Retrieved bacteria from Noctiluca miliaris (green) bloom of the northeastern Arabian Sea

Abstract

In recent years, seasonal blooms of the dinoflagellate Noctiluca miliaris have appeared in the open-waters of the northern Arabian Sea (NAS). This study provides the first characterization of bacteria from a seasonal bloom of green Noctiluca of NAS (20 degree N-17 degree N and 64 degree E-70 degree E), during the spring-inter-monsoon cruise of Sagar Sampada 253, in March 2007. Bacterial growth as assessed by most-probable number (MPN) and plate counts, revealed 'variable-physiotypes' over a wide range of salinities (0 percent-25 percent w/v NaCl), pH levels (5-8.5), and organic nutrient strengths, in comparison to non-bloom waters. MPN indices of bacteria in surface waters of bloom stations DWK and PRB, corresponded to (3.08-4.41)x103 cells/mL at 3.5 percent NaCl (w/v), and (2.82-9.49)x102 cells/mL at 25 percent (w/v) NaCl in tryptone-yeast extract broth (TYE). Plate counts were (1.12-4)x106 CFU/mL at 0 percent (w/v) NaCl, (1.28-3.9)x106 CFU/mL at 3.5 percent (w/v) NaCl, and (0.4-7)x104 CFU/mL at 25 percent NaCl (w/v) on TYE. One-tenth-strength Zobell's gave (0.6-3.74)x105 CFU/mL at pH 5 to (3.58-7.5)x105 CFU/mL at pH 8.5. These bacteria were identified to the genera Bacillus, Cellulomonas, Staphylococcus, Planococcus, Dietzia, Virgibacillus, Micrococcus, Sporosarcinae, Leucobacter, and Halomonas. The identity of three strains (GUFBSS253N2, GUFBSS253N30, and GUFBSS253N84) was confirmed through 16S rDNA sequence homology as Bacillus cohnii, Bacillus flexus, and Bacillus cereus. The approx 2-3-fold higher plate counts of culturable bacteria from the open-waters of the NAS indicate that these bacteria could critically determine the biogeochemical dynamics of the bloom and its milieu. The role of these bacteria in sustaining/terminating the bloom is under evaluation.