Alteration of chromophoric dissolved organic matter by solar UV radiation causes rapid changes in bacterial community composition

Abstract

We evaluated the effect of photochemical alterations of chromophoric dissolved organic matter (CDOM) on bacterial abundance, activity and community composition in a coastal lagoon of the Atlantic Ocean with high dissolved organic carbon concentration. On two occasions during the austral summer, bacteria-free water of the lagoon was exposed to different regions of the solar spectrum (full solar radiation, UV-A+PAR, PAR) or kept in the dark. Subsequently, dilution cultures were established with bacterioplankton from the lagoon that were incubated in the pre-exposed water for 5 h in the dark. Cell abundance, activity, and community composition of bacterioplankton were assessed before and after incubation in the different treatments. Changes in absorption, fluorescence, and DOC concentration were used as proxies for CDOM photoalteration. We found a significant CDOM photobleaching signal, DOC loss, as well as a stimulation of bacterial activity in the treatments pre-exposed to UV radiation, suggesting increased bioavailability of DOM. Bacterial community analysis by fluorescence in situ hybridization revealed that this stimulation was mainly accompanied by the specific enrichment of Alpha- and Betaproteobacteria. Thus, our results suggest that CDOM photoalteration not only stimulates bacterioplankton growth, but also induces rapid changes in bacterioplankton composition, which can be of relevance for ecosystem functioning, particularly considering present and future changes in the input of terrestrial CDOM to aquatic systems.

Fig. 1

Bacterial abundance (cells mL⁻¹ × 10⁶; average ± 1 SD) measured at T₀ and after 5 h incubation in each treatment for January (A) and February (B). Letters over the bars indicate those treatments showing significant differences.

Fig. 2

Bacterial activity (pM [³H]-L-leucine h⁻¹; average ± 1 SD) in January (A) and February (B) for each water treatment. Bacterial activity in situ and after GF/C filtration (T₀) are also shown (same unit as in the main graphs). Bars showing the same letters indicate those treatments where significant differences were found.

Fig. 3

Bacterial community composition (CARD-FISH) in the different treatments of both experiments (% of DAPI-stained objects; average ± 1 SD). Assayed probes: BET42a (Betaproteobacteria), Alf968 (Alphaproteobacteria), Gam42a (Gammaproteobacteria), CF319a (Cytophaga-like). Panel A, January; B, February. Bars belonging to each probe showing the same letters indicate those treatments where significant differences were found for those bacterial groups.