The death and lysis of microbial cells leads to the release of cytoplasmic contents, many of which are rapidly degraded by enzymes. However, some macromolecules survive intact and find new functions in the extracellular environment. There is now strong evidence that DNA released from cells during lysis, or sometimes by active secretion, becomes a key component of the macromolecular scaffold in many different biofilms. Enzymatic degradation of extracellular DNA can weaken the biofilm structure and release microbial cells from the surface. Many bacteria produce extracellular deoxyribonuclease (DNase) enzymes that are apparently tightly regulated to avoid excessive degradation of the biofilm matrix. Interfering with these control mechanisms, or adding exogenous DNases, could prove a potent strategy for controlling biofilm growth.

The vast majority of bacteria present in the natural environment are present in the form of aggregates and/or biofilms. Microbial aggregates are ubiquitous in the marine environment and are inhabited by diverse microbial communities which often express intense extracellular enzymatic activities. However, the secretion of an important group of enzymes, DNases, by bacteria from marine aggregates has not been studied, despite the importance of these aggregates in biogeochemical cycling of nutrients in the oceans. In this work, we therefore, employed both culture-based and bioinformatics approaches to understand the diversity of bacterial DNases in marine bacterioplankton. We found that 34% of 345 strains of attached and non-attached marine bacteria showed extracellular DNase activity. Most of these isolates belong to Proteobacteria (53%) and Firmicutes (34%). Secretion of DNases by bacteria isolated from marine gel particles (MGP) is reported here for the first time. Then, to further understand the wider diversity of the potential to produce DNases, sequences were compared using 2316 whole genome and 42 metagenome datasets. Thirty-nine different taxonomic groups corresponding to 10 bacterial phyla were found to encode genes responsible for DNase secretion. This study highlights the unexpected and widespread presence of DNase secretion in bacteria in general and in MGP more specifically. This has important implications for understanding the dynamics and fate of marine microbial aggregates in the oceans.

In the present study, we examined the interactions between the algal species Tetraselmis indica and strains of bacteria with which it is closely associated. Three bacterial strains were isolated and sequence analysis of the 16S rDNA indicated that the organisms belong to the genera Pseudomonas, Acinetobacter and Ruegeria. Morphologies of the bacterial strains were studied using epifluorescence microscopy and scanning electron microscopy. Reassociation experiments were conducted with axenic cultures inoculated with the 3 bacterial strains in concentrations comparable to natural conditions, and the effect of each bacterial population on the growth of T. indica was determined. T. indica exhibited differential growth with the various bacterial cultures, and in particular Acinetobacter sp. was observed to promote growth of the algae. These experiments revealed that microbes associated with the alga differentially influence algal growth dynamics. Bacterial presence on the cast-off cell wall products of the alga suggested the likely utilisation of algal cell wall by bacteria. The bacterial strains were tested for carbohydrate metabolism using various sugars and screened for carbohydrase activity. Bacterial strains were found to produce carbohydrases for degradation of polysaccharides generally present in the cell wall of Tetraselmis (glucans, galactans, galactomannans and pectins), whereas no such utilisation was observed for other wall substrates (such as cellulose, arabinoxylan, rhamnogalacturonan). Pseudomonas sp. and Acineto bacter sp. showed carbohydrase activity with glucans, galactans, galactomannans and pectin, whereas Ruegeria sp. showed much less carbohydrase activity and only with pectin. The carbohydrate utilisation studies using artificial substrates suggested the potential utilisation of cast-off algal cell wall products.

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