Competition between toxic and non-toxic strains within the genus Microcystis under nitrogen limitation

Abstract

Microcystis blooms are implicated in toxic algal blooms worldwide due to the production of toxins microcystins. Blooms have caused poisoning of humans and other animals, and annually contribute to economic loss. The biological role of microcystin synthesis by the genus Microcystis remains an unanswered question. Consequently, the factors that select for microcystin producing (toxic) phenotypes over non-toxin producing phenotypes are unknown. The aim of my thesis was to investigate if toxin producing strains of Microcystis are selected for over non-toxin producing strains in nitrogen limited, low N:P ratio, environments. In an effort to investigate the universality of the relationship between toxicity competition under nitrogen stress, strains investigated span 3 species: M. aeruginosa, M. viridis, and M. botrys. The first part of my thesis work included an investigation of 17 strains of M. botrys. I demonstrated that 10 strains which initially produced microcystins have lost large portions of the mcy gene cluster and the corresponding toxic phenotype while under culture. In the second part of my thesis work I conducted batch culture experiments, both monoculture and co-culture, in order to test the hypothesis that microcystin producing strains have a competitive advantage and be selected for under nitrogen limitation. There was no evidence that nitrogen limitation selected for toxic phenotypes. However, cell volume and microcystin production may have interacted to influence competition outcomes under nitrogen limitation.

First a review of the literature will be provided followed by a detailed description of the experimental design. Results of the experiments will be presented and discussed in the following sections.

Popular Abstract

Competition under nitrogen limitation in genus Microcystis

Microcystis is a genus of cyanobacteria. Cyanobacteria are photosynthetic bacterial plankton that have been present for approximately three billon years. These single celled organisms inhabit fresh and saltwater, and through photosynthesis contribute significantly to global oxygen production. Some cyanobacteria produce toxins which have caused illness and death in animals including humans. Microcystis which produces the toxin microcystin is most commonly responsible for dangerous harmful algal blooms in fresh waters. Scientist have been studying Microcystis for decades to better understand how these blooms form. Interestingly, not all of the cells in a population produce microcystins. It is mystery why these two different types of cells coexist in populations at the same time.

To explain this, it suggested that producing microcystins has a cost and must in some situations provide a benefit. Scientist have been able demonstrate a cost by showing that cells that produce microcystins grow more slowly. But determining a clear benefit has been more difficult. A possible explanation is that microcystins improve the ability for cells to cope with resource limitation. It is a widely observed pattern that the proportion of cells that produce microcystins increase as the bloom develops and nitrogen become limiting. Studies investigating the potential role of microcystins in providing tolerance to nitrogen limitation have provided some promising but also contradictory results. There is need for more research in order to determine if there is a meaningful relationship.

The aim of my thesis was to investigate if toxic strains of Microcystis are selected for over non-toxic strains in reduced nitrate environments. This was investigated by determining the presence of genes which code for toxin production in several different strains and determining if these strains were producing toxins. I then tested my hypothesis that toxic strains were favored by nitrogen limitation through monoculture and co-culture competition experiments. These culture experiments were designed to compare treatment conditions of nitrogen limitation to nutrient replete conditions.

There were 3 main findings of this work. First, it was demonstrated that the genes which code for microcystin production are unstable and can be lost, so toxic strains can become non-toxic. Second, there was no support that nitrogen limitation selects for toxic strains. Third, microcystin production may interact with cell size to explain competition outcomes. A review of the literature and ecological theory demonstrates how this interaction may occur. Smaller cells are generally more competitive for a limiting nutrient, and the metabolic cost of synthesis may cause toxic strains to have a higher demand for nitrogen. This is a possible explanation for the results of my competition experiments which showed that smaller cells were only at a competitive advantage when limited by nitrogen if they were non-toxic.

My thesis work has suggested directions for further research. There should be a though investigation of the metabolic cost of microcystin production and the minimum nitrogen cell quota of toxic strains should be compared that of non-toxic strains. There is also a need to further investigate the mechanisms for loss of genes which code for microcystins. It should be explored whether this loss is reversable and whether there are conditions that promote it.

Advisor: Karin Rengefors PhD
Aquatic Ecology, Biology Department, Lund University

• author: Wood, Ethan
• supervisor: Karin Rengefors
• organization: Degree Projects in Biology
• course: BION03 20192
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9028778
• date added to LUP: 2020-09-09 16:17:57
• date last changed: 2020-09-09 16:17:57