High carbon dioxide and silicon-aluminum interactions remodel cell walls: Surface architecture and adhesion in the marine diatom Phaeodactylum tricornutum.

Anthropogenic carbon dioxide (CO2) elevation increases pressure on ocean ecosystems. As the key primary producers in marine ecosystems, diatoms rely heavily on silicon (Si) to form their cell walls, while aluminum (Al) can compete with or modulate Si uptake. Nevertheless, the interactions among CO2, Si, and Al in marine diatoms remain unclear. This study elucidates how CO2, Si, and Al influence the model marine diatom Phaeodactylum tricornutum through single-element (C, Si, Al) and combined treatments (+Si+CO2, +Al+CO2). Multilevel indicators from growth rate to surface nano structures and elemental composition were characterized using interdisciplinary methods-such as atomic force microscopy and X-ray photoelectron spectroscopy. Results demonstrate that elevated CO2 and exogenous Si synergistically increased the growth rate of diatoms by 53 %, but the cell size significantly decreased by 5.4 %. Critically, this co-stimulation concurrently increased surface roughness and enhanced silicate polymerization via higher silanol groups (Si-OH) ratio in cell walls, thereby accelerating the sinking velocity. Al's impact emerged only under elevated CO2, promoting organic groups synthesis such as elevated amide-N functional groups on cell surfaces. These findings demonstrate that rising CO2, interacting with Si/Al availability, alters diatom adaptability and thereby potentially impacts diatom-mediated C-Si-Al biogeochemical cycles.