| In this study, surface seawater samples from Shenzhen Dameisha Bay during an Akashiwo sanguinea bloom were collected to study the chlorophyll concentration, bacterial culturability and population structure. We also isolated several algae-lysing bacterial species from seawater samples and studied the algae-lysing characteristics, algae-lysing compound extraction, and VBNC state transformation of the algae-lysing bacterium D84-1. The main findings were as follow:(1) Surface seawater samples from Shenzhen Dameisha area were continuously collected from June 19 to June 22, 2014, right after the peak of an Akashiwo sanguinea bloom on June 18. The chlorophyll concentrations, total bacteria numbers, and culturable bacterial numbers in these water samples were measured. Results showed that the chlorphyll concentration kept reducing during the 4-days-period while the total bacteria and culturable bacteria numbers increased suddenly on the 3rd day(June 21); On the other hand, 0.22 ?m filtered seawater samples were used to extract total bacterial DNA and performed metagenomic sequencing using Illumina Mi Seq 16 S r RNA gene amplicons to analyze bacterial community structures. Data indicated the dominant bacterial groups switched from Gammaproteobacteria on day 1 to Alpha- and Betaproteobacteria on day 2 and 3, back to Gammaproteobacteria on day 4. Other groups, such as Firmicutes and Cyanobacteria, also expressed remarkable fluctuation during this termination stage. The most abundant bacterial genera were Vibrio and Pseudoalteromonas in Gammaproteobacteria, represented 20.82% and 25.76 % of total bacterial sequences on day 3, respectively;(2) Several algae-lysing bacteria were isolated from seawater samples during the bloom, among which the most potent strain D84-1 was identified as Vibrio sp. through 16 S r DNA sequence analysis. This was consistent with the dominance of Vibrio sp. on total bacterial population in metagenomics data;(3) The possible algae-lysing mode of D84-1 was tested on A. sanguinea, Chattonella marina, and a couple of other harmful algal bloom-formal algal species. Results showed that D84-1 lysed algal cells through indirectly by secreting substances into supernatant. D84-1 has a broad algae-lysing spectrum that showed inhibitious effect on 10 marine algal species and one freshwater cyanobacterium. When treated with D84-1 supernatant, the C. marina cells showed decreased chlorophyll concentrations and lower activities of MDA, peroxidase and antioxidase enzymes. These results indicated that D84-1 caused lipid peroxidation of C. marina cell membrane, resulted in the supersession of reactive oxygen metabolism and cell damage;(4) The VBNC(viable but non-culturable) state transformation of D84-1 under different environmental conditions were also investigated. D84-1 cells entered VBNC state under 4 degrees and oligotrophic conditions within 3 days. When cultured in aged seawater under 4 degrees, it took 6 days for D84-1 to enter VBNC state; under 22 degrees, only 80 % cells entered VBNC state even after 12 days. This was consistant with the increased culturable bacterial numbers and Vibrio dominance during bloom termination stage. We also took SEM photos of D84-1 cells under normal and VBNC state to check the morphology change during state transformation. Under normal conditions, D84-1 cells were rod-shaped with a length of 1-2 ?m and smooth surface; while under VBNC state, cells were 1?m or less in length, significantly reduced cell volume with the shape changed from rod-like into cocci with concave. |
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