From sea shells to sea change Features

microscopic single-celled plank- but a suite of other trace elements ton, the most important groups and their isotopes. An element is of which are the foraminifera and defined by the number of protons coccolithophores (Figs 1 and 2). in its nucleus. Most elements also The foraminifera are amoeboid have neutrons in their nucleus, An introduction to protozoa that produce a multi- but the number of neutrons can biomineralization by chamber calcite shell of up to vary, making the element more Gerald Langer and Charlotte Walker. Photo by DAVID ILIFF. several millimetres in size. The or less heavy. We speak of heavy License: CC-BY-SA 3.0 coccolithophores are characterized and light isotopes of an element. by calcite platelets, termed coc- Oxygen, for example, has different n first inspection it would be hard to unite the bones movement. Endoskeletons (internal to an organism) are coliths, which are extruded to the isotopes; the most common ones of a whale, the shell of a barnacle, the Egyptian composed mainly of the protein collagen and the mineral cell surface and form a shell-like are the heavier 18O and the lighter Opyramids and the White Cliffs of Dover. However, all hydroxyapatite—a substance more commonly known as covering called a coccosphere. 16O. Since isotopes have different these marvels are connected by biomineralization, a natural bone. The bones of a turtle are an interesting curiosity These coccospheres are only masses they react slightly differ- process that has been occurring for millennia. Bio- because they form both an endo- and an exoskeleton (exter- 5–25 micrometres across, but ently in many chemical reactions, mineralization is the production of inorganic substances, in nal to an organism). The endoskeleton provides the structure owing to their abundance they leading to different isotopic this case minerals, by living organisms. Biominerals are and mobility while the exoskeleton provides protection. substantially influence the global compositions of the product. found within the hard parts of organisms—the bones, shells Protection is also an important function of biomineral- carbon cycle on very different The isotopic compositions are and teeth—where they perform many crucial functions. ized structures, for example, the hard shells of crustaceans timescales. In the short-term, the influenced by physico-chemical Moreover, the process of bio-mineralization can greatly which protect their soft bodies from physical damage production of calcium carbonate parameters and can therefore be influence the environment in which they are produced. and predation. The biomineralization process in crusta- in sea surface waters changes the used to get information about In most cases biomineralization is controlled by the ceans occurs by precipitating calcium carbonate within carbon chemistry of the water these parameters. For instance, organism. By directing the composi- an organic matrix of chitin protein most significantly by increas- the oxygen isotopic composition Figure 3. Top: Calcification by microalgae captures tion, location, orientation, size, and fibres. This process is energetically ing the carbon dioxide (CO ) and the magnesium/calcium 2 atmospheric carbon over short (annual) timescales. Middle: shape of the minerals it produces, the expensive and has associated risks—for concentration which affects gas Sinking of coccoliths removes carbon over geological ratio are used to reconstruct organism creates functional biomineral example, crustaceans are exposed to exchange between the ocean and timescales. Bottom: The global carbon cycle (Global carbon temperature. The boron isotopic structures. Controlled precipitation predators when they moult—but the atmosphere. An analogy of this cycle sketch © Karina Kaczmarek). composition is related to seawater of minerals often involves organic benefit provided by the long-term effect is opening a bottle of spar- pH, an important parameter materials, for example, limpet shells protection outweighs the cost. kling water. The bubbles are gaseous CO2 which was of the seawater carbon system. The reconstruction of are dominated by the minerals calcite Biominerals fulfil other functions previously dissolved in the water. In the ocean this effect temperature and seawater carbon chemistry over geologi- and aragonite but they also contain in animals. Interesting examples are acts on short timescales of approximately one year. By cal timescales can inform our predictions of present and various organic components such the statoliths and otoliths which contrast, the sinking of these shells to the deep sea and their future climate change and its anthropogenic component. as proteins and polysaccharides. are components of gravity-sensing eventual burial and subduction removes carbon from the Another group of phytoplankton that have inspired Another, less common, mode is mechanisms. Statoliths are found in system for hundreds of millions of years, thereby affecting remarkable biomineralization applications are the Figure 1. Amphistegina lessonii, a tropical biologically induced biomineralization. benthic foraminifer. The picture shows an a broad range of marine invertebrates the carbon cycle over geological timescales (see Fig 3). diatoms. Diatom silica frustules (Fig. 4) are thought This refers to processes initiated by an electron micrograph of the shell, and a light including bivalves, cnidarians and Striking examples of mass sedimentation of these micro- to have evolved to protect the cell body beneath. organism which lead to the precipita- micrograph of the so-called pseudopodia (© cephalopods. The statolith is a mineral- scopic organisms are the White Cliffs of Dover, which are They produce intricate, lightweight silica structures at tion of a mineral, but the organism University of Cambridge, UK), which are a part ized mass within a statocyst, a sac-like mainly composed of coccoliths and foraminifera shells. various scales that may have applications as templates exerts no control over the form, and of the amoeboid cell. Images: Gerald Langer. structure containing many inverted These huge deposits were formed in the age of the dino- for drug delivery carriers, optical devices, tools, and sometimes even the particular mineral sensory hairs. The statolith shifts as the saurs, approximately 70 million years ago. Extremely large lightweight parts for cars, ships and spacecraft. produced. For instance, some cyanobac- animal moves, brushing the hairs which (frequently several centimetres across) The study of biomineralization is teria (marine photosynthetic bacteria) provide the information to correct disc-shaped foraminifera called Num- exciting and rapidly evolving. It spans alter the seawater chemistry in their the organism’s balance. Otoliths are mulites accumulated on the more recent areas of research from biochemical vicinity in a way that causes calcium tiny particles of calcium carbonate Eocene seabed, and gave their name and cell physiological mechanisms to magnesium carbonate minerals such found in the inner ear of vertebrates. to nummulitic limestones—famous as geological processes. Today, research is as aragonite to precipitate. This type The impact of biomineralization the stone used to build the pyramids. not confined to fundamental bioscience, of biomineralization can result in large The production of biominerals greatly Applying the science of but is central to applications such as structures known as stromatolites. influences the environment. This is biomineralization construction of lightweight materials The role of biominerals particularly true in the case of calcite, Extracted from sediments millions and past climate reconstructions. Biominerals fulfil many different largely because of the chemistry behind of years old, the shells of planktonic Gerald Langer ([email protected]) is roles within organisms, the most its creation and the density it gives foraminifera have played a crucial role as an MBA Research Associate. Figure 2. Calcidiscus leptoporus, a obvious of which are structural and to organisms that produce it. In this a ‘climate archive’ for reconstructing past Charlotte Walker (chawal@mba. coccolithophore. The picture shows an Figure 4. Asterionellopsis, a chain-forming ac.uk) is an MBA and University of mechanical, for example skeletons context we refer not to the skeletons climates. The foraminifera calcite not electron micrograph of the shell, the diatom. Image: © Charlotte E. Walker /Marine Southampton PhD Student. which provide support and enable so-called coccosphere. © Gerald Langer. of large animals but to the shells of only contains calcium and carbonate, Biological Association.

26 The Marine Biologist | May 2018 May 2018 | The Marine Biologist 27