative even when the copepod was fed different axenic algal species, indicating that the food per se did not much affect BCC associated with the copepod. In xenic algal treatments, however, copepod-associated BCC differed with each alga fed, even though the same bacterial source was used to inoculate the algae. In addition, starved copepods taken at the same location but at different times significantly differed in their BCC. Algal species composition and copepod life history therefore serve to regulate BCC associated with copepods, and spatial and temporal variations in algal species composition and copepod origin would alter bacteria–copepod interactions.

Individual bacterial populations are known to respond differently toward substrate availability. To test how the availability of either pure phenol or natural humic matter (HM) selects for specific pelagic bacteria phylotypes from a humic lake (Lake Grosse Fuchskuhle, northeastern Germany), we used culture-dependent and -independent approaches. Using a batch approach, the bacterial community composition (BCC) differed depending on both the quantity and the quality of added substrates. Using a dilution-to-extinction approach, distinct BCC were detected by eliminating less abundant species. Most bacteria that were common in the lake were favored by phenol, and yet different subsets of the native BCC were enriched by HM. Specific bacterial groups with different growth requirements were consistently present, negatively influenced, or positively enriched following substrate additions. This study comprises the first explicit demonstration that bacteria such as Methylobacterium, Methylophilus, and Methylosinus spp. can be enriched on phenol or HM. Our isolation approaches led to the successful cultivation of a variety of native bacteria from the lake, such as Novosphingobium (Alphaproteobacteria) and Flexibacter (Bacteroidetes), or phenol-utilizing bacteria such as members of Actinobacteria or Burkholderia (Betaproteobacteria). Enrichment and cultivation on phenol and HM as substrates revealed highly specialized bacterial communities that resemble those found in many HM-rich lakes.

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In an effort to better understand the dynamics of members of the bacterioplankton community in relation to humic matter (HM) addition, and to provide insight into the ecology of common and persistent as well as transient freshwater bacteria, we designed a study with a batch and a dilution approach. We used single vs. repeated HM additions in incubations with bacterial communities from the epilimnion (0-10 m) and hypolimnion (40 m) of oligotrophic Lake Stechlin (northeastern Germany). Molecular methods were applied for detailed phylogenetic characterization of bacterial community composition (BCC) every 2 wk over 8 wk of incubation at in situ temperature. Whereas no significant differences in the development of BCC in batch vs. dilution cultures were observed, the BCC of epilimnic and hypolimnic samples greatly differed. This indicates that HM addition led to the establishment of a highly specific but different BCC depending on the source community in combination with the respective in situ temperature. Further, DGGE banding patterns revealed a high variability in the BCC of epilimnic and hypolimnic samples. Betaproteobacteria were consistently present and specific Alphaproteobacteria, such as members of the Roseisalinus group, Bacteroidetes, and Deltaproteobacteria were enriched only after HM addition. Other phylogenetic groups, including Actinobacteria and Gammaproteobacteria, were only sporadically present. Our approach resulted in the cultivation of a variety of bacteria such as Lysobacter, Methylobacterium, Pseudomonas, Rhodopila, and Variovorax species. The addition of HM selected for specific HM-degrading bacterial phylotypes, which are found at different depths even in the clear waters of Lake Stechlin.KEY WORDS: Humic matter · Bacterioplankton community · Lake Stechlin · DGGE · CARD-FISH Resale or republication not permitted without written consent of the publisher

In many limnetic systems, the input of allochthonous organic matter, e.g., leaf litter, is a substantial source of dissolved organic carbon (DOC) for pelagic bacteria, especially in fall and winter when autochthonous DOC production is low. However, relatively little is known about community changes of pelagic lake bacteria due to leaf litter input which includes both the release of leaf leachates and microorganisms from the leaf litter into the surrounding water. Therefore, we have experimentally studied the effects of different types of leaf litter (Betula pendula, Fagus silvatica, and Pinus silvestris) on the pelagic bacterial community composition by adding leaves to different treatments of epilimnic water samples (unfiltered, 0.2 µm and 5.0 µm-pre-filtered) from humic Lake Grosse Fuchskuhle (Northeastern Germany). The addition of leaf litter led to a significant increase in DOC concentration in lake water, and each leaf litter type produced significantly different amounts of DOC (p = <0.001) as well as of specific DOC fractions (p = <0.001), except of polysaccharides. DGGE banding patterns varied over time, between types of leaf litter, and among treatments. Bacteria belonging to known bacterial phylotypes in the southwest basin of Lake Grosse Fuchskuhle were frequently found and even persisted after leaf litter additions. Upon leaf litter addition, α-proteobacteria (Azospirillum, Novosphingobium, and Sphingopyxis) as well as β-proteobacteria (Curvibacter and Polynucleobacter) were enriched. Our results indicate that supply of leaf litter DOM shifted the bacterial community in the surrounding water towards specific phylotypes including species capable of assimilating the more recalcitrant DOC pools. Statistical analyses, however, show that DGGE banding patterns are not only affected by DOC pools but also by treatment. This indicates that biological factors such as source community and grazing may be also important for shifts in bacterial community structure following leaf litter input into different lakes.