Metal ion assimilation is crucial for all life forms because metal ions are simple yet remarkably versatile protein cofactors engaged in a variety of physiological and biochemical processes. Eukaryotic cells benefit from intracellular storage organelles such as vacuoles in plants and yeast, acidocalcisomes (ACs), and other lysosome-related organelles, providing an additional strategy to maintain metal ion homeostasis.    

Lysosome-related organelles (LROs) are a class of heterogeneous organelles that are conserved in eukaryotes and largely function in storage and secretion. Mediating metal homeostasis is one of LROs key roles. A model organism for studying metal ion metabolism is Chlamydomonas reinhardtii, yet structural and functional analyses of its LROs are insufficient.   

Recently, a research group led by Prof. HUANG Kaiyao from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences optimized a method for purifying LROs from Chlamydomonas cells, and proved that LROs have different morphology and protein composition in response to different environmental conditions. This study was published in Plant Physiology.   

In this study, the researchers first purified LROs from two populations of cells which are stationary phase or overloaded with iron. They found that the morphology, elemental content, and lysosomal activities differed between the two preparations, even though both have phosphorus and metal ion storage functions.    

By using different microscopy technologies combined with immunostaining, the researchers further discovered that LROs in stationary-phase cells had multiple non-membrane-bound polyphosphate granules to store phosphorus. LROs in iron-overloaded cells were similar to acidocalcisomes, which have a boundary membrane and contain one or two large polyphosphate granules to store more phosphorus.   

In addition, the researchers established a method to quantify the capacity of LROs to sequester individual trace metals. Based on a comparative proteomic analysis of these two types of LROs, they presented a comprehensive acidocalcisome proteome and identified 113 putative acidocalcisome proteins.   

This study provides a new direction for developing microalgae as a material for bioremediation of heavy metal pollution in the environment. 

(Editor: MA Yun)