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Investigating the Mechanism Behind the Release of Microcystins in Freshwater Cyanobacteria

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Version 2 2023-09-22, 02:15
Version 1 2021-12-09, 10:39
thesis
posted on 2023-09-22, 02:15 authored by Nicolai, Rossella

The frequency and distribution of toxic cyanobacterial blooms are increasing globally, creating the need for a better understanding of the processes involved in toxic secondary metabolite production. Microcystins (MCs) are potent hepatotoxins produced by a wide range of bloom-forming cyanobacteria genera such as Microcystis and Planktothrix. Although the release of MCs to the extracellular environment has long been considered a by-product of cell lysis and death, several studies suggest the presence of a mechanism that actively transports these toxins outside the cell membrane. The aim of the present study was to find evidence for a link between cell lysis and concentrations of extracellular MCs. A dual-fluorescence cell viability assay using the nucleic acid stain SYTOX Green was optimised for use on Microcystis and Planktothrix. A SYTOX Green concentration of 1 µM, and an incubation time of 30 minutes, yielded a bright and even fluorescent signal that readily identified lysed cells.  The improved staining technique, in conjunction with liquid chromatography-mass spectrometry analyses, was employed in a culturing experiment to track the transfer of MCs to the extracellular environment in relation to the amount of cell lysis. For Microcystis, there was a strong and significant positive relationship between cell lysis and the concentration of extracellular MC. When the extracellular MC was predicted according to cell lysis levels and the MC content per cell, lysed cells were a major contributor of MCs to the extracellular environment, although the model overestimated the concentrations. Relationships for Planktothrix were significant but weaker, possibly due to reduced accuracy in the cell enumeration step, which would have altered the calculated MC content per cell.  Whilst these findings support the hypothesis that cell lysis is the main contributor of extracellular MCs, the results do not exclude a role of MCs as signalling molecules. The recent finding that programmed cell death may occur in Microcystis under various environmental conditions may explain the commonly observed increase in extracellular MCs. Understanding the mechanisms involved in the transfer of MCs to the extracellular environment will provide further clarification on the function of these secondary metabolites and lead to the improvement of water quality management strategies.

History

Copyright Date

2020-01-01

Date of Award

2020-01-01

Publisher

Te Herenga Waka—Victoria University of Wellington

Rights License

CC BY-ND 4.0

Degree Discipline

Ecology and Biodiversity

Degree Grantor

Te Herenga Waka—Victoria University of Wellington

Degree Level

Masters

Degree Name

Master of Science

ANZSRC Type Of Activity code

1 PURE BASIC RESEARCH

Victoria University of Wellington Item Type

Awarded Research Masters Thesis

Language

en_NZ

Victoria University of Wellington School

School of Biological Sciences

Advisors

Ryan, Ken; Puddick, Jonathan