The Changes Of Microbial Community Along The Different Trophic Gradient Of Seawater In Zhanjiang Bay

Bacterioplankton biomass(BB), phytoplankton biomass(PB), main physiological groups of bacteria namely nitrate-reducing bacteria(NB)and fecal coliform(FC)were investigated in Zhanjiang Bay and bacterial community structure and function were analyzed along five different trophic seawater stations. The results show as following:1. Bacterioplankton biomass was surveyed for two years including eight seasons. The BB presented a minimum (5.22×10^-2mgC/L) in winter, Feb. 2009, however reached a peak(3631.43×10^-2mgC/L)in spring, May 2009, subsequently, BB declined. A low BB remained in autumn, Nov. 2009 and winter, Jan. 2010. Afterwards, BB rose in spring, April 2010 and reached a peak in 2010. In summer, July 2010 BB decreased, and BB remained a lower level in autumn, Dec. 2010 than in autumn, Nov. 2009. The BB decreased horizontally from inner to outer bay and from landward to seaward during investigation.2. During investigation, average of PB was lower in 2009. A high PB exhibited in winter, Feb. 2009, and then, PB declined in spring, May 2009, but went up slightly in Aug. 2009. Subsequently, PB decreased in autumn, Nov. 2009 and remained a lower level(45.57mgC/m³)in winter, Jan. 2010. However, the PB increased in spring, April 2010 and reached a peak(2278.69mgC/m³)during this investigation. And then, the PB declined in summer, July 2010 and decreased to a lower level in autumn, Oct. 2010 than in autumn, Nov. 2009. The PB was horizontally higher in the northern and middle of Zhanjiang Bay in this investigation, annual average of PB decreased from northern and middle area to core area, afterwards, the PB increased seaward and reached the maximum in outer bay.3. Minimal density of FC always showed at sampling stations in outer bay in each season. A relatively higher density of FC appeared in spring, may 2009, however it declined in summer, Aug. 2009 and decreased to 45.6 ind./mL in autumn, Nov. 2009. Subsequently, the density of FC increased unexpectedly in winter, Jan. 2010 and reached a peak of 159.5 ind./mL in spring, April 2010 in the period of investigation. During investigation, there was a lower density of NB in spring, May 2009. Afterwards, the density of NB increased rapidly and reached a peak of 243.7×10² ind./ mL in summer, Aug. 2009. And then, it came back to decline in autumn, Nov. 2009, and decreased to a minimum of 22.8×10² ind./mL in winter, Jan. 2010. The density of NB increased in spring, April 2010 and it was higher in this season than in autumn, May 2009. The density of FC was higher along landward sea area, and decreased horizontally from landward sea area to middle Nansan Island sea area and outer bay, it also declined from core area to the outer bay. But the density of NB decreased horizontally from southwest sea area to north and east sea area in Zhanjiang Bay.4. In Zhanjiang Bay, eutrophication has reached a very high level according to the indicator of heterobacterial average density getting from comprehensive investigation, however, assessment criterion of chlorophyll a representing phytoplankton biomass indicated that the seawater was middle or poor trophic in 2009, and reached an eutrophication level in 2010. Because of the complexity of marine ecosystem, an integrated method should be applied to assess trophic status of seawater since a single eco-factor as an index of eutrophication is unable to reflect the actual situation.5. The BB was highly significant positive relative to temperature and TOC, and was highly significant negative relative to NO₂^--N, SiO₃^2--Si. The BB was significant positive relative to DO and PO₄^3--P in Zhanjiang Bay. Correlation analysis showed that the PB was highly significant positive relative to PO₄^3--P, NO₃^--N, pH and TOC, but highly significant negative correlation with NH4+-N and SiO₃^2--Si, and was significant negative relative to COD and DIN. The nitrate-reducing bacteria was highly significant positive relative to temperature, while was highly significant negative relative to PO₄^3--P and pH, but significant positive correlation with chlorophyl-a and NO₃^--N. However, no significant correlation was found between FC and these environmental factors, which is attributed to a fact that input and load matter from land influence and regulate the population of FC.6. There were various bacterial classification groups, and they had different percentage in the different trophic gradient of seawater in Zhanjiang Bay. The Proteobacteria was a dominant bacterial population, followed by the Bacteroidetes and Actinobacteria in turn. The unclassified sequences also occupied the larger proportion. Comparing the bacterial classification groups of surface sample with the bottom sample in the same station, no significant difference was found among all samples except sampling station 24 (S24 and B24). Statistical analysis showed that the Alpha proteobacteria, Beta proteobacteria, Delta proteobacteria and Epsilon proteobacteria were frequent in Zhanjiang Bay, and they also had many phylogenetic types. The presence of TM7, Verrucomicrobia, Opitutae, Clostridia, Mollicutes and corresponding less abundance implied the complicated marine habitat satisfying metabolic requirement of different bacterial groups in Zhanjiang Bay.7. The relationship between different bacteria groups and main environmental factors showed that the Actinobacteria was significant positive relative to NH4+-N, NQI, SiO₃^2--Si, NO₃^--N, PO₄^3--P and TOC, however, the Cyanobacteria, unclassified sequences and Verrucomicrobia were significant positive correlation with DO and salinity. The Proteobacteria, Bacteroidetes, Cyanobacteria, Verrucomicrobia, Firmicutes, Tenericutes and Fusobacteria were significant positive relative to COD, BB, PB, temperature and pH, while various bacterial classification groups were less affected by NO₂^--N and suspended solids in Zhanjiang Bay.8. The number of Beta proteobacteria, Epsilon proteobacteria, Delta proteobacteria, Actinobacteria, Clostridia, unclassified sequences and Cyanobacteria raised with increasing trophic gradient of seawater, on the contrary, the number of Flavobacteria, Gamma proteobacteria and Opitutae declined. The less Sphingobacteria was found in the oligotrophic seawater, and the increasing abundance revealed in mesotrophic seawater, however, the number of Sphingobacteria declined in eutrophic seawater adversely. Simpson's diversity index, species richness index, OTUs, and species evenness index characterizing the community structure declined with increasing trophic gradient of seawater. The NQI and BB were at same direction. The BB, the density of NB and FC, Actinobacteria, bacterial respiration rate, unclassified sequences, Flavobacteria, Cyanobacteria and index of coverage (the same 70 clones test) increased accompanying increase of NQI, in which the changes of BB was most remarkable, followed by density of NB and FC in turn, and the bacterial respiration rate grew slowly, nevertheless the PB declined with increasing NQI.