Major advances in our understanding of marine bacterial diversity have been gained through studies of bacterioplankton, the vast majority of which appear to be gram negative. Less effort has been devoted to studies of bacteria inhabiting marine sediments, yet there is evidence to suggest that gram-positive bacteria comprise a relatively large proportion of these communities. To further expand our understanding of the aerobic gram-positive bacteria present in tropical marine sediments, a culture-dependent approach was applied to sediments collected in the Republic of Palau from the intertidal zone to depths of 500 m. This investigation resulted in the isolation of 1,624 diverse gram-positive bacteria spanning 22 families, including many that appear to represent new taxa. Phylogenetic analysis of 189 representative isolates, based on 16S rRNA gene sequence data, indicated that 124 (65.6%) belonged to the class Actinobacteria while the remaining 65 (34.4%) were members of the class Bacilli. Using a sequence identity value of >98%, the 189 isolates grouped into 78 operational taxonomic units, of which 29 (37.2%) are likely to represent new taxa. The high degree of phylogenetic novelty observed during this study highlights the fact that a great deal remains to be learned about the diversity of gram-positive bacteria in marine sediments.Gram-positive bacteria can be divided into two major subdivisions: the phylum Actinobacteria, also described as the high-GϩC gram-positives, and the phylum Firmicutes, also known as the low-GϩC gram-positives, a group that includes such well-known genera as Bacillus and Clostridium (21). Gram-positive bacteria typically have a cell wall consisting of a thick layer of peptidoglycan (19), while a few rather unusual genera lack a cell wall entirely (42). Many in this large group of primarily chemoorganotrophic bacteria are also known to produce spores in response to starvation or harsh chemical or physical conditions (17,40,50). Aerobic gram-positive bacteria, specifically actinomycetes (defined here as bacteria within the order Actinomycetales) and members of the order Bacillales, are generally saprophytic and include well-known producers of important secondary metabolites (23,53).While the most thoroughly studied gram-positive bacteria include human pathogens (e.g., Mycobacterium tuberculosis, Bacillus anthracis) and soil-derived, antibiotic-producing actinomycetes (2), relatively little is known about the diversity and distribution of gram-positive bacteria in the marine environment. This lack of information persists despite the fact that gram-positive bacteria have been cultured from the ocean for decades (5,26,32,43,68) and consistently appear in culture-independent studies (e.g., references 62 and 66), including the report of a new and as-yetuncultured order within the class Actinobacteria (54). Gram-positive bacteria are likely to play important microbiological roles in the marine environment, yet without a fundamental understanding of their diversity and ecophysiology, it is diffi...

Genomic islands have been shown to harbor functional traits that differentiate ecologically distinct populations of environmental bacteria. A comparative analysis of the complete genome sequences of the marine Actinobacteria Salinispora tropica and S. arenicola reveals that 75% of the species-specific genes are located in 21 genomic islands. These islands are enriched in genes associated with secondary metabolite biosynthesis providing evidence that secondary metabolism is linked to functional adaptation. Secondary metabolism accounts for 8.8% and 10.9% of the genes in the S. tropica and S. arenicola genomes, respectively, and represents the major functional category of annotated genes that differentiates the two species. Genomic islands harbor all 25 of the species-specific biosynthetic pathways, the majority of which occur in S. arenicola and may contribute to the cosmopolitan distribution of this species. Genome evolution is dominated by gene duplication and acquisition, which in the case of secondary metabolism provide immediate opportunities for the production of new bioactive products. Evidence that secondary metabolic pathways are exchanged horizontally, coupled with prior evidence for fixation among globally distributed populations, supports a functional role and suggests that the acquisition of natural product biosynthetic gene clusters represents a previously unrecognized force driving bacterial diversification. Species-specific differences observed in CRISPR (clustered regularly interspaced short palindromic repeat) sequences suggest that S. arenicola may possess a higher level of phage immunity, while a highly duplicated family of polymorphic membrane proteins provides evidence of a new mechanism of marine adaptation in Gram-positive bacteria.

Actinomycetes were cultivated using a variety of media and selective isolation techniques from 275 marine samples collected around the island of Guam. In total, 6425 actinomycete colonies were observed and 983 (15%) of these, representing the range of morphological diversity observed from each sample, were obtained in pure culture. The majority of the strains isolated (58%) required seawater for growth indicating a high degree of marine adaptation. The dominant actinomycete recovered (568 strains) belonged to the seawater-requiring marine taxon 'Salinospora', a new genus within the family Micromonosporaceae. A formal description of this taxon has been accepted for publication (Maldonado et al., 2005) and includes a revision of the generic epithet to Salinispora gen. nov. Members of two major new clades related to Streptomyces spp., tentatively called MAR2 and MAR3, were cultivated and appear to represent new genera within the Streptomycetaceae. In total, five new marine phylotypes, including two within the Thermomonosporaceae that appear to represent new taxa, were obtained in culture. These results support the existence of taxonomically diverse populations of phylogenetically distinct actinomycetes residing in the marine environment. These bacteria can be readily cultured using low nutrient media and represent an unexplored resource for pharmaceutical drug discovery.