|Title||Microbial activity and community structure in a net pen aquaculture area|
|Authers||Tomoko SAKAMI, Katsuyuki ABO and Kazufumi TAKAYANAGI|
|Keywords||Bacterial production, hydrolytic enzyme, particle-associated bacteria, DGGE|
|Citation||Bull. Fish. Res. Agen. supplement No. 19,-, 2007|
Excess organic matter production from net-pen aquaculture farms operated in open water causes serious seawater pollution. To develop sustainable aquaculture systems we need to consider the mechanisms of organic matter cycling. Microbial communities have very important roles in cycling organic matter in seawater. To elucidate how microbial communities are affected by aquaculture activities, we examined the bacterial activity and community structure in a red sea bream aquaculture area and a neighboring, non-aquaculture, reference area in Gokasho Bay, Japan.
The bacterial activity parameters examined - abundance, production rate, and extracellular hydrolytic enzyme activity- were always higher in the aquaculture area than in the reference area, and these differences were most pronounced in surface waters in summer. The annual mean bacterial abundance and production rate in the aquaculture area were 4.7×109 cells L－1 and 82 mg C m－3 day－1 : about 1.4 and 3.5 times, respectively, those in the reference area. The annual bacterial production per unit area was estimated as 608 g C m－2 y－1 in the aquaculture area. The difference of bacterial production between the two examined areas was 444 g （37 mol） C m－2 y－1, which was equivalent to the organic matter loads from fish farming. This fact suggests that bacterial community in seawater could utilize organic matters as much as fish farms released into the surrounding water in this aquaculture area.
The annual mean extracellular leucine aminopeptidase activity （which represents bacterial protein degrading activity） in seawater in the aquaculture area was about twice that in the reference area. On the other hand, the activity of β-glucosidase （which represents polyhydrocarbon degrading activity） in the aquaculture area was about five times that in the reference area, indicating that, overall, β-glucosidase activity was promoted more than leucine aminopeptidase activity. These microbial activity parameters were positively correlated with the organic matter concentrations in the water, suggesting that input of organic matter from the fish farms to the surrounding waters promoted microbial activity.
We examined bacterial community structure in seawater by using a PCR-denaturant gradient gel electrophoresis （DGGE） method based on 16S-rRNA gene fragment fingerprinting. Since the level of leucine aminopeptidase activity was closely correlated with the particulate organic matter concentration, the bacterial community was separated into two categories, particle-associated （>1μm） and free-living （<1μm）. In the free-living bacterial community, the number of DGGE bands （which corresponded to bacterial species） ranged from 2 to 12, and the DGGE profiles were similar in the aquaculture area and in the reference area. However, some bands identified as representing alpha and gamma proteobacteria were observed only in the aquaculture area from spring to autumn. In the particle-associated bacterial community, the number of DGGE bands was less than in the free-living one, and most of the bacteria were identified as cyanobacteria. However, in summer, when the particle-associated bacterial community had high hydrolytic enzyme activity, bands identified as representingthe Cytophaga-Flavobacterium-Bacteroides group, most isolates of which have the ability to degrade biomacromolecules, were detected in the aquaculture area, together with those of alpha and gamma proteobacteria. These results suggest that the variation in bacterial activity was related to bacterial community structure and that aquaculture activity affects the bacterial community in seawater, both quantitatively and qualitatively.