Pathways of Bioavailability

The notion of biogeochemical cycles usually conjures up images of carbon, nitrogen and phosphorus and their intricate interactions, but the concept can be expanded to include most elements in the periodic table. It is probably obvious that there is a biogeochemical calcium cycle owing to its use in shells and bones, as well as in regulation of cellular functions. Likewise, there is a magnesium biogeochemical cycle given its presence in chlorophyll, its intentional inclusion in biogenic carbonates, and its presence as a major cation in cellular fluids. There also has to be a strontium biogeochemical cycle, as it is incorporated with other alkaline earth elements and can even be used as a proxy for calcium. But, what is known about the pathways and processes in the strontium cycle? Or those of cobalt, nickel, zinc, molybdenum, tungsten and copper that are all essential for certain metal enzymes?

Human activities introduce new variations on biogeochemical cycles, not only of carbon, nitrogen and phosphorus through fuel consumption, pollution and land-use changes, but across the periodic table. We reduce aluminum hydroxide ores into metallic aluminum and introduce a new oxidation state of aluminum at the surface of the Earth. This is the beginning of a biogeochemical cycle of aluminum in which oxidation-reduction processes are involved. Is there a biological role in its subsequent re-oxidation back to bauxite minerals?

The transport and transformations of potentially toxic compounds introduced into the environment by human activities dictate whether those compounds make the transition into the biosphere. The pathways of bioavailability are as diverse and complex as any other biogeochemical processes, and may best be understood from a biogeochemical perspective. In many cases, biological uptake of toxins occurs because pathways already exist for transport of other similar compounds. The response of organisms to toxins can be suppressed or magnified owing to other reactions implicitly or explicitly involved in biogeochemical cycles. Multiple interactions may need to be characterized to understand pathways of bioavailability - something facilitated by current analytical capabilities. As an example, using the same analytical approaches on samples of rain, soil, groundwater, roots, fungi, wood, leaves and insects allows researchers to connect changes in the chemical speciation of toxic trace elements and organic compounds with their bioavailability in forested ecosystems. Similarly integrated approaches to aquatic ecosystems are easily imagined.