Can a bassline act as the melody of a song or be the focal point over a melody in a song? Listen to the instrumental below for example. It sounds like the bass-guitar is the lead tune throughout the whole song with the exception of the riff that occurs every 8-bars. By the way what instrument is playing that riff to me it sounds like a guitar.
It's almost like you answered your own question! Yes, a single line of music can act as both the "bass line" and the "melody".
The term "bass line" usually refers to the set of lowest sounding notes in a piece of music that has some form of harmony or counterpoint or other tonal structure. If there isn't much of a tonal structure, then that meaning has less meaning. Even when there is a tonal structure, the lowest sounding notes can be the focal point of the music. This is rare because our ears and hearing processes tend to hear the higher notes in the audible range more clearly, so melodies tend to be the highest sounding notes, or at least notes in the octaves above middle C, where our hearing really tends to focus. That said, there are contrapuntal pieces like Bach fugues and inventions where the themes (basically, "main melodies") are traded back and forth between the highest and lowest sounding notes (and middle notes, when three or more parts are written).
"Bass line" could also, or instead, refer to all the notes played by a bass guitar or some other instrument that is acting as a bass guitar. Since any instrument can play a melody, including a bass instrument, it's entirely possible for the "bass line" to also be the melody, regardless of whether the notes are the lowest sounding, highest sounding, or are intermixed with any harmony that may exist.
People don't normally call a bassline 'the melody' as such, because... it's a 'bassline'! Nevertheless, it's absolutely possible for a bassline to be a focal point of a tune, and it's a standard feature of funk, disco, and especially hip-hop genres, some reggae, dubstep.... and more.
It's hardly what one could (or would) call a melody. It's just a recurring pattern - an ostinato. But, I suppose, yes, in the absence of any other instrument, it's the 'lead'. Although to me, it's crying out for something else to go over it all, with a melody...
Sounds to me like a bass guitar going through a synth patch, or indeed, a synth itself.
A bass can absolutely play the melody. It's not "standard," but that doesn't mean an aspiring bassist can't do it.
Rush would be an excellent source of countless examples.