in short: I assume chlorella is more cost-effective for you and producers, and more aligned with our environment.
When you want to, I can recommend to further look into it, and there probably is a lot I did not mention: Chlorella, in natural circumstances, contains vitamin B12 — seemingly synthesized by bacteriae in chlorella's habitat — and according to the publication (I linked below) even contains precursors to, ánd vitamin D(2) itself.
It has an outstanding nutrient profile — as mentioned in a previous comment 3g of it fulfills the Reference Daily Intake (RDI) of B12.
I will follow up with a few assumptions mixed with references and a quote from the study regarding advantages of chlorella:
- I see the more natural way of chlorella production as an advantage.
- I assume it grows cost-effective as it "can be mass-cultured" (according to the linked publication).
- It contains a super dense and rich nutrient profile, covering a wide variety of micronutrients as the publication shows — a more convenient view on the (micro-)nutrients and how they relate to RDI can easily get accessed with a (Brave) Leo Community Intelligence prompt (yes, I refer to 'AI' that way) or any other Community Intelligence prompt.
- I (not knowing the prices of fabricated supplements) assume it costs less for the consumer to add 3g of algae to the diet than a plethora of pills to cover all the nutrients chlorella provides.
Bonus: As also mentioned in the publication: shiitake and other mushrooms can contain Vitamin D after Sun (or other UV-light exposure). I see a complete plant-based, natural, and vegan diet as perfectly possible and eat that way myself, pretty happily so.
I hope I provided valuable assistance in any way.
https://pmc.ncbi.nlm.nih.gov/articles/PMC7551956/
[1](Bito T, Okumura E, Fujishima M, Watanabe F. Potential of Chlorella as a Dietary Supplement to Promote Human Health. Nutrients. 2020 Aug 20;12(9):2524. doi: 10.3390/nu12092524. PMID: 32825362; PMCID: PMC7551956.)