HABs are serious environmental hazards in many coastal areas because of the growing human population size and increasing industrialization, and they threaten co-occurring aquatic organisms by producing toxins. Algal–bacterial interactions are key to regulating and decreasing K. mikimotoi blooms. In this study, a novel indigenous bacterial strain of Paracoccus sp. (O-1) with potent algicidal activity was isolated from a K. mikimotoi culture. After algal cells were treated with strain O-1 with the 3 and 5% dosage, they were significantly inhibited, and algicidal activity of this bacterial strain reached 90% in 72 h, suggesting that this strain effectively killed algal cells. While reports on the application of Paracoccus to control HABs are limited, the algicidal activity of this species may play an important role in bacterial–algal interactions and natural attenuation of HABs.
The O-1 culture used in this study exhibited strong algicidal activity (> 90%), but the specific mechanism remains unfamiliar. Therefore, the mode of algicidal action should be identified. Algicidal mechanisms can be classified into two general modes, namely a direct mode, where bacterial cells must be in contact with algal cells for successful cell lysis, or an indirect mode, where an extracellular metabolite produced by algicidal bacteria is responsible for the algicidal activity (Mayali and Azam 2004; Pokrzywinski et al. 2012). The O-1 algicidal mode was found to be indirect, as the algicidal effects were likely expressed via the production of extracellular algicidal compounds, as opposed to direct cell–cell contact. These results agree with those of another study, which found that Deinococcus sp. strain Y35 exhibited a strong algicidal effect on the toxic dinoflagellate Alexandrium tamarense by producing extracellular pigments (Li et al. 2015).
To characterize the algicidal activity of the extracellular compounds of strain O-1, our results suggest that these algicidal compounds tolerated a relatively wide range of temperatures and pH conditions, but the algicidal activity was diminished under strong acidic conditions, implying that these extracellular substances were thermally stable but sensitive to acidic conditions. Other studies have reported the temperature and acidic/alkaline stability of bacterial algicides. An algicidal compound produced by Pseudomonas aeruginosa showed no change in algicidal activity after being autoclaved at 121°C for 15 min and stored at 4°C for 3 months (Dakhama et al. 1993). In contrast, Lee et al. (2000) showed that an algicidal serine protease released by Pseudoalteromonas sp. strain A28 had diminished activity after being heated for 15 min at 100°C (Lee et al. 2000). Li et al. (2016) also noted that algicidal substances produced by Exiguobacterium sp. strain h10 exhibited high thermal stability and pH instability (Li et al. 2016).
Bacterial compounds with algicidal activity can be classified into three main types: proteins or proteases (Lee et al. 2002), pigments (Li et al. 2015), and surfactants (Wang et al. 2005). The algicidal compound produced by strain O-1 in this study has yet to be identified, but its thermal stability and pH instability suggest that it is not a nucleic acid or protein. This algicidal compound is likely to be a pigment or glycolipid (Kaytee et al. 2012). This result agree with those of another study, Malimas et al. (2008) identified the pigments produced by Gluconobacter sphaericus (Malimas et al. 2008). The algicidal compound produced by strain O-1 may be a novel algicide for K. mikimotoi blooms and should be identified and further characterized in future studies.
To determine the specificity of the algicidal activity of strain O-1, its effects on two other Chlorophyta species were tested. Strain O-1 had a strong inhibitory and lethal effect on K. mikimotoi, whereas it was ineffective on the other two tested species. The algicidal activity of strain O-1 was therefore exhibited species-specific. These results conclude that fibrous cell wall of chlorophytes may make them resistant to the “non-nucleic acid or non-protein” metabolite. Similarly, Doucette et al. (1999) revealed that strain N-3 had a relatively specific inhibitory effect on the dinoflagellate Gymnodinium nagasakiense (Doucette et al. 1999). Algicidal bacteria that have been described may aid the development of technologies for mitigating HABs.
The algicidal activity of strain O-1 against K. mikimotoi depended on the concentration of the bacterial culture and treatment time. Compared to the control group, the 1% bacterial dose stimulated the growth of K. mikimotoi after 5 d (Fig. 2). This may be related to hormesis (Stebbin 1982), as K. mikimotoi cells exhibited some resistance to the adverse effects of the O-1 algicide. At bacterial doses of 3 and 5%, the algicidal effect was complete after 5 d. The algicidal activity was somewhat correlated to the bacterial cell density (Zhang et al. 2016). Increased algicidal activity may have been due to increased algicidal compound production per cell with increased bacterial growth under favorable conditions.
To optimize the culture conditions of O-1 cells, a single-factor design experiment was conducted to determine the optimal thermal and pH conditions. The optimal conditions for this bacterium occurred at 30 ℃ and pH 6.5. Ma et al. (2015) investigated the influence of environmental factors, including temperature, illumination intensity, and pH, on the growth of two algicidal bacteria using the Plackett–Burman design (Ma et al. 2015). Our results suggested that environmental factors (pH and temperature) may play essential roles in the quantity of strain O-1, thereby these factors significantly affected the algicidal activity of the algicidal bacterial strain, which was consistent with the results of a previous study (Yoon et al. 2011).
In conclusion, this study showed that the screening of novel Paracoccus sp. algicide, is beneficial for increasing knowledge and understanding of bacteria with bioactive potential in the marine environment. Strain O-1 has the potential to be an algicidal bacterium against HABs caused by K. mikimotoi in future.