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WAX AND WANE OF MICROCYSTIS (CYANOPHYCEAE) AND MICROCYSTINS IN LAKE SEDIMENTS: A CASE STUDY IN QUITZDORF RESERVOIR (GERMANY)1

Benthic stages of the annual life cycle of the meroplanktonic cyanobacterium Microcystis spp. in relation to microcystin (MCYST) dynamics in sediments of a shallow lake (Quitzdorf Reservoir, Germany) were investigated. Based on changes in the absolute abundance of benthic Microcystis, the annual life cycle was subdivided into four phenological stages: reinvasion, pelagic growth, sedimentation, and overwintering. Habitat‐coupling processes, such as reinvasion of the pelagic zone in spring as well as autumnal sedimentation, were particularly triggered by changes in water temperature. During reinvasion substantial losses of Microcystis were detected. Only a minor part of benthic Microcystis (about 3%) formed the inoculum for pelagic growth. Between 65% and 85% of the benthic Microcystis stock disappeared during the reinvasion phase. Because these colonies were neither detected within the sediments nor in the pelagic inoculum, it was concluded that they were subjected to decay. The occurrence of extracellular MCYSTs in the pelagic zone during this period, which cannot solely originate from the pelagic Microcystis population, supports this conclusion. Dynamics of benthic Microcystis and MCYSTs were characterized by almost identical successions with a decrease during reinvasion, an increase during sedimentation, and remarkable invariability throughout pelagic growth and overwintering. It can be deduced that MCYSTs are preserved within benthic resting stages of Microcystis because they could play a role during overwintering or reinvasion.

Impact of inorganic carbon availability on microcystin production by Microcystis aeruginosa PCC 7806.

ABSTRACT Batch culture experiments with the cyanobacterium Microcystis aeruginosa PCC 7806 were performed in order to test the hypothesis that microcystins (MCYSTs) are produced in response to a relative deficiency of intracellular inorganic carbon (Ci,i). In the first experiment, MCYST production was studied under increased Ci,i deficiency conditions, achieved by restricting sodium-dependent bicarbonate uptake through replacement of sodium bicarbonate in the medium with its potassium analog. The same experimental approach was used in a second experiment to compare the response of the wild-type strain M. aeruginosa PCC 7806 with its mcyB mutant, which lacks the ability to produce MCYSTs. In a third experiment, the impact of varying the Ci,i status on MCYST production was examined without suppressing the sodium-dependent bicarbonate transporter; instead, a detailed investigation of a dark-light cycle was performed. In all experiments, a relative Ci,i deficiency was indicated by an elevated variable fluorescence signal and led to enhanced phycocyanin cell quotas. Higher MCYST cell quotas (in the first and third experiments) and increased total (intracellular plus extracellular) MCYST production (in the first experiment) were detected with increased Ci,i deficiency. Furthermore, the MCYST-producing wild-type strain and its mcyB mutant showed basically the same response to restrained inorganic carbon uptake, with elevated variable fluorescence and phycocyanin cell quotas with increased Ci,i deficiency. The response of the wild type, however, was distinctly stronger and also included elevated chlorophyll a cell quotas. These differences indicate the limited ability of the mutant to adapt to low-Ci,i conditions. We concluded that MCYSTs may be involved in enhancing the efficiency of the adaptation of the photosynthetic apparatus to fluctuating inorganic carbon conditions in cyanobacterial cells.

Phytoplankton appearance in particle size spectra – Deriving conversion functions between microscopic and particle counter measurements

Analysis of aquatic field samples by particle counters are a widespread method but the representation of phytoplankton abundance and of size classes in which phytoplankton appears in the resulting size spectra is not well studied. To address this gap, two freshwater phytoplankton species were analysed in a particle counter and using a microscope: the colony forming Asterionella formosa (Bacillariophyceae) and the single-celled Microcystis aeruginosa (Cyanobacteria). Field samples, growth experiments and model approaches were used to study the image of phytoplankton derived by two different commonly used and standardized counting methods. In our results, the colony forming A. formosa had to be considered in units of colonies because the counting device enumerated only 23% of the single cells but 85% of the colonies that were determined under the microscope. Furthermore, the size class representation in the particle counter of both taxa appeared in much smaller ranges than expected from microscopic size measurements. Model simulations of movements and rotations of phytoplankton in the measuring device can explain half of the size shift. We deduce that about 86% of the cell areas of both studied species are transparent from two approaches. First, areas derived from simulations of rotated phytoplankton colonies equal the measured particle spectra of the laboratory cultures when the shadow areas are reduced to 14%. Secondly, field counts of A. formosa can be integrated into particle size spectra of the total particulate material when the same reduction factor is applied. For the considered optical counting device, field samples of A. formosa can be detected in particle size spectra when colony sizes as well as transparency of the cells and reduction of cell sizes by rotations are taken into account.