How do marine plants right at the bottom of the ocean survive (don't they need a source of sunlight)?

I understand that most aquatic plants are found in the surface layers where light is more available.

But wouldn't being right at the bottom of the ocean be a major problem when it comes to photosynthesis? But obviously, something must happen to keep these plants alive.

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Orignal From: How do marine plants right at the bottom of the ocean survive (don't they need a source of sunlight)?