The response mechanism of alkalophilic Nitzschia sp. NW129 to low alkalinity-A study combining physiological and transcriptional analysis

Global aquatic acidification significantly threatens alkaline lake ecosystems. The mechanisms by which alkaliphilic microalgae, the key producers in these ecosystems, respond to reduced environmental alkalinity remain poorly understood. Here, we investigated the responses of alkalophilic Nitzschia sp. NW129 to low alkalinity (pH 9.2) through integrated physiological-biochemical and transcriptomic analyses. Relative to the control (pH 11.5), we observed a 60.1 % decrease in polysaccharide content, while total lipids and proteins increased by 1.74-fold and 2-fold, respectively. Transcriptome analysis revealed up-regulation of genes encoding carbonic anhydrase (CA) and malic enzyme (ME), along with those involved in glycolysis and fatty acid (FA) synthesis, compensating for carbon supply and shifting carbon flux from carbohydrate synthesis to lipid accumulation. Enhanced expression of TCA cycle genes and those encoding F-ATP synthase and inorganic pyrophosphatase (PPase) provided sufficient energy for cellular homeostasis, further facilitated by the up-regulated expression of ATP-dependent V-ATPase and ABC transporter genes. Temporal analysis revealed that the expression of genes involved in protein synthesis pathways was up-regulated on days 1 and 4 but notably down-regulated on day 2, suggesting protein degradation at this time to balance energy supply for adaptation. Despite these coping shifts, impairments in photosynthetic energy dissipation and electron transport, along with transcriptional changes including down-regulating cell cycle and inducing apoptotic pathways, ultimately caused a substantial reduction in biomass. These findings provide a basic understanding of the response mechanisms of alkalophilic microalgae to low alkalinity stress, which should aid to develop strategies to improve microalgal tolerance against acidification.