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  • Writer's pictureJonathan Simonds

Heavy Metal Detox: the Future of Pesky Chemicals in Cultivated Algae

As an algae industry professional, talking about heavy metals can feel a bit taboo. I love seaweed! Shouldn’t I expound on its nitrogen-fixing symbioses instead, or share videos of sea otters wrapping their pups in kelp to keep them safe?

A stylized sketch of an adorable baby sea otter in the center of a dense kelp mat
An otter pup tucked in a splash of kelp

Part of our responsibility as an industry is to recognize hurdles and work to overcome them. The reality is that despite seaweed’s status as a superfood, boasting a buffet of nutrients and proteins, it tends to absorb more arsenic, cadmium, and lead than most of us would be comfortable eating in large quantities for every meal. Even though there is an incredible diversity in seaweeds–some have been evolving independently since before mushrooms and animals diverged–the general trend to soak up nasty metals is hard to deny.

Luckily, facing the problem head-on and diving into the dizzying array of potential solutions is exciting and could unlock significant new market potential as the possible use-cases for seaweed are expanded.

Before we blow the heavy metals problem out of proportion, let’s keep some perspective. Japan, boasting some of the highest seaweed consumption per capita globally, is also the country with the greatest life expectancy according to the World Health Organization’s most recent data. At the rates that most people consume it, seaweed is good for us. Those who eat the most live the longest.

With that in mind, why write this piece at all? As a lover of seaweed, I’m not satisfied with the rates that most people consume it today. To see seaweed expand to become a drop-in protein replacement that competes with the likes of meat and grain (rather than okra or zucchini) for a share of global caloric intake, it is our responsibility to make sure that what we grow is not just safe but actively beneficial in those quantities. Solving the heavy metals question will help open the door for a gastric phyconomy on a different scale than we can easily picture today.

The three main areas I’m thinking about are:

  1. Growth conditions

  2. Processing and treatment, and

  3. Selection, breeding and genetics

If your team is tackling this problem from another angle, let me know!

Growth conditions

If seaweed is grown in water that does not contain arsenic, the seaweed will not contain arsenic. That much has been made obvious by generations of failed alchemists. However, eliminating the specific elements we don’t want from water on an industrial scale is no simple feat. If it were, we would have eliminated these elements from our drinking water long before the technology made its way to aquaculture. What’s more, eliminating lead or cadmium from drinking water should be easier than from algae culture media because we don’t care quite so much whether we eliminate other micronutrients in the process–humans rely on food for most of these.

Entirely eliminating target metals from a culture medium may be challenging and we shouldn’t count on this technology to solve our woes in the near term, but we shouldn’t count it out in the future. In the meantime, we can explore other ways to modify growth conditions that accept the presence of these metals in the growing media without accepting their uptake into tissues.

Phosphate and arsenate can be difficult for cellular machinery to distinguish, and arsenate is commonly taken up through phosphate absorption pathways. Studies have shown that increasing phosphate in culture media can significantly reduce arsenate uptake and consequently total arsenic tissue concentrations.

A cadmium ion on the left is represented as a pink sphere, while a calcium ion on the right is represented as a purple sphere. Each is labeled with it's respective chemical symbol and charge.
A relative comparison of the sizes of cadmium and calcium ions (drawn to scale)

Promising as providing excess competing elements is, it may be unfeasible for ocean-based farmers to grow under excess phosphorus or excess calcium regimes, to say nothing of the environmental impact. Even in tanks, a delicate balance has to be struck. If a nutrient is added to seawater that the target seaweed cannot fully utilize, chances are good that another opportunistic species will find a use for it.

Processing and Treatment

There remains the possibility that algae that has already taken up undesirable elements can be purged of them before being consumed. There is some promising research here, but it is worth noting that most of the interventions we currently know about are not particularly specific, they may also eliminate desirable elements or compounds or have other side effects.

Hijiki is infamous for high levels of arsenic and brown seaweeds in general (including the “kelps”) are higher than average. However, Hijiki is traditionally washed and soaked before preparing, which studies have shown significantly reduces arsenic content. Temperature of the water is important, and near boiling temperatures tend to be more effective.

Another consideration is that many elements tend to partition preferentially to specific parts of the cell or be incorporated into specific molecules. Instead of removing a specific element from all biomass, it may be possible to sort desirable fractions of the biomass while leaving the undesirable element behind in a waste (or differentially useful) fraction. For example, when proteins, lipids, starches, and carbohydrates are extracted from algae, in which extraction is cadmium found primarily? What about arsenic? Mercury? I have found limited literature to answer these questions, if you have answers please send them my way. Extracts with lower concentrations of metals may serve as better food supplements and additives (potentially in large quantities) while the remaining fractions may find better uses in non-food applications.

Selection, breeding, and genetics

Selection, breeding, and genetic work are highly promising, and the ease of growing a low-toxin cultivar may be more attractive than adding steps to one’s processing train or using expensive fertilizers in excess.

A lead atom cannot be picked out of a solution with a pair of tweezers; this kind of precision is better suited to cellular machinery built on the molecular scale. Sargassum species tend to accumulate significantly more arsenic than ulva species, while both can accumulate biomass at similar rates. Can we give sargassum the tools that ulva uses to limit As uptake? If we do this, what unexpected consequences might we encounter?

It is possible that the genes responsible for the higher uptake in sargassum could be identified and snipped out. Or perhaps genes in ulva that encode proteins that are able to discriminate between arsenate and phosphate could be snipped in. Certainly there will be tradeoffs, but we won’t know without learning more. Genetic work to reduce As uptake is already being done in rice, and that work will clear some of the barriers for phycologists as it progresses.

Gene editing is all well and good, but the requisite background work is years in the making. There is a lower-tech option with a more reasonable time horizon. We don’t actually care about the genes, we just care about whether algae takes up toxins. Here, simple selection of seedstock is a powerful tool. For species for which breeding technology exists, selective breeding can compound and cement gains. For the record, where breeding technology does not exist, I would encourage companies and academics to make this a top priority: it is the bare minimum of control that we exert consistently for almost every other agricultural crop.

Right now, there is risk associated with making improvements in heavy metal concentrations because there is not a universal certification or even many existing use-cases where low-toxin algae is more marketable. Slightly elevated heavy metal concentrations do not generally get in the way of enjoying a 25-calorie nori snack every once in a while. However, I believe that they are a key factor limiting expansion of the market into new territories and new use-cases. Luckily, marginal costs for selection can be low, and prioritizing selection may position your company uniquely amongst the competition, able to brand and market differently.

Small seed stalks give the impression of corn but are much smaller, with only about six kernels each
Teosinte, a wild grass from which corn was bred
A two dried corn cobs stand in a field ready for harvest
Modern corn, after many generations of selection

What might a selection program look like in a cost-conscious, industrially-minded macroalgae lab? If your lab is lucky enough to be outfitted with the right chromatography equipment, put it to use on the wild-type seed tissue you collect. This is especially cost-effective if you are using a sporelation-based propagation technique (as opposed to a vegetative one) because the information you collect about one parent blade might shed light on hundreds or thousands of offspring crop plants.

Once you have more information about your wild-type algae, plant the best seed! I cannot tell you how many times I have seen companies collect sorus or other spore tissue indiscriminately and plant out any offspring they get. For tens of thousands of years our ancestors have known to plant the best seeds to get the best crops. Why do we forget this wisdom now?

In the end, heavy metal concentrations might be among a litany of traits you are testing for, but they should be considered. Upfront investments in selection will pay dividends down the road.

In conclusion

Let’s keep our eye on the prize. Before all this talk of toxins makes you worried, remember that most seaweeds are already plenty edible and in fact are particularly healthy inclusions into our daily diets. However, paying attention to undesirable elements, even in low concentrations, may unlock the doors for massive expansion and adoption that have so far been out of reach. Next-generation seaweeds could be included into diets as more than an accent but rather as a staple.

Agree? Disagree? Have questions? Feel free to leave a comment below or send me an email!



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