## 4 Discussion
### Microbial Food Web Structure  
Installed mesocosms maintained an environment similar to the natural condition during experiments. There was little difference between the surrounding water temperature and salinity compared to that measured within the mesocosms. The DO and pH, however, varied slightly between the natural seawater and the mesocosms, but the pattern of these variables over time remained the same in each. The abrupt dissipation of phytoplankton communities could lead to a decrease in DO and pH in the BMs. The major differences in biological factors between the CMs and BMs was the abrupt decrease in the abundance of phytoplankton, followed by the ciliate and zooplankton of the BMs. The abundance of phytoplankton began to decrease on day 1 of _T. striata_ cultivation, while the ciliates began to decrease over the following two days and the zooplankton over the following three days. The initial decrease in phytoplankton coincided with a rapid decrease in \(n\) and \(p\) after _T. striata_ inoculation. Especially, the mechanism of \"luxury uptake\" is known to occur when phytoplankton including _Tetraselmis_ rapidly uptake and store excess phosphorus as inorganic polyphosphate granules [73; 74; 75; 76]. In contrast, silicate rapidly decreased in the CMs after day 3 due to the dominance of the diatom _Chaetoceros_ spp. Since inorganic nutrients are continuously supplied in natural marine environments, it is difficult to partition the rapid nutrient uptake of _T. striata_ and its effect on the growth of other phytoplankton. Another possible influence on phytoplankton growth/succession patterns could be due to alleopathy and algicidal bacteria. Although this was not the focus of the present study, it is known that many phytoplankton species are able to secrete an allelopathic substance that can inhibit the growth of other organisms [77]. A rapid increase in DOC was associated with the growth of _T. striata_, and the possibility of the presence of a large amount of allelopathic material could be a possibility to explain this increase. On day 1 when growth of phytoplankton began to decrease, the proportion of the Alternomonadaceae bacterial group increased noticeably. The dominant genus group was _Alteromonas_. Many species of _Alteromonas_ show an algicidal effect on various phytoplankton communities such as dinoflagellate, raphidophyte, and diatom, even working on phytoplankton species of freshwater [78; 79; 80; 81; 82; 83].  
The decrease in phytoplankton was followed by a decrease in the population of zooplankton and ciliates, which are higher trophic levels in nature. There may, however, be an alternate reason in our study. The cultivation of _T. striata_ using semi-permeable membranes may have physically inhibited the transfer of energy from phytoplankton to higher trophic levels. Bacterial-sized organisms are able to migrate, but the semi-permeable membrane obstructed the direct interaction of phytoplankton (_T. striata_) with ciliates and zooplankton, such that activities including feeding and decomposition were disabled. This may have resulted in an imbalanced trophic structure within the BMs, whereby the higher trophic levels were removed (or extremely reduced).