Monday, May 27, 2013

My graduation speech: an ode to good education.

A month or so ago, I was delighted to give the graduation speech on behalf of my cohort of science graduates at Macquarie University.

And here, through the magic of the Intertubes, it is! I was a touch nervous, as is clearly apparent. But I meant every word.

Sunday, January 29, 2012

Another frog species spotted in our garden....

Last night, we encountered this gorgeous creature on the other side of our sun-room window. It's Litoria phyllochroa, Leaf Green Tree Frog, family Hylidae. Not an uncommon species in this part of Sydney.



That brings to five the number of frog species we've identified in our garden, listed in our garden census. The wet weather we've been having is such a joy!


Thursday, January 26, 2012

When Plants Parasitise Fungi: myco-heterotrophy

Deep down on the forest floor, sunlight is the scarcest of all resources for plants. They struggle to reach it, lianas and vines climbing up to reach the top and seedlings exploit a gap in the canopy to shoot up and out-compete other plants. But some species have evolved a way to do without sunlight entirely. These plants don't photosynthesise at all. They have no (or very few non-functional) chloroplasts and so lack the green colouration associated with plants.

Instead, they "steal" the carbon they require from fungi, and frequently, indirectly through fungi from other plants. Plants that parasitise fungi are called myco-trophic (myco=fungal, trophic=relating to feeding or nutrition).


The Australian orchid, Rhizanthella gardneri, does not photosynthesise, but
relies on its fungal host for nutrients and carbon.
Photo: Bert Wells, WA Government Recovery Plan.


80% of land plant families are mycorrhizal, having a symbiotic relationship with fungi. Normally, the relationship is mutualistic, with the plant providing the fungus with energy by way of carbon products from photosynthesis, and the fungus supplying the plant with nutrients from the soil such as nitrogen and phosphorous; see (a) in figure below. Species such as the orchid Rhizanthella gardneri, above, "cheat" on this relationship, taking both nutrients and carbon from the host mycorrhizae; see (b). Because many mycorrhizal fungi and their hosts are generalists, sometimes an entire underground network of hyphae from a single fungus connects multiple individual plants of different species. So while the "cheater" parasitises the fungus, its ultimate hosts are the other mutualistic plants in the network, with the fungus acting as a bridge.

Of course, sometimes fungi are pests of plants and take nutrients away from the plant. In (c) we see how that works. Carbon and nutrients are obtained by a pathogenic fungus. But here, the tables are turned. Another plant is parasitising the parasitic fungus. It's an epiparasite: the parasite of a parasite.

So far, the process is relatively straightforward. The parasitic plant is simply exploiting mechanisms that have evolved many times in plant evolution, the elegant, microscopic interaction of plant root cells and fungal hyphae. It's an example of how mutualisms can be unstable, able to switch to parasitism as environmental pressures alter. As with much in biology, it represents the ongoing "arms race" that occurs between species competing for resources.

The fourth mode (d), is a little understood but quite extraordinary relationship: the recruitment by a plant of a fungal host that is normally free-living. In this case, the fungi is saprotrophic: it obtains its nutrients by digesting dead plant material, such as logs.


© Margaret Morgan


Saprotrophic fungi are a common sight in forests and woodlands. Their fruiting bodies (the "mushrooms") are frequently seen growing out of dead logs or leaf litter, but most of the organism is beneath the surface, masses of filamentous hyphae exuding enzymes to break down the organic material. 

Mycena galericulata, a saprotrophic fungus growing on a dead log.
Image (CC) by Dan Molter, Wikimedia.  

The Asian orchid Gastrodia confusa, below, has recently been found to be an example of myco-heterotrophy of a saprotroph, gaining its carbon and nutrients from a Mycena species (Ogura-Tsujita et al. 2009).


Gastrodia confusa, Taiwan. Image © JJ-Merry. Reproduced with permission. 

For many years, it has been assumed that plants in this genus were themselves saprophytes, gaining nutrients directly from decaying organic matter. Indeed, this claim is still made. Molecular studies employing isotopes of carbon and nutrients have now shown that in this species at least, the fungal host is playing an essential role. Gastrodia species are broadly distributed from Africa and India, to South-East Asia, Australia and Pacific islands. Three species are endemic to New South Wales. It would be fascinating to establish whether in these species myco-heterotrophy is also occurring. 

I'd bet good money that it is.

_____________________________________________

This post is adapted from a scientific poster I made for BIOL341 Parasitology, a unit at Macquarie University convened by Dr Michelle Power.

Special thanks to JJ-Merry for permission to include his image of Gastrodia confusa. There are very few images of this species online, and most are his work. His excellent photography of South-East Asian tropical plants can be found here

References:

Bidartondo, MI 2005, 'The evolutionary ecology of myco-heterotrophy', New Phytologist, vol. 167, no. 2, pp. 335-52.


Bidartondo, MI, Redecker, D, Hijri, I, Wiemken, A, Bruns, TD, Dominguez, L, Sérsic, A, Leake, JR, Read, DJ 2002, 'Epiparasitic plants specialized on arbuscular mycorrhizal fungi', Nature, vol. 419, no. 6905, pp. 389-92.


Hibbett, DS 2002, 'When good relationships go bad', Nature, vol. 419, no. 6905, p. 345.


Kikuchi, G, Higuchi, M, Yoshimura, H, Morota, T, Suzuki, A 2008, 'In vitro symbiosis between Gastrodia elata Blume (Orchidaceae) and Armillaria Kummer (Tricholomataceae) species isolated from the orchid tuber', Journal of Japanese Botany, vol. 83, no. 2, p. 77.

Leake, JR 2005, 'Plants parasitic on fungi: unearthing the fungi in myco-heterotrophs and debunking the 'saprophytic' plant myth', Mycologist, vol. 19, no. 3, pp. 113-22.


Merckx, V, Bidartondo, MI , Hynson, NA 2009, 'Myco-heterotrophy: when fungi host plants', Annals of botany, vol. 104, no. 7, pp. 1255-61.


Ogura-Tsujita, Y, Gebauer, G, Hashimoto, T, Umata, H, Yukawa, T 2009, 'Evidence for novel and specialized mycorrhizal parasitism: the orchid Gastrodia confusa gains carbon from saprotrophic Mycena', Proceedings of the Royal Society B: Biological Sciences, vol. 276, no. 1657, pp. 761-7.


Wednesday, November 09, 2011

Saltmarsh: regeneration of an endangered habitat.

Lane Cove National Park is a narrow, irregularly-shaped ecological reserve surrounded by Sydney suburbs. It is marked in dark green in the map below. Although it extends over 10 kilometres in length, it is only 6 sq kilometres in area. It surrounds the Lane Cover River, which begins in Hornsby to the north and opens into Sydney Harbour.


Map from National Parks and Wildlife Service

Its location, topography and perimeter present major ecological impacts on the park, specifically storm-water run-off from residences and weed invasion.

A key feature of natural Sydney vegetation is that it is largely adapted to soils derived from ancient sandstone, notoriously low in nutrients--particularly phosphorus. While seeds of invasive plant species evolved in high nutrient environments are likely found in all urban Sydney bushland, they generally cannot develop to maturity where nutrient levels are poor. Where run-off occurs, however, nutrients from human activity fertilise the soil and weeds can thrive. That's why weeds are more likely to be found near suburban bushland creeks than in the bush beyond. In addition, cuttings and seeds from weeds can be washed into the area, to propagate and invade. Weeds can then out-compete native plant species and destroy habitat and food sources of native fauna.

Sugarloaf Point is a region in the park that was once a thriving saltmarsh ecosystem. In 1940, 1.2 ha of the Point (below) were mangrove swamp, and 0.2 ha were saltmarsh.


Sugarloaf Point in 1940. Image © NSW Dept of Lands 2011.

Midway through last century, the river was dredged to improve boat access, and the excavated silt deposited onto the salt marshes.

Sugarloaf Point in the 1969s, at the height of dredging operations.
Photo courtesy of National Parks and Wildlife.
Over time, bushland has regenerated on the former saltmarsh site.

Casuarinas now growing on the site where the dredged silt was dumped.

Saltmarsh ecosystems provide a crucial role in coastal estuarine systems. They provide habitat for a range of molluscs and crustaceans, and act as nurseries for species that find their way into harbours. Microbats feast nocturnally on the high insect populations they support. And marshes recycle nutrients to make them available to other species. These environments are filled with water at high tide, and completely washed out during king tides. At king tides, fish enter to feed on the abundant prey available there.


National Parks ranger, Andrew Duffy, shows plant ecology staff and students a
coastal saltmarsh at Lane Cove National Park. The bridge is part of the
Great North Walk, stretching from central Sydney to
Newcastle, 250 kilometres north.

Sarconornia quinqueflora is a key species in Australian coastal saltmarsh. 

In New South Wales, coastal saltmarsh is listed as an endangered ecological community under the Threatened Species Conservation Act 1995. Frequently associated with this saltmarsh in saline wetlands are mangrove swamps, which provide habitat to a range of endangered and threatened species of flora and fauna, including the critically endangered Beach Stone-Curlew, Esacus neglectus.


Beach Stone-Curlew. Image CC, Avicida

Mangrove swamp, Lane Cove National Park.


Given the vulnerability to disturbance and pollution in Lane Cove National Park, one might imagine that its saltmarshes don't have a hope in Hades. However, thanks to the work of the National Parks and Wildlife Service, its local ranger, Andrew Duffy, and groups like Friends of Lane Cove National Park, extraordinary saltmarsh restoration is taking place there.


Saltmarsh restoration at Lane Cove NP.


This photo shows the work done thus far on the re-creation of a saltmarsh in the park. The soil has been removed to make the site low enough to be thoroughly inundated twice a month, with the gradient calculated to keep the water at an appropriate level across the site. Volunteers have planted tubestock of the signature species, including Suaeda australis, Tecticornia arbuscula, Juncus kraussii and Isolepis nodosa (Knobby Club-Rush, one of my favourite monocots, and not just because of its common name!)

Creating physical conditions is only part of the story. There's also chemistry. The environment needs to be kept free of the pollution that encourages weeds. To this end, nutrient traps have been created to decrease harm from storm water from adjacent residential areas. Below is a gross pollutant trap, which collects leaf litter and rubbish. The content from the trap is removed and disposed of off-site.


Gross pollutant trap.


More particulate pollutants need a finer trap, so crushed sandstone storm-water treatment traps have been set up, immediately adjacent to the nearby major road.

Crushed sandstone storm-water treatment trap,
immediately adjacent to Pittwater Road.

The crushed sandstone is obtained from construction sites in the Sydney area, and makes an ideal medium to suppress weeds and provide suitable substrate for native plants. It is placed on terraces constructed across the embankment. Beneath the trap is a settlement pond, where escaped nutrients settle and are physically removed with an excavator when around five tonnes of sediment have accumulated.

One of the predictions of climate change, already observed, is rising ocean water levels which will obviously have an effect on both saltmarsh and mangrove ecological communities. To this end, thought is being given to planning the restoration of these ecosystems at slightly higher altitudes. Water levels are not the only issue these communities must contend with, however. There is a strong inverse relationship between saltmarsh diversity and temperature in Australia, which does not bode well. (See link below for more.)

Thanks to Dr Michelle Leishman of Macquarie University's Department of Biological Sciences and to NPWS Ranger, Andrew Duffy, for introducing me to his magnificent habitat and restoration work. If I were a rich woman (ya ha didle deedle, didle deedle didle deedle dum) I'd donate to this cause in a heartbeat.

Further reading:

Coastal saltmarsh in the NSW North Coast, Sydney Basin and South East Corner bioregions - endangered ecological community listing

Protecting and Restoring Coastal Saltmarsh

Coastal saltmarsh vulnerability to climate change in SE Australia

[Edit: Thanks to Doug Beckers for his correction of the ID of Sarcocornia quinqueflora!]