Phylogenetic diversity of aggregate‐attached vs. free‐living marine bacterial assemblages

The phylogenetic diversity of macroaggregate‐attached vs. free‐living marine bacteria, co‐occurring in the same water mass, was compared. Bacterial diversity and phylogenetic identity were inferred by analyzing polymerase chain reaction (PCR) amplified, cloned ribosomal RNA (rRNA) genes. Ribosomal RNA genes from macroaggregate‐associated bacteria were fundamentally different from those of free‐living bacterioplankton. Most rRNA types recovered from the free‐living bacterioplankton were closely related to a phenotypically undescribed α Proteobacteria group, previously detected in surface waters of North Pacific and Atlantic central ocean gyres. The results suggest that members of this phylogenetically distinct, a proteobacterial group are abundant free‐living bacterioplankters in coastal, as well as open‐ocean habitats. In contrast, most macroaggregate‐associated rRNA clones were closely related to Cytophaga , Planctomyces, or γ Proteobacteria , within the domain Bacteria. These data indicate that specific bacterial populations, different from those which predominate in free‐living bacterioplankton, develop on marine phytodetrital aggregates. The inferred properties of attached bacterial assemblages have significant implications for models of microbially mediated transformation of particulate organic material.