Investigations on the relationships between algal blooms and bacterial populations in the Schlei Fjord (western Baltic Sea)

Abstract

1. Investigations were carried out on monthly voyages to the Schlei, a fjord of the western Baltic Sea, from 1972 to 1974. 2. The Schlei is characterized by two successive water blooms - one in the spring caused by Chlorella sp., and a Cyanophyceae bloom in late summer with Microcystis aeruginosa as the dominant species. 3. The blooms are accompanied by different bacteria populations. It was noted, e. g., that the proportion of red pigmented bacteria is greatest during the spring, whereas in late summer the numbers of pigmented bacteria decrease and white or colorless forms dominate. 4. In order to determine the relationship of the bacteria to the phytoplankton blooms, the predominant algae and bacteria species in the Schlei were isolated, and laboratory experiments with these microorganisms were carried out. 5. The results of these experiments showed that the growth of the red bacteria was clearly enhanced when cultivated together with Chlorella sp. 6. The enhancement takes place immediately with actively growing Chlorella cultures. Apparently these algae secrete organic substances into the culture medium during their exponential phase of growth, which are utilized by the red bacteria. It is likely that a similar process occurs in the Schlei. The rapid increase in red bacteria has been shown to take place parallel to the development of the Chlorella water bloom in the spring. The proportion of pigmented bacteria decreases in the late summer with the breakdown of the Chlorella bloom, and a new bacterial population becomes dominant. 7. Other experiments with Chlorella sp. showed that Escherichia coli is strongly inhibited by the presence of these algae. The inhibition takes place only after Chlorella has reached the end of the exponential phase of growth, however. The inhibitory substances are probably autolysis or degradation products which accumulate in the culture medium during the stationary phase of algal growth. 8. None of the bacteria tested had any influence on the growth rate of Chlorella sp., nor were the algae cells colonized by bacteria at any time. 9. With the development of the Microcystis bloom in late summer, white or colorless bacteria are more frequently found in the waters of the Schlei. At the same time, there is generally a decrease in the saprophyte counts including pigmented bacteria. 10. Two strains of white bacteria commonly associated with Microcystis were able to suppress the growth of many other bacteria on agar plates, especially red, yellow and brown pigmented species. 11. Sterile filtrates prepared from the culture solutions of the white bacteria had no effect on the strains tested, however. The suppression on agar plates is therefore more likely due to competition for nutrients. There is no evidence that antibiotic substances are involved. 12. Filtrates prepared from non-axenic cultures of Microcystis aeruginosa during the exponential as well as stationary phase of growth also did not inhibit the growth of the bacteria tested. 13. The decrease in the numbers of saprophytic bacteria in the Schlei during the summer may be due in part to the presence of the large white colonies on agar plates which prevent other bacteria from developing. These white bacteria are apparently associated with Microcystis water blooms; possibly a symbiotic or mutualistic relationship exists between these microorganisms. © 1976 Biologischen Anstalt Helgoland.