Dynamics of bacterial community composition and activity during a mesocosm diatom bloom

Abstract

Bacterial community composition, enzymatic activities, and carbon dynamics were examined during diatom blooms in four 200-liter laboratory seawater mesocosms. The objective was to determine whether the dramatic shifts in growth rates and ectoenzyme activities, which are commonly observed during the course of phytoplankton blooms and their subsequent demise, could result from shifts in bacterial community composition. Nutrient enrichment of metazoan-free seawater resulted in diatom blooms dominated by a Thalassiosira sp., which peaked 9 days after enrichment ( approximately 24 microg of chlorophyll a liter(-1)). At this time bacterial abundance abruptly decreased from 2.8 x 10(6) to 0.75 x 10(6) ml(-1), and an analysis of bacterial community composition, by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments, revealed the disappearance of three dominant phylotypes. Increased viral and flagellate abundances suggested that both lysis and grazing could have played a role in the observed phylotype-specific mortality. Subsequently, new phylotypes appeared and bacterial production, abundance, and enzyme activities shifted from being predominantly associated with the <1.0-microm size fraction towards the >1.0-microm size fraction, indicating a pronounced microbial colonization of particles. Sequencing of DGGE bands suggested that the observed rapid and extensive colonization of particulate matter was mainly by specialized alpha-Proteobacteria- and Cytophagales-related phylotypes. These particle-associated bacteria had high growth rates as well as high cell-specific aminopeptidase, beta-glucosidase, and lipase activities. Rate measurements as well as bacterial population dynamics were almost identical among the mesocosms indicating that the observed bacterial community dynamics were systematic and repeatable responses to the manipulated conditions.

FIG. 1

(a and b) Chlorophyll a (Chl. a), phosphate, and POC in tank 1 (a) and tank 3 (b), which were amended with NO₃⁻ and NH₄⁺, respectively. (c and d) Abundances of free-living and attached bacteria in tank 1 (c) and tank 3 (d) are presented as stacked bars, which sum to total bacterial abundance. (e and f) Abundances of virus and flagellates in tank 1 (e) and tank 3 (f). Error bars indicate SD.

FIG. 2

(a and b) Stacked bar graphs showing carbon production of free-living and attached bacteria over time in tank 1 (a) and tank 3 (b). (c and d) Biomass-specific growth rates of free-living and attached bacteria versus time in tank 1 (c) and tank 3 (d). Error bars indicate SD.

FIG. 3

Time course of potential ectoenzyme activities in tank 1 (a to d) and tank 3 (e to h). (a and b) Total potential enzyme activities presented as percentages of their peak values. (c to h) Cell-specific hydrolysis rates for attached and free-living bacteria. Error bars indicate SD.

FIG. 4

DGGE profiles of the bacterial community composition over time in tanks 1 and 3. Separate gels (each with 30 to 46% denaturant gradients) were used for the two tanks. Each excised, cloned, and sequenced band is numbered on the left. The relationships of excised band sequences to other sequences in the GenBank database are indicated in the table under the gels.

FIG. 5

Phylogenetic tree showing relationships of the sequences found in our samples to representative bacterial 16S rRNA genes. Sequences of the cultured bacteria were retrieved from GenBank. The tree was inferred by the maximum-likelihood method using full-length genes for the cultured bacteria and the entire length of the sequenced bands (169 to 194 bp) beginning at the equivalent to base 341 and going towards the 3′ end of the 16S rRNA molecule (Escherichia coli numbers). An Archaea (Sulfolobus solfataricus) was used as an outgroup. GenBank accession numbers of the cultured bacteria are as follows: R. algicola, X78314; R. denitrificans, M96746; A. macleodii, Y18228; F. maritimus, M64629; C. lytica, M62796; L. cohaerens, AF039292; A. nitrofigilis, L14627; P. marinus, X63140; Synechococcus sp. strain WH8103, AF001479; O. neapolitana, X80390. The scale bar indicates substitutions per nucleotide position.