by Daniel Rossie
When thinking about life and the growth of organisms, Arsenic is not what comes to mind for your first choice of element to introduce into your organism’s medium. However, many organisms not only have the ability to handle the toxicity of Arsenic (As), but also incorporate this element into their biochemical makeup. Arsenic has been shown to support “diverse microbial life” and it could have been an early means of energy propagation on the primordial Earth1). Evidence shows that Arsenic is present in relatively large quantities in aquatic organisms, even though As does not substitute itself for its group partner on the periodic table, Phosphorus (P). A strong contrast is seen, however, in terrestrial organisms. The consensus generally revolves around, and affirms that, organisms are able to adapt to and tolerate high As levels, but not being able to sustain themselves on As alone, in place of P.
Toxicity of arsenic in an organism is defined as being high when it is in its inorganic form, and increasingly less as it moves down to its more organic forms. Inorganic Arsenic can be found as arsenate and arsenite, which are the most toxic forms. The methylated forms of arsenic are moderately toxic. These include methylarsonate (MMA), dimethylarsinate (DMA), trimethylarsine oxide (TMAO), and tetramethylarsonium (TETRA). These moderately toxic forms are the precursors to the organic forms of arsenic found as arsenobetaine (AsB), arsenocholine (AsC), and arsenosugars (AsS)2). These three compounds are simply the arsenic form of these normal compounds in a cell. A betaine molecule is a specialized zwitterion; a choline molecule is involved in lipid formation and composition; and sugars are broken down for energy. Methylation serves as a sort of “shell” around the toxic arsenic, creating an effective barrier between the As and the cell.
3) The pathway of Arsenic Methylation in the body as the compound becomes more organic, finally reaching a form ready for excretion.
Arsenic is integrated into biomolecules using the same pathways in which zwitterions, lipids, and sugars are created. Specifically in the production of phospholipids, the hypothesis relies on the replacement of nitrogen by As4). However, the differences in Phosphorus and Arsenic should be taken into consideration. Even though both are group 15 elements, their properties can only overlap to a certain extent. Phosphorus is a much more abundant element in nature, partially due to its capability to form closer bonds. This parallels the study of Carbon and Silicon where the bond distances between C-C and Si-Si were quite different, even though they are both group 14 elements, because of the simple observation that as you move down a column/group, the atomic radius of the element increases. This forms a barrier for stronger bonds because the atoms cannot get closer together5). Thus, the majority of scientific scholars reject the idea that arsenic can fully replace phosphorus as the structural basis for all biomolecules.
In addition, the two reactive oxidative states of arsenic contribute to its inability to form stable ester bonds, where as phosphate esters are much stronger6)7). The process of detoxifying arsenic involves methylation of the inorganic arsenate or arsenite into molecules like MMA and DMA. The systematic methylation of the more inorganic arsenic compounds is the pathway in which the cells process arsenic in order to convert it to the non-toxic forms (AsB, AsC, and AsS), which are suitable for interactions with the biomolecules inside the cell 8).
Much evidence suggests that there is a significant difference between both level and chemical forms of arsenic between terrestrial organisms and marine organisms. Terrestrial organisms rarely have a level of arsenic above 1 ppm whereas marine animals have levels that vary from a few ppm to more than 100 ppm 9)10)11).
Extremophiles, but more specifically Arsenophiles, are bacterial organisms that have biological pathways to deal with elevated levels of arsenic12). It has been shown that photosynthetic organisms may play a significant role in As geochemical cycling by methylation of more inorganic arsenic forms down to the more organic forms, but little is known about the mechanisms of methylation13).
In addition, other marine organisms, like the bivalve mollusc, algae, crustaceans, and fish, among others “generally bioaccumulate arsenic as complex organic compounds” 14)15), resulting in overall elevated levels of arsenic biological incorporation. Furthermoe, in a study on plaice, the arsenic content varied between 3 and 166mg/kg 16).
Moreover, terrestrial organisms respond in a more defensive mode. The critical detoxification of arsenic from the body is created through the methylation of the inorganic forms until the transformed arsenic compound is in the most organic form. Trace amounts of arsenic are found in the urine of humans, when arsenic exposure is induced, as a defense mechanism for the toxicity of arsenic 17). One experiment showed that after the consumption of fish with a high arsenic content, 69-85% of the arsenic was excreted from the human subjects within 5 days 18). Terrestrial organisms attempt to expel as much arsenic as they possibly can, but it is inevitable that there must a threshold value in which the body's removal process cannot keep up with the arsenic influx and some is incorporated into biomolecules. Arsenocholine, among other compounds, have been isolated from terrestrial organisms, such as the mushroom Amanita muscaria, in order to show the necessity for arsenic incorporation when soil treatments are rich in arsenic 19). However, this incorporation is solely as assistance to the biomolecules and not as a replacement for critical elements that form these molecules.
Life based on arsenic stretches our scientific understanding of this toxic element, and implications of its properties sustaining life attack our very knowledge of biochemical truths. It is not a major player in the structure of cellular biomolecules, but it plays a major role in support of these macromolecular structures. Incorporation of arsenic-based compounds are seen mainly in marine life, and are more readily involved in supporting macromolecules in these aquatic organisms. However, terrestrial life prefers to rid itself of the toxic compounds by making the compounds more organic and excrete the compounds when possible. Incorporation of organo-arsenic compounds occurs only when the organism cannot resist the influx of the toxic element, and the organism has no choice but to adapt to its new environment. Overall, arsenic is not readily accepted into the structures of important molecules but has been shown to be in the cells of marine organisms in larger quantities.