The influence of elevated SiO₂(aq) on intracellular silica uptake and microbial metabolism

Microbes are known to accumulate intracellular SiO₂(aq) up to 100s of mmol/l from modern seawater (SiO₂(aq) <100 µmol/l), despite having no known nutrient requirement for Si. Before the evolution of siliceous skeletons, marine silica concentrations were likely an order of magnitude higher than the modern ocean, raising the possibility that intracellular SiO₂(aq) accumulation interfered with normal cellular function in non-silicifying algae. Yet, because few culturing studies have isolated the effects of SiO₂(aq) at high concentration, the potential impact of elevated marine silica on early microbial evolution is unknown. Here, we test the influence of elevated SiO₂(aq) on eukaryotic algae, as well as a prokaryote species. Our results demonstrate that under SiO₂(aq) concentrations relevant to ancient seawater, intracellular Si accumulates to concentrations comparable to those found in siliceous algae such as diatoms. In addition, all eukaryotic algae showed a statistically significant response to the high-Si treatment, including reduced average cell sizes and/or a reduction in the maximum growth rate. In contrast, there was no consistent response to the high-Si treatment by the prokaryote species. Our results highlight the possibility that elevated marine SiO₂(aq) may have been an environmental stressor during early eukaryotic evolution.