AME 78:93-106 (2017)  -  DOI: https://doi.org/10.3354/ame01805

ABSTRACT: Our understanding of the impact of phytoplankton on bacterial communities is largely based on studies showing that a few bacteria have interactions with the phytoplankton class, which is often diatoms, dominating phytoplankton communities. To determine the effect of the complete phytoplankton community on the entire bacterial community, we used tag pyrosequences of the 16S rRNA gene and phytoplankton pigments measured by high performance liquid chromatography along with Chemtax analyses to examine bacterial and phytoplankton communities along the salinity gradient of the Delaware Estuary, USA, in August and November of 3 years (2011-2013). Salinity had a large effect on the composition, taxon richness, and evenness of bacterial communities in the estuary, but so too did the composition and biomass of the phytoplankton community. Phytoplankton classes had a larger effect in shaping the composition of bacterial communities than did total chlorophyll a. Although diatoms and cryptophytes dominated the phytoplankton communities in both August and November, less common phytoplankton classes, such as dinoflagellates, haptophytes, and prasinophytes, had more significant relationships with the entire bacterial community and with individual bacterial taxa. In contrast, the 2 most abundant bacterial subclades in the estuary, SAR11 IIIa and SAR 11 IIIb, had few significant relationships with chlorophyll a or with phytoplankton classes. These data on bacterial and phytoplankton community composition help to explain the weak coupling between bacteria and phytoplankton communities often observed in estuarine and other aquatic systems.

KEY WORDS: 16S rRNA · Chemtax · SAR11 · Diatoms · Bacteroidetes · Planctomycetes