Blooms of single bacterial species in a coastal lagoon of the southwestern Atlantic Ocean

Abstract

We investigated seasonal differences in community structure and activity (leucine incorporation) of the planktonic bacterial assemblage in the freshwater and brackish-water zones of a shallow coastal lagoon of the southwestern Atlantic Ocean. Alphaproteobacteria formed the dominant microbial group in both zones throughout the sampling period. After an intrusion of marine water, members of the SAR11 lineage became abundant in the brackish-water zone. These bacteria were apparently distributed over the lagoon during the following months until they constituted almost 30% of all prokaryotic cells at both sampling sites. At the first sampling date (March 2003) a single alphaproteobacterial species unrelated to SAR11, Sphingomonas echinoides, dominated the microbial assemblages in both zones of the lagoon concomitantly with a bloom of filamentous cyanobacteria. Pronounced maxima of leucine incorporation were observed once in each zone of the lagoon. In the freshwater zone, this highly active microbial assemblage was a mix of the typical bacteria lineages expected in aquatic systems. By contrast, a single bacterial genotype with >99% similarity to the facultative pathogen gammaproteobacterial species Stenotrophomonas maltophilia formed >90% of the bacterial assemblage (>10(7) cell ml(-1)) in the brackish-water zone at the time point of highest bacterial leucine incorporation. Moreover, these bacteria were equally dominant, albeit less active, in the freshwater zone. Thus, the pelagic zone of the studied lagoon harbored repeated short-term blooms of single bacterial species. This finding may have consequences for environmental protection.

FIG. 1.

(A) Total prokaryotic cell numbers in the two zones of the lagoon. (B) Bulk leucine incorporation rates (bars) and concentrations of chlorophyll a (Chl a; circles). On two occasions, samples were lost: March 2003 (leucine incorporation) and August 2003 (chlorophyll a).

FIG. 2.

Relative abundances (percentages of total cell counts) of subphyla of Proteobacteria, of bacteria from the Cytophaga-Flavobacteria lineage, and of Actinobacteria in the two zones of the lagoon.

FIG. 4.

Photomicrographs of bacteria targeted by CARD-FISH with probe SteMal_439 in the brackish zone of the lagoon in December 2003. (Left) All bacteria (DAPI staining). (Right) Hybridized cells. The large object shown in panels C and D represents a heavily colonized phytoplankton cell.

FIG. 3.

Phylogenetic affiliations of 16S rRNA gene sequences obtained from the Sphingomonas and SAR11 (“Candidatus Pelagibacter”) lineages (boldface). Only nearly complete sequences are depicted. The square brackets indicate the target ranges of the FISH probes SphEch_1249 and SAR11_441 (see reference 38). Bar, 10% estimated sequence divergence.

FIG. 5.

Numbers of cells (A) and relative abundances (B) of bacteria from the marine SAR11 clade in the two zones of the lagoon. For comparison, panel B also depicts the conductivity data (circles) reported in Table 1.