Good things come in small packages

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And few eukaryotic things are smaller than diatoms, unicellular algae that are common, numerous and taxonomically extremely diverse, particularly in the oceans. Small? Yes, typically 20 – 200 µm in diameter. Good? Yes; their annual explosion in numbers during the first quarter of the year – the so-called spring phytoplankton bloom – essentially kick-starts productivity and ecology in a large part of the world’s oceans.

Diatoms
Suspended particulate matter (SPM) in Thermaikos Gulf, Greece, 2004. Scanning electron microscope (SEM) backscattered images of SPM collected on a membrane filter (pore size 0.4 μm). Diatoms Chaetoceros spp. can be identified.. Image: Kostas Tsobanoglou / Wikipedia

By dint of their photosynthetic efforts, it is estimated that diatoms not only contribute approx. 25% of total (i.e. aquatic and terrestrial habitats combined) primary productivity on the planet, but also produce between 20 and 40% of the Earth’s atmospheric oxygen (!!). All of which is good; but there’s more. Bahman Delatat et al. have used genetically engineered diatoms as a delivery system for drugs that target cancerous cells in mammals.

Cancers pose a major threat to human health and the successful treatment of those affected is a matter of global biomedical research effort. A key goal of anti-cancer treatments is to ‘target’ – a euphemism that means to kill – the cancerous cells but avoiding harm to healthy cells. In an attempt to provide that specificity, the frustule (the outer casing of the alga which in large part is composed of hydrated silica) of the diatom Thalassiosira pseudonana was genetically engineered to contain antibody recognition sites.

When introduced into the body of the animal to be treated, and having linked to the appropriate antibodies, the diatom cells are then able to attach specifically to cancerous cells. Once the package has been delivered to the correct site, its toxic cargo of anti-cancer drugs is released, in the vicinity of its target cells. Having been successfully tested in mice, the team optimistically conclude that “genetically engineered biosilica frustules may be used as versatile ‘backpacks’ for the targeted delivery of poorly water-soluble anticancer drugs to tumour sites.”

Certainly a good outcome from the things in that small package! But – and there’s usually always a ‘but’ – the association of diatoms and toxic compounds also has a bad side.

Although blooms of diatoms are generally a good thing, if those diatoms are ones that produce domoic acid (a naturally-occurring neurotoxic amino acid) – such as Pseudo-nitzschia spp.they are not so good. For example, Peter Cook et al. show that California sea lions exposed to domoic acid can suffer brain damage that leads to significant deficits in spatial memory.

Not too surprisingly, such blooms – and which include many more groups of toxin-producing algae than diatoms – that can cause harm to other organisms are termed harmful algal blooms (HABs). Understandably, HABs are of particular concern when toxic algal metabolites are involved because these compounds can impact upon all organisms that either feed on the algae, or consume those organisms that have done so. In that way, toxin levels can accumulate quite markedly as one ascends a food chain from the primary producing algae to the herbivores that consume them directly, to various levels of carnivorous animals that consume members of the trophic level below.

This phenomenon of biomagnification is why animals towards the top of that trophic pyramid – such as California Sea Lions – are susceptible to this form of ‘chemical warfare’ (however unintentional it may be on the part of the diatom). HABs are therefore an example of the ‘dark side’ of primary productivity in the oceans (and we are reminded that “spread of algal toxin [domoic acid specifically] through marine food web broke records in 2015”), and proof that – sometimes – bad things can also come in small packages.

[Ed. – appetite now ‘wetted’? For more on diatom biology and ecology, try E. Virginia Armbrust’s review* and Amanda Hopes & Thomas Mock’s article.]

* Currently available without a paywall at http://baliga.systemsbiology.net/drupal/education/sites/baliga.systemsbiology.net.drupal.education/files/L5ATeacherResource-primarylit_Lifeofdiatoms.pdf


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