Microbial Diversity and Biogeochemistry of Deep-Sea Sediments

The biochemistry of piezophilic bacteria is unique in that the piezophiles produce polyunsaturated fatty acids (PUFAs), EPA (20:5, cis-5,8,11,14,17-eicosapentaenoic acid) and DHA (22:6, cis-4,7,10,13,16,19-docosahexaenoic acid). The biosynthesis of PUFAs is believed to be a mechanism of microbial response to the high hydrostatic pressure in the deep sea. DeLong and Yayanos (1985) were among the first to test the responses to pressures from 30 to 50 MPa (at 2oC) of the gram-negative and facultative anaerobic bacterium CNPT3. The concentration of saturated fatty acids decreased from 34 to 25% with pressure, whereas the concentration of unsaturated fatty acids increased from 45% to 75%. There is a striking correlation between growth at high pressure and fatty acid unsaturation index (DeLong and Yayanos, 1985; Allen et al., 1999). Allen et al. (1999) investigated the fatty acid composition of piezotolerant bacterium Photobacterium profundum SS9 in response to changes in hydrostatic pressure (0.1, 28, and 50 MPa). Concentrations of eicosapentaenoic acid (EPA) increased with decreasing temperature; pressure increases resulted in more significant increases in EPA and 18:1. Despite the significant advances in the past two decades, the piezophilic bacterial cellular lipid biochemistry remains to be fully characterized. A pertinent question is if piezophilic bacteria synthesize PUFA de novo, through dietary uptake, or both.

We examined the biosynthesis and cellular uptake of PUFAs in a moderately piezophilic (Shewanella violacea DSS12) and two hyperpiezophilic bacteria (S. benthica DB21MT-2 and Moritella yayanosii DB21MT-5) that were grown under 50 MPa (megapascal) and 100 MPa in media containing marine broth 2216 supplemented with arachidonic acid (AA, sodium salt) and/or antibiotic cerulenin. There was active uptake and cellular incorporation of AA in the hyperpiezophilic bacteria DB21MT-2 (14.7% of total fatty acids) and DB21MT-5 (1.4%), but no uptake was observed in DSS12 (Fig. 1). When cells were treated with antibiotic cerulenin, all three strains incorporated AA into cell membranes (13 to 19%). The biosynthesis of even-numbered monounsaturated fatty acids was significantly inhibited (10 to 37%) by the addition of cerulenin, whereas the concentrations of PUFAs increased by 2-4 times. These results suggest that piezophilic bacteria biosynthesize and/or incorporate dietary polyunsaturated fatty acids that are required for the growth and piezoadaptation.

Fig. 1. RFLP fingerprinting of DNA isolated from the Japan Trench sediment that was incubated at 30 MPa (top left), 50 MPa (top right) and atmospheric pressure (right).