focus on Microscopy Microtechniques & Tabletop Ele on Microscopy at the British Antarctic Survey


Dr. Katrin Linse, British Antarctic Survey, Cambridge. Tel: +44 1223 221400, E-mail: kl@bas.ac.uk, Web: www.antarctica.ac.uk, Patrick Marks, Hitachi High-Technologies Corporation, Maidenhead. Tel: + 44 800 316 1500, E-mail: eminfo@hht-eu.com, Web: www.hht-eu.com


The seas that encircle the continent of Antarctica are known as the Southern Ocean. Occupying the only band of latitudes on the planet where ocean waters encircle the globe, the Southern Ocean connects the Atlantic, the Pacific and the Indian oceans, and is a crucial cog in the Earth's climate system. The Southern Ocean is one of the largest marine ecosystems in the world. Its cold and often ice-covered waters harbour a web of species from the smallest single-celled plants and animals to the largest mammal on Earth, the blue whale. Every new species discovered needs to be properly described and microscopy plays an important role in this process.


Working from the Royal Research Ship James Clark Ross and research stations on South Georgia - Bird Island and King Edward Point – as well as bringing samples back to their Cambridge headquarters, scientists from British Antarctic Survey (BAS) are studying many crucial aspects of the Southern Ocean and the species that depend on it. One of the key challenges for BAS scientists today is to predict how human activity and climate change will affect the Southern Ocean, and how the creatures that depend on it will respond. Parts of Antarctica are among the most rapidly warming areas of the planet. At BAS, scientists are monitoring the effects of this climate change on the Southern Ocean's ecosystem. BAS marine biologists utilise a number of techniques to inspect and characterise species found in the Southern Ocean, and variable pressure electron microscopy is a powerful recent addition to their capabilities.


Variable Pressure Scanning Electron Microscopy


The benefits of variable pressure electron microscopy are well known. Scanning electron microscopy offers much better resolution and depth of focus than optical microscopy and allows high resolution studies of sample morphology. Conventional scanning electron microscopy is a high-vacuum technique, which means that examination of biological material requires special preparation techniques. These range from cooling, freezing or drying to avoid the loss of water vapour from the sample (and subsequent deformation) in the vacuum environment, to coating the sample with a thin carbon or metallic layer to conduct away any electrical charge induced by the electron beam. With variable pressure scanning electron microscopy, however, the pressure in the sample chamber is maintained at a much higher value than in a conventional SEM. This has a two-fold benefit. Firstly, the higher pressure reduces the rate of water evaporation from the sample and secondly, the ionisation of the gas molecules in the chamber by the electron beam results in dissipation of the excessive charge that would otherwise build up on the surface of the sample. The nett effect is that the variable pressure instrument greatly simplifies the examination of biological samples. The emergence of the tabletop variable pressure scanning electron microscope has made this powerful technique much more readily available to hands-on scientists, since a dedicated microscopist or microscopy technician is not needed for operation. Samples can be mounted ready for use and high quality images produced by non-specialists in just a matter of minutes. Such is the compact size and ease of use, that in principle, the microscope could be installed on board the research ship for use during a research cruise!


Tabletop Electron Microscopy at BAS


The tabletop microscope contributes greatly to the morphological studies of many of the diverse species found in the seas around Antarctica and helps enhance existing knowledge about biodiversity. Of particular interest are gastropod and bivalve molluscs and isopods. Marine biologists at BAS use the TM3000 tabletop microscope from Hitachi [8], typically at magnifications between 30x - 5000x to view, measure and record morphological details of the hard components of these fauna, such as shells, opercula, radulae and ctenidia. The morphology of the shell of a gastropod (e.g. Figure 1) shows the whorls of the adult shell, which starts forming when the larval gastropod settles and becomes a juvenile. The operculum is an anatomical structure found in some marine gastropods. The operculum is also known as a ‘little lid’ and is attached to the upper surface of the gastropod’s foot. In its most complete state, it serves as a sort of "trapdoor" to close the aperture of the shell when the soft parts of the animal are retracted [1]. An example is shown in Figure 2. The radula [2] is a minutely toothed ribbon, which is typically used by gastropods for scraping or cutting food before the food enters the oesophagus (analagous to the mouth parts). The arrangement of teeth on the radula ribbon varies considerably from one group to another.


Whilst examination of these structures is a powerful tool in charactering species, the microscope is also used to compare fauna currently existing in the region with previously discovered examples. For example cores from marine sediment retrieved from the region contain diatoms that can be


Figure 1. Apical view of Iheyaspira bathycodon shell


thousands of years old. Diatoms are one of the largest and ecologically most significant groups of organisms on Earth. Comparisons with current examples and changes in species abundance within a sediment core can be used to infer possible changes in climate over the period, since changing climate could alter the mixing depths and delivery of nutrients to diatoms and their subsequent sizes. Shorter term comparisons can also be made with archived fauna collected since the earliest sample collecting expeditions in the 1940s.


Identification of a New Species of Gastropod


While the Antarctic region is naturally the key area of research for BAS scientists, they also collaborate extensively with the environmental science community in the UK and overseas to share best practice and to maximise the scientific impact. One such collaboration has led to the identification of a new gastropod from the Von Damm Vent Field, an active, high-temperature hydrothermal system, on the world’s deepest spreading centre, the Mid-Cayman Spreading Centre in the Caribbean, at 2300 m depth [4]. Gastropods are one of the most species-rich macrofaunal taxa in hydrothermal vents.


INTERNATIONAL LABMATE - JANUARY/FEBRUARY 2013


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