Vitamins are organic (carbon- and hydrogen-based) molecules that are essential nutrients for humans. There are 13 different vitamins, of which the B vitamin group has eight, and they all function as co-factors (non-protein components) for various cellular enzymes. Vitamin B12 is a unique molecule that is not made in animals or plants, but it is vital for cell metabolism and formation of red blood cells. Our dependency on B12, especially for those on a vegan diet, determines our health.

The ABC of B12

Vitamin B12 (also known as cobalamin) is an organometallic compound, with cobalt as the metal ion component in its complex C-H structure. This was unravelled by Dorothy Hodgkin in the 1960s, for which she won a Nobel prize. Organisms that can synthesise it do so via approximately 30 enzymatic reactions, making it technically challenging to manufacture synthetically.

Vitamin B12 is an essential co-factor for various enzymes in the one-carbon metabolic pathways in human cells, driven by the folate (vitamin B9) and methionine biochemical cycles. These cycles produce the amino acid methionine, required for synthesis and methylation of DNA. B12 is also essential for the Kreb’s cycle that functions in the mitochondrion to produce ATP, the energy molecule. Given its essential role, B12 deficiency can, in adverse circumstances, lead to neurological disorders, depression, cancer, and severe anaemia, among others.

Ruminating its roots

Vitamin B12 is unusual, as it is not synthesised in humans, other animals, fungi, or plants. Research shows that the human genes responsible for synthesis of other vitamins, like vitamin C, were lost by mutations thousands of years ago, due to adaptations of our diet: if the diet is rich in a particular vitamin that is easy to obtain, then there is no selection pressure for the body to make it – but we do need it.

Microbes are the sole biological source of vitamin B12. Evolutionary studies have revealed that similar B12-producing bacteria inhabited the gut of chimpanzees and ancient humans living in a natural (non-polluted) environment. There is some debate about if and where B12-producing bacteria reside in the gut of a modern-day human, because a sterile environment and pesticides (that kill microbes in soil and on crops) diminish this possibility.

A bovine meat-rich diet contains B12, but how? Soil contains B12-producing bacteria (which could arise from animal faeces), and cows grazing on organic pastures have more of these bacteria in their gut that enable digestion, than those cooped up and fed supplements, or lacking B12 in their diet. Hence B12 is present, either naturally or as synthetic supplements that the animals get, in their meat and milk they produce. In addition, B12 synthesis by bacteria is dependent on cobalt; therefore, a diet with cobalt-rich foods (such as greens and nuts) is beneficial. Interestingly, while higher plants do not make B12, lower plants such as algae possess B12-dependent and independent routes to make methionine. These plants must form relationships with bacteria to obtain the necessary B12.

Beneficial botanical bacteria

In a vegan diet, B12 sources include fermented foods such as tempeh, stinky tofu, and pickled vegetables (because of fermenting bacteria), B12-fortified cereals and plant-based milk, nutritional yeast, and certain mushrooms. There are a few plants that contain B12, but this is because of their symbiotic association with bacteria. Algae such as nori, a B12-containing plant food, lack B12 biosynthetic genes, but do have B12-dependent enzymes.

Vitamin B12 is unusual, as it is not synthesised in humans, other animals, fungi, or plants.

Depending on growth conditions, B12-producing bacteria live symbiotically inside these algal cells, which in turn supply carbon for the bacteria. In another plant in the duckweed family, ‘Mankai’, which grows on the surface of freshwater bodies, the bioavailable B12 content is high, but its source is likely to be endophytic bacteria which live inside the plant cells. In higher land plants, the lack of B12-dependent enzymes might explain why they do not need to produce B12 (they make methionine through other routes). But leguminous plants such as alfalfa form a symbiotic association with special nitrogen-fixing bacteria (rhizobia) that convert atmospheric nitrogen to a simpler form for the plant, in special root structures called nodules. These endophytic rhizobia produce B12 (when cobalt is present) so they can survive the defence molecules produced by the plant host. Therefore, there is a potential bacterial source of B12 inside certain plant cells.

Boosting B12 in crops

There are hopes for strengthening plants with B12 using biofortification methods, by feeding edible seeds and plants with B12 from bacterial cultures via hydroponics or in enriched growth medium and, more recently, via aeroponics. The use of B12-producing lactic acid bacteria (LAB) waste (that can be obtained from the probiotics industry) as fertiliser would be an economical and natural way to boost B12 in edible crops.

Given that externally supplied microbial B12 can be absorbed by roots and transported to leaves through the xylem, it should be possible to enrich edible plant parts with B12. Another approach might be to explore bacterial endophytes (such as LAB) that reside within crops, screen for B12-producing strains, and provide the right environment to enhance such bacteria. Studies on the bioavailability of B12 in these crops, and its stability following cooking, will be necessary. By exploiting plant–bacteria interactions, the future might be brighter for a B12 boost in edible plants.

Dr Radhika Desikan