Greenspace: Every Fifth Breath

While acknowledging that some botanists might disavow algae as plants, I’m still inclined to corral them into Greenspace – after all they do have chlorophyll.

A year ago in this column, we explored the wonders of south-eastern Australia’s most spectacular seaweeds – bull kelp washed in the havoc of high energy rocky coasts and the giant kelp marine forests tethered and submerged in waters up to 35 metres. As well as chlorophyll, the kelps contain the accessory pigments fucoxanthin and betacarotene, giving them a characteristic golden colour – a characteristic that provides a clue to link them to their counterpoint in this month’s focus – the diatoms. Diatoms are unicellular with the smallest only two microns (twothousandths of a millimeter) and the largest perhaps 500 microns (0.5 mm). In contrast to the wild dishevelment of the kelps, the minute diatoms are the most elegant jewels of the ‘plant’ world. The common simile for diatoms as ‘algae in glasshouses’ references their transparent cell walls. However, the simile doesn’t do justice to their striking beauty. Perhaps Oscar and Lucinda’s conception of the glass church in Peter Carey’s novel is more adequate – “All of their emotions were fused together in this glass vision in which they saw that which cannot be seen”. Victorians fell in love with diatoms and constructed impossible arrangements of them under the microscope – The Diatomist is a short film that tells this story from the perspective of Klaus Kemp – a contemporary diatomist who’s rediscovered the Victorian art. As Klaus observes, “The first time I saw a diatom I was 16 – it was love at first sight. I could not get over how nature could produce something that beautiful … most of the world never sees that beauty.” While most of the world never sees that beauty, the impact of diatoms on our lives is profound. David Mann at the Royal Botanic Gardens Edinburgh observes that diatoms are responsible for nearly a quarter of global carbon sequestration – more than the contribution for all of the tropical rainforests and brings this reality home by observing, that diatoms, “give us every fifth breath by the oxygen they liberate during photosynthesis”. About half of this harvesting of sunlight through photosynthesis is from oceans and the other half from terrestrial freshwater. The significance for Earth’s carbon budget is clear (yes, it’s the economy, stupid). Diatoms provide a key foundation for climate security, and for the ecosystems that drive food and water security. About 30,000 species are known – David Mann suggests there may be 100,000 species in total. The cell walls of diatoms are referred to as frustules and are constructed from silicon dioxide and water (a more efficient process than the construction of organic cell walls in most plants). In a mathematical sense, they are always ‘closed generalised cylinders’ and usually straight (‘right’) although the cross section of the cylinder varies from circular to elliptical to spicular to complex lobed shapes. These elegant glass houses provide some challenges for reproducing diatoms and cell division sees a decline in average cell size over several years during the life cycles of most diatoms (and while the basic architecture is maintained the shape changes too). However, sooner or later there is an abrupt restitution of size, taking a few days, involving formation of a special cell, called an auxospore. In poor environmental conditions or in the absence of a suitable ‘mate’, cells continue to divide and get smaller and smaller until they die. This behaviour is unique in the plant world – and seems quite strange. The architectural precision of the transparent frustules – maintained regardless of the environmental conditions, provides inspiration for nanotechnology. The ability of diatoms to reliably manufacture valves of various shapes and sizes is being explored for application in micro- or nano-scale structures for optical systems and semiconductor nanolithography. Radical proposals see diatom valves as potential vehicles for drug delivery and providing the architecture for constructing titanium dioxide solar cells. Diatoms appear early in the fossil record and are certainly evident in the Lower Cretaceous, about 125 million years ago. Exactly when diatoms first appeared remains unclear. The frustules of dead diatoms endure in the stratigraphic record where they provide clues for geological exploration, and, form remarkably pure deposits as diatomite. The largest single atmospheric dust source on Earth is the Bodélé depression in Chad (up until 6,000 years ago Lake Megachad). Here, storms push diatomite gravel over diatomite dunes generating dust by abrasion and serendipitously raining fossil diatom nutrients on the Amazon’s rainforests. The variously fractured frustules comprising diatomaceous earth are utilised industrially as an extremely fine filter, a mild abrasive and as thermal insulation. Alfred Nobel recognised the value of diatoms in stabilising nitroglycerine in 1866 – a dynamite discovery! Diatomaceous earth is also used as an insecticide – the fine dust can extract lipids from arthropod exoskeletons and minute sections of frustule can pierce exoskeletons during grooming resulting in dehydration and death. Diatomaceous earth is sometimes added to stock feed to prevent insect infestation with evidence suggesting that the additive, when dosed appropriately, can improve weight gain. While non-toxic, the fine dust of crystalline forms can be hazardous with prolonged exposure leading to silicosis. Diatomaceous earth is registered in Australia as a pesticide for bed-bug control. Worth considering as part of an integrated pest management plan but I’d seek professional advice from a pest controller. Stephen Forbes, Director, Botanic Gardens of South Australia

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