Cross-species transcriptomics and transgenic analyses implicate major LHCII genes in microalgal responses to ammonium stress.

INTRODUCTION: Nitrogen is an essential macronutrient for photosynthetic organisms, but excessive ammonium causes ammonium toxicity and severely restricts growth. Although ammonium stress has been widely documented, the conserved response modules shared by microalgae and higher plants remain poorly understood.

METHODS: We performed a cross-species transcriptomic comparison using Auxenochlorella pyrenoidosa, Chlamydomonas reinhardtii and rice (Oryza sativa) to identify conserved pathways associated with ammonium stress. To test its functional relevance, we generated overexpression lines of the major LHCII gene in A. pyrenoidosa and C. reinhardtii.

RESULTS: The photosynthesis-antenna protein pathway was detected in all three species and significantly enriched in C. reinhardtii and rice, and major LHCII antenna genes were consistently downregulated under ammonium stress. Under ammonium stress, overexpression of the major LHCII gene improved algal performance in both species, but the dominant beneficial traits differed: increased cell density and chlorophyll accumulation in A. pyrenoidosa, and increased fresh weight together with improved chlorophyll accumulation in C. reinhardtii. Physiological analyses further showed species-dependent effects under the tested conditions on ROS levels and PSII effective photochemical efficiency [Y(II)], suggesting that the major LHCII gene contributes to ammonium tolerance through conserved photosynthetic functions, although the downstream physiological responses differed between the two algae under the tested conditions.

DISCUSSION: These results identify a shared transcriptional response of major LHCII genes to ammonium stress and demonstrate that their overexpression improves growth-related traits in two microalgal species.