Integrated Environmental and Molecular Mechanisms of Navicula sp. Biofilm Induced Settlement and Metamorphosis in Mizuhopecten yessoensis Larvae.

Many marine invertebrate larvae are influenced by environmental factors during development, with diatom biofilms playing a crucial role in the settlement and metamorphosis of bivalve larvae. This study found that Navicula sp. biofilms alter the larval microenvironment by increasing dissolved oxygen levels through photosynthesis and decreasing nitrate, ammonium, and phosphate levels in the surrounding water. Exposure to Navicula sp. biofilms induced directional swimming in larvae and significantly shortened the time required for settlement and metamorphosis. Biofilms formed by Navicula sp. contained effective substances and key infochemicals that promoted the settlement and metamorphosis of Mizuhopecten yessoensis larvae. Soluble polysaccharides containing β-1,4-glycosidic bonds secreted by the biofilm were recognized by the larvae, triggering settlement and metamorphosis signaling. Untargeted and targeted metabolomic analyses revealed increased levels of cGMP (Cyclic guanosine monophosphate), and GMP (Guanosine monophosphate), suggesting that larval settlement and metamorphosis may be associated with cGMP regulation. Based on these results, cGMP was selected for subsequent functional analyses. Treatments with the NO donor SNAP, the cGMP analog 8-Br-cGMP, and the sGC activator BAY 41-2272 significantly promoted metamorphosis, whereas the sGC inhibitor ODQ suppressed metamorphosis in a dose-dependent manner. These findings demonstrate that the NO-sGC-cGMP pathway positively regulates the settlement and metamorphosis of M. yessoensis larvae, with cGMP serving as a key effector. This study provides new insights into the mechanisms underlying larval settlement and metamorphosis in bivalves.