About the project
The aim of this project was to explain the significance of morphological changes of filamentous cyanobacteria when filter-feeding daphnids are present. We proposed two hypotheses that could explain why cyanobacterial filaments increased in thickness and simultaneously decreased in length in the presence of daphnids and their infochemicals (the phenomena observed in preliminary experiments). According to the first hypothesis, thicker and simultaneously shorter filaments are more resistant to grazing. To test this hypothesis, the effect of thick and thin cyanobacterial filaments on life history response of daphnids was checked. We also checked how the presence of Daphnia infochemicals affects the cell ultrastructure in cyanobacteria. – in other words, whether the thickness of cyanobacterial cell wall increases in response to grazer infochemicals. According to the second hypothesis, cyanobacteria with thicker filaments gain competitive advantage over those with thinner, unaltered filaments. In this scenario, morphological response of cyaobacteria to Daphnia was perceived as the result of availability of additional amount of nutrients, especially nitrogen that Daphnia can supply to the pool of resources. Thus, we expected that cyanobacteria should grow faster in the presence of Daphnia exudates. We also checked how Daphnia exudates affect the competition between filamentous cyanobacteria and green algae (Acutodesmus obliquus).
This study was conducted under controlled laboratory conditions on different species and strains of filamentous cyanobacteria, strains obtained from culture collections as well as newly isolated ones from lakes localized in Wielkopolska region (Poland). Experiments were carried out in batch cultures (photo 1) and in continuous culture chemostat systems (photo 2). Experimental cultures were maintained in phytotrons (special culture rooms) under controlled temperature, humidity, photoperiod. Cyanobacterial samples collected during the experiments were analyzed using light microscope. To analyze ultrastructure of cyanobacterial cells, cyanobacterial filaments were observed under transmission electron microscope (TEM).
The results of this study provided answers on the key questions posed in the project. Firstly, the presence of Daphnia exudates cause not only thickening of cyanobacterial filaments but they also promote fast growth of cyanobacteria; these changes in morphology and physiology of cyanobacterial cultures are due to the availability of ammonium ions that Daphnia supply to pool of the food source. Secondly, filament thickening induced by Daphnia is common type of a response for nostocalean cyanobacteria (order Nostocales), whereas cyanobacteria of the order Oscillatoriales do not thicken their filaments in such conditions. Thirdly, competition experiments showed that green algae can effectively compete with filamentous cyanobacteria in the absence of Daphniainfochemicals. However, the presence of Daphnia cues affects negatively the competitive abilities of green algae and leads to the exclusion of these organisms from the arena of competition. Fourthly, filament thickening induced by Daphnia infochemicals does not strengthen markedly the negative influence of cyanobacteria on life history of Daphnia. Fifthly, Daphnia infochemicals do not affect significantly the thickness of a cell wall in cyanobacteria. Sixthly, filament thickness can determine the resistance of filaments to grazing, as one of our experiments showed that thin filaments are grazed more effectively than thicker ones.
The results of this project expand the knowledge about cyanobacteria-Daphnia interactions. They also show how particular factors like light availability, nutrients availability, competitor presence or grazer presence can affect the morphology, ultrastructure and physiology of filamentous cyanobacteria.