Strong coupling between natural Planctomycetes and changes in the quality of dissolved organic matter in freshwater samples

Abstract

Dilution-regrowth experiments coupled to fluorescence in situ hybridization were conducted with samples from two humic reservoirs in order to examine how inorganic nutrients (N, P) affect free-living bacterioplankton phylogenetic groups and subsequently the quality of dissolved organic matter (DOM). The experiments were complemented by analyses of the empirical relationships between the targeted bacteria, nutrients, DOM and grazers. The ratio of absorbance of waters (A) at 250 and 365 nm (A(250 nm):A(365 nm)), which has been found to increase with the proportion of small molecules in the DOM pool in other humic waters, was used as an index of DOM quality. When nutrient stimulated bacteria, both the responses of bacterial groups (in the absence of grazers) and the ratio A(250 nm):A(365 nm) were generally different between treatments (+N, +P, +NP), suggesting that in nutrient-poor systems, differences in the type of inorganic nutrient supply will ultimately cause differences in DOM quality. The ratio A(250 nm),:A(365 nm) peaked in the +N treatments where members of the Planctomycetes (PLA) were the most stimulated group, and across treatments, PLA best explained (positive relationship) variations in this ratio. Consistent with this, the in situ data showed that the removal of the negative effects of flagellates on PLA yielded the highest R(2) in attempts to use bacterial groups to explain variations in A(250 nm):A(365 nm). These findings provide lines of evidence, not previously demonstrated in natural waters, that Planctomycetes may be an important factor changing the DOM quality, particularly in nutrient-poor systems when supplied with inorganic N.

Planctomycetes in Lakes: Poor or Strong Competitors for Phosphorus?

“…However, the fact that Planctomycetes abundances increase during or after phytoplankton blooms (which are known to release large amounts of carbon, nitrogen, and phosphorus) indicates that dissolved organic matter (DOM) and inorganic-nutrient availability are key factors enhancing Planctomycetes growth and suggests that their low abundance might also be an indication that they are poor competitors for some nutrients. An experimental study has also shown strong increases in the abundance of pelagic Planctomycetes (estimated by fluorescent in situ hybridization) following additions of inorganic nitrogen and phosphorus, alone or in combination, in the absence of grazers (6). In many cases in that study, the increase in Planctomycetes abundances was greater than (N addition) or similar to those of dominant groups, such as betaproteobacteria (6).…”

“…In aquatic ecosystems, they represent between 0 and 11% of planktonic prokaryotic communities (3,4,5,6,7). Despite this low abundance, the role of Planctomycetes in aquatic ecosystem functioning is increasingly recognized, given their involvement in nitrogen and carbon cycling (6,8,9,10).…”

“…Such information is scarce for Planctomycetes, partly because most such studies on freshwater bacterial communities have used general bacterial primers for amplification of 16S rRNA genes; these primers generally amplify the most represented bacterial phyla (Actinobacteria, Proteobacteria, and Bacteroidetes) well, but not Planctomycetes. Available data suggest that Planctomycetes might actually be maintained at low abundance in aquatic systems by mortality induced by viruses (5) and heterotrophic nanoflagellates (HNF) (6). On the other hand, the very few studies that have examined Planctomycetes growth have reported generation times above 100 h (14), suggesting that Planctomycetes might be slow-growing bacteria.…”

“…An experimental study has also shown strong increases in the abundance of pelagic Planctomycetes (estimated by fluorescent in situ hybridization) following additions of inorganic nitrogen and phosphorus, alone or in combination, in the absence of grazers (6). In many cases in that study, the increase in Planctomycetes abundances was greater than (N addition) or similar to those of dominant groups, such as betaproteobacteria (6). The few studies reporting on factors that might control Planctomycetes members also suggest an important role for inorganic nutrients (1,15).…”

Vertical Distribution of Functional Potential and Active Microbial Communities in Meromictic Lake Kivu

“…4). Many studies have reported high relative abundance of Bacteroidetes or Planctomycetes following algal blooms [56][57][58][59][60]. Both phyla are known to be present and attached to aggregates, and they have various extracellular enzymes that can degrade complex organic molecules and give a competitive advantage with regards to other microbial groups [61][62][63].…”