The first thing that strikes me about this submission is that it doesn't seem close-up enough to be considered a macro shot. However, I'm reminded that there are some *really* small cactii out there so perhaps this really is a very close up shot of a very small cactus. If so I think you really needed to put something else in-shot to give us the impression of that.
Your lighting on this shot is excellent with a good range of light and dark all across the frame. The light picks out the shapes in the cactus very nicely. The black and white is fine - it adds a little to the stark feeling of the shot which enhances the ferocity of the cactus. Focus seems just a shade too soft. It wouldn't be an issue at all except for those needles that somehow don't look as fearsome as they should. Your composition is fine if a little conventional - that central placement is a little rigid and I'm wondering why I can see a little tabletop in the bottom right. It's fine, I think, to see some table surface but if you're going to show it I think you should show more - in for a penney, in for a pound as they say.
Overall you've taken a technically good shot of a cactus. I think the next step with this shot would be to decide what you're trying to portray and really go all out to capture that aspect of it. If it's a fierce cactus you're trying to show then really let us see those needles. Perhaps even zoom right up to them and fill the frame with needles. If it's a small cactus you're trying to show then really give us the feeling of it being small by putting something larger that we can identify in shot with it.
I'd have drawn a lot tighter to the subject on this shot. I don't know if I'd really call this a macro. The B&W was a good choice for this photo. THe lighting is also really well done. I think my biggest issue is that this isn't relaly a close enough shot.