But I also wouldn’t shun algae just because they occupy some B12 transporters. There’s all kinds of metabolic tradeoffs with every kind of food, and we are designed to tolerate that. Most metabolic pathways are 4-5x redundant IIRC.
Seemed hard to find numbers, but one source said 331mg of EPA per gram of Spirulina platensis (https://www.ripublication.com/ijac16/ijacv12n4_05.pdf).
Was considering what it would be like to eat like 6 grams of spirulina for 2g of EPA, versus that in one teaspoon of fish oil (high EPA type).
But got stuck on the B12 blocking stuff. I wonder if your point is true that there's enough redundancy it doesn't matter. It would be nice if it was easier/cheaper to get bloodwork to do some experiments with this kind of stuff.
Re testing, the article you linked has another article linked on the same site: https://www.b12-vitamin.com/analogues/ They mention MMA testing here (you could get one as part of the Organic Acids Test at „The Great Plains“ lab for example), but that’s quite expensive ($299 right now). It’s maybe the most accurate B12 test out there. Technically, it’s a test by proxy.
If you are currently serum B12 deficient, then there’s probably not that much to worry about, because the B12 analogues in algae also won’t be absorbed properly (maybe due to a lack of intrinsic factor). Supplemental methyl- and adenosyl-B12 don’t have that problem.