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 phylogcnetic identity were inferred by analyzing polymerase chain reaction (PCR) amplified, cloned ribosomal RNA (rRNA) genes. Ribosomal RNA genes from macroaggregatc-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 undcscribcd (Y Proteobacteria group, previously detected in surface waters of North Pacific and Atlantic central ocean gyres. The results suggest that members of this phylogenetically distinct, (Y proteobacterial group are abundant free-living bactcrioplankters in coastal, as well as open-ocean habitats. In contrast, most macroaggregate-associated rRNA clones were closely related to Cytophuga, Planctomyce.s, or y 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. Macroscopic detrital aggregates > 0.5 mm (marine snow) are considered important agents in the flux of biogenic carbon from the sea surface to the deep ocean. Biological and chemical transformations which occur on sinking aggregates, including microbial processes, have a profound influence on the quantity and quality of organic matter reaching the deep sea. Chemolithotrophy, primary production, and complex microbial succession have all been observed in association with macroscopic organic detritus. Aggregates may also serve as localized microhabitats, highly enriched in a variety of micronutrients and microbial assemblages (Alldredge and Silver 1988). Presumably due to enhanced microbial metabolism, stable pH and redox gradients can I Corresponding author. Acknowledgments This research was supported by Office of Naval Research Young Investigator Award NO00 14-90-J19 17 to E.F.D., and NSF OCE 89-22825 to A.L.A. We are indebted to Chris Gottschalk, Lisa Dilling, Uta Passow, and Ken Cracker for diving assistance. We thank Linda Amaral and Ee Lin Lim for technical assistance. be maintained in large particulate material. These and other data have led to the hypothesis that transient microacrophilic or anaerobic microhabitats, in otherwise aerobic environments, can be maintained in macroaggregates. Microscopic and metabolic studies conducted with naturally occurring macroaggregates have revealed some significant differences between attached and free-living bacterial assemblages. Attached bacteria are often larger in size, and present in higher local concentrations, than those found free-living in the water (Caron et al. 1982). However, with respect to total cell numbers or total cell carbon, freeliving bacteria are thought to represent the predominant fraction in aquatic environments (Kirchman and Mitchell 1982; Simon et al. 1990). Uptake of specific nutrients by free-living vs. aggregate-attached bacterial populations can also be significantly different between the two populations. Cellular uptake rates of sugars and amino acids by attached bacteria are sometimes greater than those of surrounding, free-living bacteria (Kirchman and Mitchell 1982). Higher concentrations of specific