Algicidal and growth-inhibiting bacteria (GIB) are promising tools for mitigating the negative impacts of harmful algal blooms in coastal environments; however, there is a paucity of quantitative data describing their ecological effects. Here, we investigate their temporal and spatial distribution in the southwest Yatsushiro Sea during the summer of 2011, by co-culturing bacterial isolates with an axenic culture of a fish-killing raphidophyte Chattonella antiqua. This novel co-culturing method excludes other biological influences, such as promotion of algal growth by some bacteria. The highest density (1.6 × 10⁴ cells mL⁻¹) of algicidal bacteria (AB) was detected at the surface of the nearshore station on 3 August 2011 when the maximum chlorophyll a concentration was observed, associated with the high abundance of two centric diatoms, low abundance of dinoflagellates and the absence of C. antiqua. On 10 August, total bacterial abundance increased at 10 m, suggesting that the bacteria used decaying organic matter derived from dead microalgae sinking to the bottom to promote their growth. Most of AB (87.5%) were found to be particle associated, suggesting these bacteria attached to the dead phytoplankton particles after killing them. Two types of GIB impaired co-cultured C. antiqua by inducing unusual cell deformations, including round and elongated cell morphologies. Partial 16S rRNA sequencing showed that algicidal and GIB were mainly comprised of “γ- and a-proteobacteria”. This study provides a new perspective on the dynamics of AB, suggesting that they play a significant role in regulating microalgal composition, physiology and abundance in coastal marine environments.

The intensity and frequency of harmful algal blooms (HABs) have increased, posing a threat to human seafood resources due to massive kills of cultured fish and toxin contamination of bivalves. In recent years, bacteria that inhibit the growth of HAB species were found to be densely populated on the biofilms of some macroalgal species, indicating the possible biological control of HABs by the artificial introduction of macroalgal beds. In this study, an artificially created Ulva pertusa bed using mobile floating cages and a natural macroalgal bed were studied to elucidate the distribution of algal growth-limiting bacteria (GLB). The density of GLB affecting fish-killing raphidophyte Chattonella antiqua, and two harmful dinoflagellates, were detected between 10⁶ and 10⁷ CFU g⁻¹ wet weight on the biofilm of artificially introduced U. pertusa and 10 to 10² CFU mL⁻¹ from adjacent seawater; however, GLB found from natural macroalgal species targeted all tested HAB species (five species), ranging between 10⁵ and 10⁶ CFU g⁻¹ wet weight in density. These findings provide new ecological insights of GLB at macroalgal beds, and concurrently demonstrate the possible biological control of HABs by artificially introduced Ulva beds.

The predation of commercially important Japanese sea cucumber Apostichopus japonicus by the kelp crab Pugettia ferox, widely distributed in coastal northeast Asia, was examined in field sampling and laboratory experiments. The ossicles of A. japonicus were detected from the stomach contents of a natural population of P. ferox at the rate of 32.4% of 68 individuals collected within artificial intermediate sea cucumber reefs for releasing hatchery-produced juveniles in December 2018. In the following laboratory experiments, a high mortality rate (7.7 ± 2.4 individuals day^–1) of juvenile A. japonicus (15.35 ± 2.47 mm) was observed despite the different sizes and sex of P. ferox tested. It was also confirmed that a maximum of five sea cucumbers was killed and cut into small pieces within the first 2 h. Smaller and younger P. ferox individuals (adolescent) between carapace widths of 14.2–17.8 mm actively decorated themselves using pieces of chopped sea cucumber after feeding. Attached pieces of sea cucumber were observed to be fully eaten within a week, suggesting a possible strategy by P. ferox of short-term food storage as well as mimicry. This study demonstrates considerable evidence that predation mortality by sufficiently mobile P. ferox on commercially important A. japonicus can be significant, causing high mortality at the early life stage in the natural environment, especially in areas releasing hatchery-produced juveniles. It is also worth noting that the utilization of freshly chopped sea cucumbers as decoration material and food storage is a unique and novel ecological trait of P. ferox.

Temporal changes of algicidal and growth-inhibiting bacteria on the fish-killing raphidophyte flagellate, Chattonella antiqua, at an eelgrass (Zostera marina) bed in southern Ariake Sea, Japan in 2011 was investigated. The maximum value (5.1 × 10⁷ CFU g⁻¹ wet leaf) of algicidal bacteria (AB) was detected from a biofilm formed on Z. marina on August 1 when AB in the adjacent seawater had also peaked (1.2 × 10⁴ CFU mL⁻¹). Two causative bacteria isolated from the biofilm and seawater on August 1 were both identified to be of the genus Alteromonas (γ-proteobacteria). AB and growth-inhibiting bacteria (GIB) were present from the beginning of sampling (May 20) to August 26, fluctuating between 8.6 × 10² and 1.2 × 10⁴, 1.2 × 10³ and 9.3 × 10³ CFU mL⁻¹, respectively. The highest phytoplankton density observed was 6423 cells mL⁻¹ on September 29 and was comprised of centric diatoms such as Chaetoceros, Skeletonema, and Thalassiosira and coincided with the absence of AB and GIB where the decline of Z. marina was also observed. These findings provide a new ecological insight on AB and GIB associated with Z. marina beds, indicating eelgrass beds have the important role as the nursery of those bacteria that can be utilized as mitigation measures of harmful algal blooms (HABs) in the future.