Chloroplast movements in siphonous macroalgae in response to high light and grazing

Abstract

Fast cytoplasmic streaming enables extensive chloroplast movements in the giant cells of unicellular, siphonous macroalgae. Here, we studied chloroplast movements in two such algae: the Dasycladalean Acetabularia acetabulum and the Bryopsidales Bryopsis sp.. We hypothesised that chloroplast movements function as a protective avoidance mechanism under excess light, particularly in Bryopsis sp., which lacks capacity for fast induction of photoprotective non-photochemical quenching (NPQ) and state transitions. In addition, we also investigated whether chloroplast movements are involved in responses to wounding and herbivory. The movements were studied by light microscopy, photography and pulse modulated chlorophyll a fluorescence quenching analysis. Chemical inhibitors of actin polymerization and microtubules assembly were used to confirm that the observed effects were active responses controlled by the cytoskeleton. A. acetabulum responded to high light by reversible chloroplast aggregation, probed by macro-imaging; and chemical inhibition of chloroplast movements led to an enhancement of Photosystem II photoinhibition, as probed by the fluorescence parameter FV/FM. No chloroplast movements were observed in Bryopsis sp. in response to high light. In A. acetabulum, wounding caused either by cutting or due to feeding by the sap-sucking sea slug Elysia timida triggered aggregation of chloroplasts within minutes of incurring the damage. Interestingly, the aggregation also occurred in intact cells away from the cutting site. Furthermore, the addition of media collected from the vicinity of cut algae was sufficient to induce chloroplast aggregation in intact algae, suggesting that water-borne cues or signals triggered the aggregation response in A. acetabulum. Bryopsis sp., however, responded to cutting by only local chloroplast aggregation. The relevance of chloroplast movements in protection against both abiotic and biotic stressors in A. acetabulum, and the potential reasons behind the different defence strategies of the algae, are discussed.