Cell lysis of competitors and osmotrophy enhance growth of the bloom-forming alga Gonyostomum semen

Abstract

The nuisance alga Gonyostomum semen (Raphidophyceae) is known to form dense blooms in many freshwater bodies, and has increased its distribution and abundance. However, information on the mechanisms behind bloom formation and maintenance is scarce. Field observations indicate that G. semen may be favored in humic lakes. In the present study, we performed controlled laboratory experiments on cultures to test whether G. semen growth is enhanced by humic acids. In addition, we tested the mixotrophic capability of G. semen by providing it with Rhodomonas lacustris as prey. G. semen increased its growth rate in the presence of humic substances due to either acquisition of carbon or some unknown growth factor. Moreover, growth rate was enhanced in the presence of R. lacustris, which in turn was negatively influenced when cultured together with G. semen. We found no evidence of phagotrophy, but observed lysis of R. lacustris cells exposed to G. semen pre-grown with R. lacustris. Thus, our experiments suggest that cell lysis (probably via trichocyst action) and osmotrophy could be 2 mechanisms involved in establishing and maintaining dominance of G. semen in humic lakes. © Inter-Research 2008.