Interactions Between Harmful Algae and Algicidal and Growth-Inhibiting Bacteria Associated with Seaweeds and Seagrasses

Abstract

Phytoplankton and bacteria are the main components of marine ecosystems and hence comprise abundant numbers and biomass in the sea. There exist close and diverse interactions between phytoplankton and bacteria in marine ecosystems. Among these relationships, bacteria possessing algicidal and growth-inhibiting activities against phytoplankton have been increasingly gathering attention and are expected to control phytoplankton dynamics especially harmful algal blooms in coastal environments. Algicidal activities are divided into two types, i.e., the direct attack type, needing direct attachment to host algal cells, and the indirect attack type, producing algicidal matter. Some bacteria kill part of algal populations, and they are growth-inhibiting bacteria. In harmful algal blooms (HABs) of such as the raphidophytes Heterosigma akashiwo and Chattonella spp., bacteria play an important role in the bloom terminations. Most algicidal bacteria are particle-associated forms in coastal seas. A new finding is that huge numbers of algicidal bacteria against HAB species (dinoflagellates and raphidophytes) were attached to the surface of seaweeds such as the green alga Ulva pertusa, the red alga Gelidium spp. and the brown algae Sargassum muticum and S. thunbergii. The densities of these algicidal bacteria reached as many as 106 g−1 wet weight. Further, algicidal bacteria were found in the biofilm of the seagrass Zostera marina with high densities of 107 g−1 wet blade or more. In the case of the toxic dinoflagellate Alexandrium tamarense, growth-inhibiting bacteria with strong activities were isolated from the blade of Z. marina. In developed countries, seagrass- and seaweed beds have been lost by reclamation for increasing land of commercial uses. We here propose restoration of seagrass- and seaweed beds, not only for increasing the nursery grounds of various marine lives but also for creating preventative strategies for HAB occurrences. This is a kind of activity in conformity with the Sato-Umi concept recently proposed.