[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-01] Rapid turnover and succession of seawater bacterial community induced by the presence of seagrasses: potential implication for environmental management

*Toshihiro Miyajima¹ (1.Division of Ocean-Earth System Science, Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:seagrass beds, coral reefs, environmental protection, microorganisms, ecosystem function

Seagrasses are marine vascular plants that form submarine meadows, often called seagrass beds, in shallow sedimentary coasts. By leaching dissolved organic matter from their leaves and roots, seagrasses form characteristic microbial and animal communities on their phyllosphere and in their rhizosphere, referred to as holobionts. Seagrasses have also been considered to influence planktonic microbial community in seawater distant from them, inhibiting the growth of exotic microorganisms of anthropogenic origin and coral pathogens (Lamb et al. Science 355:731, 2017). Such functions of seagrass beds are expected to have high potential in the context of maintaining the ecosystem health of, e.g., coral reefs, and also sanitation in the aquaculture industry, among others. However, it is difficult to demonstrate directly the benefits of seagrass on the microbial environment in seawater in a field with reproducibility being difficult to ensure. Therefore, the road to field implementation is thus not expected to be easy.

This study, being conducted in an attempt to harness the function of seagrass beds to improve environment of coral reefs, was aimed to demonstrate the regulating effects of subtropical seagrass species on seawater microbial communities in a simple laboratory experiment. Seagrasses were collected from seagrass beds adjacent to coral reefs on Ishigaki Island, Okinawa, and incubated under irradiation (12h:12h) in closed aquaria with circulating seawater. Changes in the bacterial community in the incubated seawater was monitored using amplicon sequence analysis for the 16S rRNA V3/V4 region. In some experiments, the protist community was also characterized using 18S rRNA amplicon sequencing. As the incubation medium, filtered seawater inoculated with a small amount of mangrove effluent water as a model of an exotic microbial community was used.

The results showed that the microbial community in seawater underwent a rapid turnover within only a few days, and that the turnover in species composition was particularly dramatic in the presence of seagrass. While changes also occurred in the control system without seagrass, the bacterial species composition was quite different from that in the presence of seagrass. Bacterial species that rapidly declined and disappeared in the presence of seagrass were identified, and many of them showed similar responses regardless of seagrass species used. On the other hand, many bacterial species were also identified that increased either rapidly or slowly in the presence of seagrass, with some of them being dependent on seagrass species, while others grew regardless of seagrass species.

These observations show that seagrass meadows exert a strong regulatory effect on the microbial community in seawater. Bacterial species adapted to individual seagrass species or to seagrasses in general were selected for growth, while those not adapted to seagrass beds were rapidly eliminated. Since exotic harmful microorganisms such as pathogens and toxic bacteria are not considered to be adapted well to the conditions in seagrass meadows, seagrasses are expected to function effectively to eliminate such harmful microorganisms. Although the mechanism by which such community turnover occurs cannot be proven from this study, experimental data suggest that the interaction between the growth-promoting effect of seagrass metabolites on certain bacteria and the non-specific suppressive effect of predatory protists, mainly ciliates, on bacteria (microbial loop) drives the rapid turnover of bacterial community. However, because turnover of bacterial community around seagrass beds takes a few days, the rate of seawater exchange in the seagrass beds, which is constrained by tides and other factors, is expected to be an important factor in determining the extent to which the microbial community in the field is actually regulated by seagrass beds