Over half of the bacterioplankton cells in ocean surface waters are capable of carrying out a demethylation of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) that routes the sulfur moiety away from the climatically active gas dimethylsulfide (DMS). In this study, we tracked changes in dmdA, the gene responsible for DMSP demethylation, over the course of an induced phytoplankton bloom in Gulf of Mexico seawater microcosms. Analysis of >91,000 amplicon sequences indicated 578 different dmdA sequence clusters at a conservative clustering criterion of ≥90% nucleotide sequence identity over the 6-day study. The representation of the major clades of dmdA, several of which are linked to specific taxa through genomes of cultured marine bacterioplankton, remained fairly constant. However, the representation of clusters within these major clades shifted significantly in response to the bloom, including two Roseobacter-like clusters and a SAR11-like cluster, and the best correlate with shifts of the dominant dmdA clades was chlorophyll a concentration. Concurrent 16S rRNA amplification and sequencing indicated the presence of Roseobacter, SAR11, OM60, and marine Rhodospirillales populations, all of which are known to harbor dmdA genes, although the largest taxonomic change was an increase in Flavobacteriaceae, a group not yet demonstrated to have DMSP-demethylating capabilities. Sequence heterogeneity in dmdA and other functional gene populations is becoming increasingly evident with the advent of high-throughput sequencing technologies, and understanding the ecological implications of this heterogeneity is a major challenge for marine microbial ecology.