Public key not available

Quote:W: GPG error: http://plex.r.worldssl.net lucid InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 43525C28E533491A
W: GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 082CCEDF94558F59
W: GPG error: http://liquorix.net sid InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3EFF4F272FB2CD80
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.linuxmint.com debian Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3EE67F3D0FF405B2

W: GPG error: http://extras.ubuntu.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192