## 1 Introduction  
Over the past two decades, the cultivation of microalgae as a source of sustainable biomass for the production of biofuels gained much attention [1; 2]. Marine microalgae cultivation is relatively low cost, has a low space requirement, and can utilize a natural source of nutrients for algal growth [3]. The green alga _Tetraselmis_ sp. (Chlorodendraceae) is regarded as a suitable candidate microcalga for bio-diesel production due to its high lipid content and fast growth [1; 4; 5]. As a consequence, various studies were carried out to search for the optimal growth conditions of this species [6; 7; 8] and to increase the lipid content using genetic, biotic, and environmental manipulations [9; 10; 11; 12; 13]. More recently, _Tetraselmis striata_ was cultured near Yeonghueung Island, Korea, in a \"marine culturing field\"--a floating marine system which uses semi-permeable membranes for cost-effective mass cultivation [14; 15] (Figure 1). This system also uses minimal power for temperature control, as well as a minimal supply of nutrients, and it requires insignificant mixing compared to conventional, large-scale algal culturing systems [16; 17; 18; 19]. This large culturing facility, however, may pose adverse effects on the surrounding marine environment, many of which are yet to be investigated. Firstly, this large-scale marine culturing field may cause detrimental effects to the underlying benthic ecosystem by obstructing light usually transmitted through the water column [20; 21; 22]. Secondly, any damage to the culturing marine culturing field due to natural disasters such as typhoons or tsunamis may cause large quantities of microalgae to be released into the natural ecosystem. Finally, large quantities of dissolved organic matter (DOM) are continually released into the environment from this facility due to the use of semi-permeable membranes [23; 24]. While reduced light transmission and natural disasters are localized events, the release of DOM into the coastal environment can be significant, resulting in coastal eutrophication and the potential for other microalgal blooms to occur including those that are harmful. The DOM released by phytoplankton is also a ubiquitous process, often resulting in 2%-50% of the carbon fixed by photosynthesis leaving the cell [25]. This supply of carbon is thought to be the main autochthonous source of dissolved organic carbon (DOC) into marine ecosystems and a major source of carbon for heterotrophic prokaryotes [25]. A subsequent change in the release of DOC into the marine environment may also cause a change in bacterial composition within the microbial ecosystem [26; 27; 28]. As these bacterial communities form the basis of microbial food webs, playing a significant role in marine biogeochemical cycling and energy transfer, any change in their community structure is likely to also affect organisms at higher trophic levels [29; 30]. Indeed, recent research by [PERSON] et al. (2017) [31] showed that the DOC measured near Yeongheung Island appeared to be an important factor affecting the abundance and composition of bacteria and phytoplankton in this area. At present, the size of the marine culturing field near Yeonghueung Island, Korea, gradually expanded to 1 ha (10,000 m\({}^{2}\)) with the capacity to cultivate 280 tons of _T. striata_. Whilst the size of the marine culturing field gradually expanded, the environmental impacts are yet to be studied. It is hypothesized that a large amount of DOC entering the region may change the composition of bacteria, and a sudden bloom of _T. striata_ may inhibit the growth of other competitive organisms and/or affect organisms at higher or lower trophic levels. With this in mind, we firstly estimated amount of DOC released from _T. striata_ culture when the marine culturing field was in operation, and we compared the bacterial composition inside and outside of the semi-permeable membrane in which _T. striata_ was cultured.