Nickel from olivine dissolution induces diel photodamage and metabolic adjustment in Gracilariopsis lemaneiformis.

Nickel (Ni) released from olivine weathering during ocean alkalinity enhancement (OAE) may threaten the carbon sequestration capacity of nearshore primary producers. Here, the holobiont of Gracilariopsis lemaneiformis was exposed to a Ni gradient (20-200 μg/L) for 15 days to simulate near-field exposure, combined with 16S rRNA sequencing and targeted metabolomics, to investigate the effects of Ni stress on algal-bacterial carbon metabolism and its light-dark dependent responses.Ni exposure disrupted carbon fixation through altered energy acquisition, metabolic allocation, and growth accumulation, with clear diurnal differences. During the light period, Ni stress mainly induced photodamage, including pigment degradation and impaired photosystem II (PSII) function, thereby limiting ATP and NADPH supply. In contrast, the dark period was characterized by metabolic repair, with enhanced glycolysis, tricarboxylic acid (TCA) cycle, and pentose phosphate pathway (PPP), accompanied by increased α-ketoglutarate and succinate accumulation to maintain energy homeostasis. Meanwhile, the associated microbial community shifted from a "photosynthetic compensation" state to a "tolerance-adaptation" state. Overall, Ni stress disturbed carbon metabolism through a diurnal "daytime photodamage-nighttime metabolic repair" mechanism, potentially affecting the carbon accumulation capacity of G. lemaneiformis. These findings provide new insights into circadian-regulated algal-microbial responses to Ni stress and contribute to ecological risk assessment of OAE in nearshore environments.