## 4 Discussion  
### DOC Released from T. striata  
Dissolved organic carbon (DOC) in the ocean is one of the largest carbon pools on Earth [46]. The extracellular release of recently fixed photosynthate is the major DOC production process in the marine ecosystem [47]. Our study revealed significant correlations between the cell density of _T. striata_ and DOC concentration in both the cultivation pond and the mesocosm experiments. When the growth rate of _T. striata_ was high, the amount of DOC secreted was high. The highest _T. striata_ cell densities were observed on day 5 (cultivation pond) and day 6 (mesocosms). Although the high cell densities were still observed on day 7 in both the cultivation pond and the mesocosms, the DOC concentrations rapidly decreased. Phytoplankton usually sequester excess carbon in the form of storage polysaccharides, such as chrysolaminarin in diatoms [48; 49] and glycogen in cyanobacteria [50]. These polymers also constitute the main fraction of phytoplankton exudates. According to [PERSON] and [PERSON] (1999), up to 37% of the DOC originating from phytoplankton consists of high-molecular-weight (>1 kDa) components, mainly hydrolyzable carbohydrates. The polysaccharides represent the largest fraction of high-molecular-weight DOC, with around 50% relative amount of the DOC in surface waters and around 25% in deep waters [51]. Various studies from the North Atlantic also support this large polysaccharide component with samples from the North Atlantic (<11% of total DOC) [52], the Norwegian Sea (<14.8%) [53], and the North Pacific (<10.1%) [54].  
In aquatic systems, bottom-up (inorganic and organic nutrient supply) factors significantly influence bacterial succession. The DOC in marine ecosystems is almost completely cycled by bacterial groups over different timescales depending on different biochemical characteristics of the water column. The DOC pool constitutes the so-called \"labile\" fraction and turns over rapidly (hours to days).  
Figure 11: Changes in inorganic nutrient concentrations: nitrate + nitrite (**A**), ammonium (**B**), phosphate (**C**), and silicate (**D**) during mesocosm experiment period (15 days). Each value of white and dark circles was averaged by two measured values of bioreactor installed mesocosms or control mesocosoms, respectively.  
The \"semi-labile\" fraction is cycled by bacteria on time scales of weeks to months [55], while the so called \"refractory\" DOC pool is biologically cycled on time scales ranging from centuries to millennia [55; 56]. Therefore, in order to understand the close variation of the bacterial community, it is necessary to examine the differing fractions of DOC. [PERSON] et al. (2011) [35] proposed that the methodology of DOC be divided into four fractions to characterize the DOC depending on the varying biological component, as well as temporal and differing regional sources. In addition, the proportion of PLF measured from phytoplankton was higher (73.9%) than the other organic matters (leaf litter, compost, and paddy water) in their experiment. In our study, a high concentration of DOC (74.1 mg\(\cdot\)L\({}^{-1}\)) and a significant correlation between the amount of DOC released from _T. striata_ and the proportion of PLF was observed in our mesocosm study; however, the PLF fraction was relatively lower than the other DOC fractions during the mesocosm experiment. Although the amount of PLF fraction was small, its changes were significantly correlated with the total amount of DOC and biomass of _T. striata_. This may be explained by the fact that the release of compounds from phytoplankton is susceptible to rapid uptake by heterotrophic bacteria [57]. This microbial degradation can induce the dramatic decrease of PLF and the enhancement of the FLF, the HLF, and the THLF fractions at the same DOC concentration [35].