Jeju lava seawater enhances biological stability, growth, and biomass quality during pilot-scale inland cultivation of Tetraselmis suecica

In large-scale microalgae cultivation, contamination from grazers, competing algae, bacteria, and viruses can lead to culture collapse and productivity loss. However, pre-treating large water volumes entails high costs and logistical challenges. Therefore, securing naturally clean, biologically safe water sources that do not require sterilization is important for stable and sustainable biomass production. In this study, we evaluated the novel use of Jeju lava seawater (JLS) as a filtered water source for pilot-scale cultivation of the marine microalga Tetraselmis suecica . T. suecica growth was monitored using flow cytometry, and bacterial, viral, archaeal and eukaryotic species were identified using metagenomic analysis. In untreated natural seawater (NS), grazer contamination was detected before culture collapse, and T. suecica cell density decreased by 60.9%–70.9% within approximately two days after grazer detection, leading to early termination of cultivation. In contrast, JLS cultures showed no contamination and reached final biomass levels 2.1–4.3 times higher than those of NS cultures. Metagenomic analysis showed that NS harbored diverse microbial communities, including 6 eukaryotic, 5 viral, and 43 bacterial taxa, whereas only 9 bacterial taxa were detected in JLS, indicating a substantially lower contamination risk. NS cultures showed altered biomass composition and decreased photosynthetic pigments. The photosynthetic pigment content was 2.6–2.9 times higher in JLS cultures than in NS cultures. Furthermore, JLS cultures maintained more efficient photosynthetic activity and lower physiological stress than grazer-contaminated NS cultures. These findings highlight JLS as a clean and productive alternative water source for large-scale and stable biomass production in inland areas.