Tuesday, 3 December 2013

Sooty mould makes a killer honey

In 1855 Reverend Berkeley remarked on an abundance of black moss in New Zealand. At least this is what the locals called it. More generally it is considered to be one of the sooty moulds or as Berkeley himself would have classified it, a pyrenomycete.

Pyrene is what we call the stone of a fruit, such as an apricot, and many of the pyrenomycetes are rough in texture. The mycetes bit tells us it's a fungus.

Black moss is formed by a collection of barely related fungi that express themselves as a black fibrous coatings on tree trunks. In New Zealand they favour species of Southern Beech, Nothofagus*. That's because these fungi thrive on honeydew, a sweet compound produced by scale insects living on the Beech.

It has been estimated that scale insects live on the trunks and branches of Nothofagus* in a million hectares of the northern half of New Zealand's South Island. Their presence is almost always associated with black moss. Although widespread, the insect and its fungal passenger are patchy in distribution.

A month or so ago, tramping near Wanaka, in the southern half of the South Island, I found this patch. I thought a bush fire had rendered every lichen and moss black until a French tourist explained to me that it was, as everyone at least since Berkeley has known, a fungus. I only saw this one small area, perhaps a few hundred metres square, infected with black moss but it was a memorable and photogenic (if you like to take pictures of black sooty things) display.

Black Moss doesn't seem to harm the plant too much although there is some debate about whether scale insects are helpful or not. A study in 2006 argued that not only do they suck up valuable sugars form the plant, they might also reduce the rate of litter breaking down and releasing much needed nutrients to the soil.

The French tourists also told me that bees feeding off the black moss produce a very expensive kind of honey. It is an acquired tasted they said, in a tone that suggested they hadn't yet acquired it.

There are various honeydew honeys produced around the world. The basic principle is that the bee collects nectar from the exudate of another insect, rather than from a flower. They are sometimes called forest honey or, when the exuding insect lives on a pine, pine honey. These honeys tend to have less glucose and fructose, and more complex sugars and minerals than flower-sourced honeys. And they don't crystallize easily.

But honeydew honey from New Zealand is different. In addition to the insect exudate it contains bits of the black moss (which sounds marginally better than 'bits of the sooty mould'). That may be one reason why it tends to be darker in colour than most other honeys, although its deep colour is more likely due to the bees storing it in darker combs.

Naturally because this honey is relatively rare, expensive and intriguingly produced, it must be good for you, right? Well according to my not entirely unbiased source, it has plenty of oligosaccarides to help you maintain good bacteria in your gut, plenty of antioxidants which everyone seems to want, and a dose of an antibacterial called glucose oxidase (I'm just assumed it kills only bad bacteria).

As yet I haven't tasted any. All the shops seemed to be selling expensive Manuka honey, produced from the flowers of the local teatree, Leptospermum scoparium. The most expensive jars of this condiment contain methylglyoxal which a German scientist swears is antibacterial. There are obviously some nasty bacteria out there and honey, in one form or another, is the way to deal with them.

*Without wishing to worry you too much, a proposal to split Nothofagus into five genera has just been published, meaning the New Zealand species would no longer be Nothofagus but either Lophozonia or Fuscospora.

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