Dr. Greger has been telling his readers to consume DHA (Docosahexaenoic acid), a type of omega-3 fatty acid obtained mostly from seafood. But seafood is some of the dirtiest food you can eat. So, he advises getting DHA from algae. Problem is, algae-derived DHA, in the amounts he recommends (250 mg/d) can only realistically be supplied in supplement form. Why is that a problem? It’s an extract, not a whole food; it’s ultra-processed; its production is energy-intensive; its often more oxidized/rancid that seafood-derived DHA; and the limiting factor for most people: it’s expensive. (Nordic Naturals cost $25 for a 30-day supply. That’s $100 for a family of 4, on top of their regular food budget. For life.)
The algae has to be grown, either inside in climate-controlled laboratories or outside in ponds. Farming algae (including feeding), harvesting and extracting its oil, discarding or repurposing the remainder, and packaging the purported valuable bit in single-serve capsules requires lots of energy. All told, eating algal DHA could be worse for the environment than eating seafood.
Do we even need to eat seafood or algae oil at all? John Langdon argues that we don’t:
A number of authors have argued that only an aquatic-based diet can provide the necessary quantity of DHA to support the human brain. … There is no evidence that human diets based on terrestrial food chains … fail to provide adequate levels of DHA or other n-3 fatty acids. Consequently, the hypothesis that DHA has been a limiting resource in human brain evolution must be considered to be unsupported.
– Has An Aquatic Diet Been Necessary For Hominin Brain Evolution And Functional Development?, John Langdon, British Journal of Nutrition, 2006
Dr. Greger is saying that everyone should take algal DHA supplements, not just vegans, because omnivores have an “omega-3 index” that’s “just as bad” as vegans.
Millions of people cannot afford to take algal supplements for the rest of their lives. But many will think they need to, to get the long-chain omega-3 fatty acid DHA. What will they do? Eat seafood.
I’ll use canned tuna as an example but my arguments apply to most seafood. … The 5 ounce can of tuna fish below costs about $0.76 on Amazon. It contains about 300 mg DHA, 20 grams of protein, and can provide the basis of a meal for 2 or more people. Unfortunately, it also contains pollutants from the water in which it lived: pesticides, flame retardants, polychlorinated biphenyls (PCBs), mercury, arsenic, lead, and other heavy metals, pharmaceuticals, teflon, microplastics. Tuna are being fished at unsustainable rates and there’s no such thing as a “dolphin-friendly” catch. Also, tuna and other fish are intelligent: “Science has shown fish to be capable of collaboration, recognition, astonishing feats of memorization, and craving physical touch.” There are so many reasons not to eat tuna fish. But many people will not get past my first two sentences.
There isn’t an RDA for DHA because the Institute of Medicine says it’s not an essential nutrient, meaning we don’t have to consume it because our bodies make it. We make it from ALA (alpha-linolenic acid) an omega-3 fatty acid that’s found in lots of plant foods (like flax, walnuts, kidney beans, oatmeal, cauliflower, spinach). One way we can make more DHA is to reduce the amount of omega-6 fatty acid in our diet (from vegetables oils like soy), or just eat a low-fat diet. That reduces competition for the converting enzyme (my second reference below).
Here are some of my past posts on this topic:
A Vegan Can Get Enough Omega-3 Without Taking A Supplement
Study: People Who Eat A Low-Fat Diet Have Higher Levels Of Omega-3s (EPA And DHA) In Their Blood
Need For Long-Chain Omega-3 Fatty Acids (EPA, DHA) Can Be Met By Plant-Based Diet
Study: Seafood Consumption Linked To Cognitive Decline
Is Dr. Greger Saying That Vegan Diets Are Inadequate? It Sounds Like He Is.
Fanatic Cook 
I wouldn’t pay more than $8.44/mo. Nordic Naturals is just another brand relabeling DSM/Martek algal oil, but one can buy the same (Ovega-3, their own brand) at a site like Vitacost for that much (500 mg every other day). Schizochytrium fermented in big stainless vats, capital and labor intensive but not especially energy intensive. Open air cultivation would offer too many risks of contamination for human use.
My literature investigation suggests:1) premenopausal women do fine producing long-chain ω-3s from ALA, but men and postmenopausal women don’t, 2) they have little effect on cardiovascular risk in the context of a Western (high arachidonic acid) diet, 3) long-chain ω-3s may still retain dietary importance in preventing or delaying dementia, and in treating inflammatory disease in the context of low-arachidonic acid, plant-based diets.
As of 5 years ago there were GMOs seed crops (“false flax”) modified to produce high levels of long-chain ω-3s. Due to market concerns about GMOs, this crop was destined for use as fish feed, and environmentally, it certainly has benefits compared to overfishing to feed farmed fish.
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