## 4 Discussion
### Changes in Bacterial Community in Cultivation Pond and Mesocosms  
The bacterial community within the cultivation pond was significantly different to that observed in the natural seawater during our experiment. A noticeable increase in the ratio of Rhodobacteraceae was recorded; however, these results do not consider the attached and/or free-living bacteria. It is, therefore, difficult to determine the direct effect of DOC released from _T. striata_ on the bacterial community as a whole. The _T. striata_ strain employed in our experiments was axenic, which may have introduced bacteria into the cultivation pond upon inoculation. It is difficult to be certain, therefore, whether a large proportion of the Rhodobacteraceae and Saprospiraceae bacterial groups was present at the time of inoculation or naturally increased, but the bacterial species diversity certainly decreased over the experimental duration. Despite the experimental set-up being a \"semi-natural\" condition, results from the mesocosm experiment suggest that there was an effect on the bacterial community from the DOC release. Evidence to support this include the following: the bacterial compositions of CMs and BMs were clearly differentiated at the end of the experiment, with a significantly decreased bacterial species diversity; the initial bacterial diversity in both mesocosms and the CMs on day 15 were high, but the diversity of the BMs on day 15 was significantly (\(p<0.05\)) lower; in the BMs, the Cellvibrionaceae group occupied a large proportion, while the uncultured alpha and gamma Proteobacteria and etc. groups noticeably decreased at the end of mesocosm experiment (Figure 9). An increase in bacterial groups which favor the DOC artificially increased by the large cultivation of _T. striata_ would have resulted in a reduction of the natural bacterial group. The Rhodobacteraceae group increased in the _T. striata_ cultivation pond. Interestingly, this bacterial group also gradually increased in the mesocosm experiment after the installation of the _T. striata_ bioreactor. The most relevant bacterial group was the Rhodobacteraceae for the PLF fraction in the analysis, followed by the Flavobacteriaceae (Figure 10). Many previous studies reported that the Rhodobacteraceae group is closely correlated with phytoplankton blooms, for example, blooms of _Akashiwo sanguinea_, _Alexandrium_ spp., _Gymnodiinium catenatum_, _Karenia mikimotoi_, _Prorocentrum lima_, and _Leptocylindrus_ spp. [58; 59; 60; 61; 62; 63; 64]. Interestingly, most of the Rhodobacteraceae bacteria (90%) contain the vitamin B12 synthesis pathway, which is suggested to help in the growth of phytoplankton [65]. Furthermore, the Flavobacteriaceae are chemo-organotrophic [66; 67], and strains were shown to synthesize proteins that actively bind and exploit algal polymeric substrates, such as carbohydrates, polypeptides, and lipids [68]. This bacterial group can be a dominant group in particle-attached communities [69] and are also closely associated with phytoplankton blooms [59; 70]. Another bacterial group, the Colwelliaceae, although not strongly correlated with the PLF fraction in our study, is highly associated with the total amount of DOC. The genus _Colwellia_, the type genus in the Colwelliaceae,only found in oceanic environments, is known as polysaccharide-degrading bacteria [71]. It is also a classic marine secondary producer and a key player in the global carbon cycle. Colwelliaceae are involved in decomposing organic material such as hydrocarbons, lipids, proteins, and polysaccharides within particulates of the pelagic zone and sea ice, and within algal, faunal, and floral associations, where it engaged in anaerobic respiration and/or fermenting [72]. The Rhodobacteraceae and Flavobacteriaceae groups, therefore, seem to be related to the growth of _T. striata_, and the Colwelliaceae group seems to be related to the degradation or termination of _T. striata_ or the other phytoplankton cells in the BMs.