Nitrogen Fixation

Colonies of the nitrogen-fixing cyanobacterium Nostoc are visible as dark spots in the thallus of the hornwort Dendroceros crispatus.

Nitrogen is essential for plant and animal growth. Nitrogen makes up about 80% of the earth’s atmosphere, but most organisms cannot use free nitrogen. They must wait until it has been fixed, or chemically transformed, into a more usable form. Cyanobacteria can fix atmospheric nitrogen and all hornworts and many lichens contain cyanobacteria. Once in the lichen or hornwort, the nitrogen can become available to plants and animals in several ways:

• Nitrogen compounds may be leached from the lichen or hornwort.
• Some invertebrates eat the lichens and hornworts and release nitrogen-enriched droppings.
• When the lichen or hornwort dies and decays the nitrogen compounds are released.

Studies in areas from the arctic tundra to the equator have shown that lichens are important nitrogen sources in most ecosystems but the hornwort inputs to ecosystems have been less well studied.

Lichens, Plants and Snail Poo

The harsh Negev Desert in Israel is strewn with limestone boulders, which have nitrogen-fixing lichens growing on them. Various species of land snails feed on the lichens during the night. Once the sun comes up the snails retreat to the sheltered areas under the boulders and release their faeces down there. Research has shown that about 11% of total soil nitrogen inputs in the Central Negev Highlands of Israel come from the snail poo!

Leptogium azureum, a foliose lichen which occurs throughout the tropical areas of the world and has a cyanobacterium, Nostoc, as the photobiont.

The foliose lichen Pseudocyphelaria neglecta, widespread in Australia, contains the nitrogen-fixing cyanobacterium, Nostoc.

Plant seeds that land in the shelter of the boulders have a better chance of growing into mature plants than seeds that germinate in the open. Moreover seeds that germinate near the boulders do so in a nitrogen-enriched area, gaining an extra advantage. There are nitrogen-fixing lichens in the arid areas of Australia, but so far no studies have been done on their interactions with snails or other animals.

This tiny land snail Euchondrus desertorum from the Negev Desert in Israel is only about 12 mm long. It is one of several species that play an important role in the nitrogen cycles in the ecology of this arid area.

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