Bacterioplankton niche partitioning in the use of phytoplankton-derived dissolved organic carbon: quantity is more important than quality

Abstract

Some prokaryotes are known to be specialized in the use of phytoplankton-derived dissolved organic carbon (DOCp) originated by exudation or cell lysis; however, direct quantification measurements are extremely rare. Several studies have described bacterial selectivity based on DOCp quality, but very few have focused on the quantity of DOCp, and the relative importance of each of these variables (for example, quantity versus quality) on prokaryote responses. We applied an adapted version of the MAR-FISH (microautoradiography coupled with catalyzed reporter deposition fluorescence in situ hybridization) protocol using radiolabelled exudates from axenic algal cultures to calculate a specialization index (d') for large bacterioplankton phylogenetic groups using DOCp from different phytoplankton species and at different concentrations to elucidate to what extent the bacterial response to DOCp is driven by resource quantity (different DOCp concentrations) or by quality (DOCp from different phytoplankton species). All bacterial phylogenetic groups studied had lower d' at higher DOCp concentration, indicating more generalist behavior at higher resource availabilities. Indeed, at increasing resource concentrations, most bacterial groups incorporated DOCp indiscriminately, regardless of its origin (or quality). At low resource concentrations, only some specialists were able to actively incorporate the various types of organic matter effectively. The variability of bacterial responses to different treatments was systematically higher at varying concentrations than at varying DOCp types, suggesting that, at least for this range of concentrations (10-100 μM), DOCp quantity affects bacterial responses more than quality does. Therefore, resource quantity may be more relevant than resource quality in the bacterial responses to DOCp and affect how bacterioplankton use phytoplankton-derived carbon.

Figure 1

Box plots presenting the percentage of MAR-FISH-positive (cells actively taking up the radiolabeled compounds) within each heterotrophic bacterial phylogenetic group (MAR+_g/CARD-FISH+_g, see text), for the different DOCp concentrations (grouping the results by the three different algal treatments). The central full line indicates the median value, the boxes indicate the lower and upper quartiles, and vertical lines indicate the 10th and 90th percentiles. Note the different scales on the y axes.

Figure 2

Bidimensional heat map of the IS between marine heterotrophic bacteria phylogenetic groups and different DOC sources at different concentrations. Darker tones of gray indicate stronger IS (‘MAR+_g/DAPI counts' data from Table 1 and Figure 2). Roseobacter and SAR11 are both subgroups of Alphaproteobacteria.

Figure 3

Type II linear regression between two different IS metrics: %+MAR+_g/DAPI counts=% +MAR+_g/MAR+_Eub × 0.154−0.036 (n=36, r=0.98; P<0.0001).

Figure 4

Specialization index d' (Blüthgen et al., 2007) of the different bacterial phylogenetic groups as a function of DOCp concentration. Roseobacter and SAR11 are both subgroups of Alphaproteobacteria.