High phylogenetic diversity in a marine-snow-associated bacterial assemblage

Large (>500 mu m), suspended particles of organic and inorganic material, often referred to as 'marine snow', are considered important in the vertical transport and biogeochemical transformation of particulate organic carbon in the marine environment. Previous work has indicated that bacterial species inhabiting marine snow particles may differ greatly from those commonly found living free in the surrounding water column. To further characterize marine-snow-attached bacterial populations, we sampled marine snow by SCUBA diving during periods of intense macroaggregate formation in the northern Adriatic Sea in August 1991. Small subunit ribosomal DNA (rDNA) fragments of bacteria were amplified from extracted nucleic acids by the polymerase chain reaction (PCR). The diversity of the recovered rDNA clones was initially assessed by comparing restriction fragment length polymorphisms (RFLPs) of individual clones. Ninety-five bacterial clones examined yielded 90 different RFLP patterns, representing an estimated sampling coverage of only 5.3%. Sequence analysis of 39 randomly chosen clones was used to assess the general phylogenetic affiliation of individual clones. Bacterial phyla represented in the library included affiliates of the Planctomyces, the Gram-positive bacteria, the Cytophaga-Flavobacteria-Bacteroides (CFB) lineage, and the alpha-, gamma-, delta-, and epsilon-subdivisions of the Proteobacteria. The results suggest that bacterial colonization of suspended marine macroaggregates can result in diverse and complex assemblages, with specific phyla, such as the CFB, being commonly associated with marine particles. Furthermore, this particle-attached bacterial assemblage was similar to other bacterial assemblages found in marine sediments and terrestrial soils, with respect to the nature of the associated phylogenetic groups.