Abstract
Microalgae can respond to natural cues from crustacean grazers, such as Daphnia, by forming colonies and aggregations called flocs. Combining microalgal biology, physiological ecology, and quantitative proteomics, we identified how infochemicals from Daphnia trigger physiological and cellular level changes in the microalga Scenedesmus subspicatus, underpinning colony formation and flocculation. We discovered that flocculation occurs at an energy-demanding ‘alarm’ phase, with an important role proposed in cysteine synthesis. Flocculation appeared to be initially stimulated by the production of an extracellular matrix where polysaccharides and fatty acids were present, and later sustained at an ‘acclimation’ stage through mitogen-activated protein kinase (MAPK) signaling cascades. Colony formation required investment into fatty acid metabolism, likely linked to separation of membranes during cell division. Higher energy demands were required at the alarm phase, which subsequently decreased at the acclimation stage, thus suggesting a trade-off between colony formation and flocculation. From an ecological and evolutionary perspective, our findings represent an improved understanding of the effect of infochemicals on microalgae-grazers interactions, and how they can therefore potentially impact on the structure of aquatic communities. Moreover, the mechanisms revealed are of interest in algal biotechnology, for exploitation in low-cost, sustainable microalgal biomass harvesting.
Highlights
Scenedesmus spp. have predominantly been isolated in freshwater bodies and have been found in soils all over the world (Trainor, 1998)
At +20 h we noticed a significant increase in the mean number of coenobia in the cultures of S. subspicatus exposed to infochemicals, whereas the Control cultures were dominated by unicells (>70%) (Figure 2C)
Flocculation efficiency (FE) differed significantly between Control, where no flocculation occurred, and S. subspicatus exposed to infochemicals (p = 0.0009), with FE = 77.37 ± 16.93% (Figure 2D)
Summary
Scenedesmus spp. have predominantly been isolated in freshwater bodies and have been found in soils all over the world (Trainor, 1998) They are cultivated in the laboratory and can tolerate a wide range of environmental conditions, making them ideal candidates for ecological, evolutionary and biotechnology research (Lürling, 2003). Zooplankton grazers of microalgae are known to induce aggregation/flocculation, and colony formation. This response of the algae is understood to be triggered by products excreted by the zooplankton grazers (Hessen and Vandonk, 1993; Lampert et al, 1994; Lürling and vanDonk, 1996) and has been reported to be induced only by herbivorous zooplankton “chemical cues” (Lürling, 2003)
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