Color Our Reef ALive

Corals are most elegant bio-geochemical engineers, building their colorful homes out of sunlight and air. But why do they bleach when stressed and how can we help them relax?

Modern, shallow (sub) tropical reefs are made by hermatypic stony ‘scleractinian’ corals. They are colonies of polyps with stingy tentacles, like the related sea anemone, secreting calcium carbonate cups; the corallite, an exoskeleton in which they can retract.

Hermatypic corals are colored by pigmented unicellular algae that they host in their tissue at concentrations of several million per square centimeter. These algae are dinoflagellates known as zooxanthellae which photosynthesize, using sunlight and carbon dioxide to produce oxygen and organic compounds, which they share with corals in exchange for nutrients and protection.

When corals are stressed, for example by extreme water temperature, salinity and light, they may expel their colored zooxanthellae and turn white (Coles & Jokiel, 1978). As the corals bleach, they become brittle and more prone to disease. When conditions return to normal, corals may incorporate the zooxanthellae again and regenerate after a while (Buddemeier & Fautin, 1993). The extreme circumstances may change the zooxanthellae from beneficial symbionts into harmful parasites that need to be expelled and if, for instance by climate change, such conditions occur too often, without time for regeneration, coral bleaching becomes permanent and entire reefs will die (Baker et al., 2018).

Coral reefs are the most diverse ecosystems on earth. They are not only a wonderful underwater world of great beauty, but they protect vast stretches of coast against wave erosion and ultimately feed billions of people with hundreds of billions of dollars’ worth of seafood. Coral reefs deserve protection and we have to:

  • Learn how to diminish environmental stress for coral reefs
  • Teach corals how to deal better with their environmental stress

We ask ourselves, for improving the well-being of coral hosts and their symbiotic guests, what bio-geochemical technology is best to use?

String of microbially precipitated nano crystals of magnetic iron oxides within the elongated cell of a magnetotactic bacteria

What is going on between minerals, metals and microbes in bio-geochemical technology?

By using minerals in bio-geochemical technology, we can influence the interaction between microbes and metals, to improve environmental management.

The health of soil and water can be managed through the interaction between microbes and metals, using minerals in bio-geochemical technology. Microbes (bacteria, fungi and algae) influence metal concentrations when deteriorating rock and minerals during bio-weathering and when forming, directly or indirectly, minerals during bio-mineralization (Gadd, 2010).

This interaction between minerals, microbes and metals determines the availability of life sustaining resources and life-threatening toxins that regulate the natural habitat of the critical zone or biosphere; the near-surface environment of rock, soil, water and air, home to living organisms.

Microbes concentrate the essential nutrients from mineral surfaces (Vaughan et al, 2002); i.e. carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur that form 95% of the biomass and other elements with essential biochemical and structural functions such as K, Ca, Mg, B, Cl, Fe, Mn, Zn, Cu, Mo, Ni, Co, Se, Na and Si. Microbes and minerals also control the concentrations and bio-availability of thirteen trace metals and metalloids that are considered priority pollutants i.e. Ag, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Se, Tl and Zn (Sparks, 2005).

Many examples show how microbes interacting with metals may operate in environmental management. For instance, microbial metal mobilization through bio-leaching can be used for metal recovery, recycling and bioremediation of mineral waste and contaminated soil (White et al, 1998). Microbes can be used to immobilize metals by bio-precipitation, for decrease of toxic concentration in bio-available phases. Reduced metal(loid)s like Se(0), Cr(III), Tc(IV) and U(IV) form insoluble precipitates through reduction in anaerobic processes. Single-oxidation-state metals are immobilized during precipitation with biologically produced sulfide and phosphate. Immobilization through microbial biosorption occurs by physico-chemical binding of metals on dead and living walls of microscopically small cells that expose a very high surface per weight to the metal containing solution. Biominerals such as microbial nano crystals of magnetic iron oxides precipitated by magnetotactic bacteria can be used for sorption of metals and their recovery from solution by magnetic separation.

For practical use of microbes and minerals in environmental management we have to ask: How can microbes manage metals better with minerals in biogeochemical technology?

featured image of geochemical phytoremediation technology showing burning roses and ash modified after an image of

Can phyto-geochemical technology promise a rose garden?

By controlling the mobility of nutrients and contaminants, using phyto-geochemical technology, we can support plants in growing in polluted environments.

We like to know what role phyto-geochemical technology can play in growing plants on strongly contaminated soil, not necessarily promising a rose garden, but at least improving success and efficiency of phytoremediation.

Phytoremediation uses higher plants to remove, immobilize or degrade contaminants such as metal(loid)s in soil and water and two approaches can be distinguished (Bolan et al., 2011):

  • Phytoextraction aims at cleaning soil and water by the uptake of contaminants and their storage in plant tissue. The contaminants are removed by harvesting the plants.
  • Phytostabilization aims at immobilizing contaminants by roots and plant cover, preventing their percolation to groundwater and their spreading by wind and water erosion.

Nutrients and contaminants enter the plants via the rhizosphere, a volume of soil or water that envelopes the roots in a few millimeters thin layer. Their mobility and uptake depend on the composition of soil and the activities of the plant and associated microbes (mycorrhizal fungi and bacteria) in the rhizosphere.

The mobility and availability of, for instance, metal(loid)s is strongly regulated by the pH of the pore water of the soil. The plant influences the pH by releasing fluxes of H+ or OH-, counterbalancing the uptake of cations and anions, respectively (Tang and Rengel 2003).

The mobility and availability of metal(loid)s is also defined by the charge of soil particles with a high surface per weight ratio. Fine-grained organic matter and clay minerals show strong capacity for adsorption and exchange of cations or anions, expressed in the Cation- and Anion Exchange Capacities (CEC and AEC) of the soil (Bolan et al., 1999).

Plants have trouble distinguishing between useful nutrients (e.g. K+, Ca2+, PO43-) and hazardous contaminants (e.g. Tl+, Cd2+, AsO43-) of the same charge and similar size (Reid and Hayes, 2003).

With phyto-geochemical technology we aim at supporting plants growing on contaminated soil by influencing the pH, CEC, AEC and bio-availability of nutrients and contaminants, using inorganic amendments (e.g. liming materials, phosphate compounds and clay materials) and organic amendments (e.g. topsoil, compost, manure, waste water treatment biosolids, peat and biochar).

We like to know: What amendments are required for effective phyto-geochemical technology and; How to administer amendments for efficient phyto-geochemical technology?

featured image of geochemical technology validation showing the canaries Fokke and Sukke in laboratory coats besides a steaming retort and Fokke asking: very impressive colleague, but does it also work in theory?

Works geochemical technology validation also in theory?

We can predict performance and validate geochemical technology before application and testing in the natural environment on large scale and long term.

When we apply geochemical technology on the small scale and short term, we can readily monitor its performance and establish to what extend the prediction of its working is correct. The theory of a geochemical process is tested by putting it into practice. Geochemical technology validation is immediate and straight forward

When designing geochemical technology for application on a larger scale and longer term, then the testability of the theory decreases. Simultaneously, the consequences of eventual malfunctioning become more serious and therefore the prediction of the course of events needs to be more accurate.

Since we can no longer readily prove the working of geochemical technology in practice, we need to ask ourselves, can we prove the working of geochemical technology in theory? How can we assure a priori geochemical technology validation? Such are not trivial questions and are subject of vigorous discussions on model validation in science and engineering of, for instance, the long-term stability of disposal sites for hazardous (radioactive) waste (Nordstrom, 2012). If we think we can indeed forecast results of geochemical technology, then what instruments are available for predicting the future most accurately?

We can distinguish four ways for obtaining high-quality geochemical technology and for improving the prediction of its working within the natural environment.

  1. Nature provides examples. By analyzing samples and explaining the distribution of measurements, we gain understanding of past and ongoing geochemical processes. This understanding is tested each time we study present processes and the way they leave traces in the fossil record. The past is the key to the present and the future.
  2. Laboratory- and pilot field tests provide another way to test our understanding of geochemical technology. Of course, only to a certain degree, because transposing technology to a different scale and into the natural environment, increases uncertainty as the effect of unknowable influences also increases.
  3. Conceptual- and scientific models provide the means to discuss our understanding of the working of geochemical technology and to put it to logical tests. Models should be elegant and simple, reflecting their limitations as an idealized representation of a complex reality and facilitating communication with a broad and critical audience.
  4. Finally, geochemical technology should be designed and applied in such a way that it automatically attains stable- or neutral equilibrium, so that it never evolves to an unpredictable unstable state.

For discussing the theory of geochemical technology in further detail, we might ask ourselves: What models most elegantly represent geochemical technology?

logo of GeoChemTec within circle of two light blue arrows

What is geochemical technology?

When using geochemical processes to improve the environment, we apply geochemical technology and perform geochemical engineering.

Schuiling (1990) defines geochemical engineering and presents an example of a very early application of geochemical technology. He writes: “If we adopt the definition that geochemical engineering is ‘the use of geochemical processes to improve the environment’, then it can be said that Hannibal practiced geochemical engineering when he poured acetic acid over limestones to secure a passageway for his elephants across the Alps (LIVIUS)”.

Geochemical technology arises from combining principles of the science of geochemistry with the practice of environmental engineering. While geochemists study the chemistry of the earth and the geochemical processes that define the distribution of minerals and chemical elements in rock, water and air; geochemical engineers apply this understanding of geochemical processes in the engineering of the natural environment.

Geochemical technology typically:

  • Fits within the course of geochemical processes and goes with the geochemical cycle;
  • Uses mineral reagents to control chemical elements in configurations that stay stable on the long term;
  • Aims at solutions for environmental problems that blend better into the natural environment and;
  • Provides innovative tools for environmental management that help to better conserve, control and protect precious natural resources.

What are examples of geochemical technology? .