Ecological Processes paper collection on biocrusts

There's so much crust news I can't possibly keep up with posting. This is one I should have addressed weeks ago. Bettina Weber and Jayne Belnap organized a crust session on biocrusts in last summer's Ecological Society of America Meeting in Portland, all authors were asked to contribute to this special issue (or paper collection, the open-access version). Some of the talks turned into papers, and other contributors were invited to fill in the gaps. It's an eclectic and high quality collection that is truly international.

Ecological Processes is a new Springer open access journal. All papers are free! Follow the links below to read the papers or download pdf's. Also, check back, I believe more papers are coming.

Biological soil crusts: their diversity, functional ecology and management.

Edited by: Dr Jayne Belnap, Dr Bettina Weber It has only recently been recognized that biological soil crusts (BSC) are a critical component of dryland ecosystems, contributing to the diversity, functionality and nutrient cycling of these regions worldwide. Formed by cyanobacteria, algae, lichens, fungi and bacteria in varying proportions, BSC grow within the uppermost millimeters of the soil, fulfilling a variety of ecosystem services. With their filaments BSC organisms glue together soil particles, thus effectively preventing erosion by both wind and water. BSC organisms in general and nitrogen-fixing cyanobacteria in particular, add nutrients to nutrient-poor desert soils, thus promoting the establishment and growth of vascular plants. Their relevance and impact on global carbon and nitrogen cycles is just starting to be considered. During the last decade, studies on BSC have intensified and diversified, creating a unique and growing research community.Collection published: 23 February 2013Last updated: 16 May 2013
Belnap J, Weber B- Biological soil crusts as an integral component of desert environments. 
Marusenko Y, Bates S, Anderson I, Johnson S, Soule T, Garcia-Pichel F. Ammonia-oxidizing archaea and bacteria are structured by geography in biological soil crusts of North American arid lands.

Weber B, Wessels DCJ, Deutschewitz, Dojani S, Reichenberger H, Budel B. Ecological characterization of soil-inhabiting and hypolithic soil crusts within the Knersvlakte, South Africa.

Budel B, Vivas M, Lange OL. Lichen species dominance and the resulting photosynthetic behavior of Sonoran Desert soil crust types (Baja California, Mexico).

Dettweiler-Robinson E, Ponzetti JM, Bakker JD. Long-term changes in biological soil crust cover and composition.

Peterson EB. Regional-scale relationship among biological soil crusts, invasive annual grasses, and disturbance.

Beraldi-Campesi H. Early life on land and the first terrestrial ecosystems.

The earliest records of internally stratified cyanobacterial and algal lichens from the Lower Devonian of the Welsh Borderland Honegger 2012 New Phytologist Wiley Online Library

The earliest records of internally stratified cyanobacterial and algal lichens from the Lower Devonian of the Welsh Borderland Honegger 2012 New Phytologist Wiley Online Library

(Very) early life on land: Just who is this Dickinsonia character, anyway?

Fig. 1. Dickinsonia (source: uonews.oregon.edu) 

My first impression upon hearing the hypothesis that Dickinsonia (Fig. 1) may be a lichen was that it does not look like any lichen I’ve ever seen. To be fair, the more normal interpretation that it is a marine animal, a segmented worm, sparks the same response…it does not look like any animal I’ve ever seen or heard of. What it actually resembles is a giant diatom, but that is pretty silly. This uncertainty is exactly what has been nagging paleontologists since the 1940’s….what are these things and what later life forms, if any, evolved from them? Dickinsonia and other Ediacaran Period (635-542 mya) fossils are enigmas. They left fossils in multiple locations world wide, then drop out of the fossil record before the Cambrian explosion in a mass extinction. 

I’m no expert on this topic, so I had to do a little looking into the history of Earth. I read Gould’s “It’s a wonderful life” several years ago purely for fun, but that is the extent of my paleontological expertise. It is common in the popular press to equate the Ediacaran time as representative of the beginnings of life. This is not true. Rather it could be the beginnings of animal life or the beginning of clearly complex multicellular life, but the Earth was already quite alive at this point. The major events are the origin of single-celled organisms and their radiation into archaea and bacteria about 3500 mya. The conventional thought is that this all occurred in the oceans, though there are other opinions. Photosynthetic bacteria showed up around this time, and maybe even were on land. Then, came the evolution of the cyanobacteria about 3000 mya which was extremely significant because their brand of photosynthesis produced oxygen as a waste product, and drew down CO2 in the atmosphere. It is plausible that they colonized land, but because they do not often leave convincing fossils on land, this is hard to prove. I would love to have Hugo Beraldi-Campesi weigh in on this, as I am out of my depth on this topic. The oxygen produced by the cyanobacteria immediately began rusting all of the iron in the Earths rocks, until this sink was saturated, i.e., all the surface iron had oxidized. At this point the great oxidation event occurred wherein the atmospheric oxygen started rising (~2500 mya) causing a mass extinction of anaerobic organisms, and also crippling the Earth’s greenhouse effect and causing a “snowball earth” episode of global glaciation, and partially frozen seas. This phenomenon happened a couple more times, possibly linked to other phenomena like super volcanos. After the last snowball Earth episode, a group of fossils called the Ediacaran fauna turn up supposedly inhabiting microbial mats on seafloors....check out the video below.

They are known from multiple locations all over the place suggesting global ubiquity. “Shortly” after, the famously rapid radiation of complex lifeforms (most of which went extinct) occurred in the Cambrian explosion, exemplified by the fauna of the Burgess shale. The Ediacaran animals are not part of this fauna, suggesting an extinction event, and plausibly leaving a bottleneck from which the Cambrian radiation occurred.

            It’s a great story, with holes, but all very reasonable given that the fossil record is always incomplete and many inferences must be made from incomplete data. What if the Ediacarans are not marine animals, and not animals at all? Then the animalian genetic lines that “exploded”, did not come form the Ediacaran…making the Cambrian explosion even more out of nowhere. The interpretation that these are lichens, marine ones (something that does actually exist today), was proposed by Retallack (1996). What if these were terrestrial organisms – including lichens and other fungi- inhabiting soils? That is what this new paper suggests (Retallack 2012). The new bit of information, that will be of great interest to people who read this blog, is the evidence that one key Ediacaran site in Australia (the type locality, actually) is composed of paleosols, not marine sediments (Fig. 2). The evidence is based on the patterns of oxidation, weathering into soil horizons, isotopic signatures, needle ice, carbonate and gypsum accumulations and cracking……..and “elephant skin” which is interpreted as a biocrust. Could there be biocrusts on land at this point, i.e. terrestrial community version 1.0? I think yes, we know cyanobacteria had been around for a very long time, and had all the tools they might need to grow on land (UV protection, desiccation tolerance, N-fixation, C-fixation). That is totally plausible, in fact they could have colonized after other photosynthetic bacteria actually making them terrestrial community version 2.0. As an ecologist, I find it hard to believe that the ocean would be teeming with microbial lifeforms in abundances great enough to overhaul the Earth's atmosphere, but the land surface would be sterile. I can easily believe that biocrusts are ancient. Retallack's (2012) interpretation is that this was a cold dry environment, so i do wonder why the putative fossil biocrusts don't have the high-relief pinnacled structure (up to 10 cm) associated with today's biocrusts in cold climates and sandy soils (e.g. the Colorado Plateau). Could the Ediacaran “fauna” be like the lichen components that colonize todays’ biocrusts? That’s the tougher one to believe. The first unambiguous fossil of a lichen is from the Devonian (Honegger et al. 2012), about the same time that plants radiated and came to dominate the continents.

Retallack's reinterpretation of depositional environment and fossils of the Ediacara Hills, Australia (source: ounews.orgon.edu). 

            It seems like Retallack has amassed multiple lines of evidence of the paleosol interpretation. As others have pointed out (Xiao & Nauth 2012), the weakness in the paleosol interpretation is that it is based on one of many localities. Even if these are paleosols, what about all of the other marine rocks which contain the same fossils, some of which are thought to be deep water formations. Some of them are clearly rippled, suggesting the action of water. Were these Ediacarans terrestrial AND marine organisms, then? It seems to me that for now the Ediacaran enigmas must remain enigmas. I do look forward to a reexamination of other Ediacaran sites, if nothing else this paper makes the Earth’s history just a little more fascinating than it already was (quite). Whether you believe him or not, Gregory Retallack seems like an intriguing scientist with a tendency towards provocative papers. This is how science works...once we get comfortable with a hypothesis, along comes a challenge. The scrutiny will likely lead us closer to the true nature of the Ediacarans, though we may have to wallow in deeper and deeper confusion for a while. As I’ve said multiple times here, this is not my expertise and many more knowledgeable people might come across this blog….. so please do light up the comments section with your impressions of this new paper.

Honegger, R., Edwards, D., Axe, L. 2012. The earliest records of internally stratified cyanobacterial and algal lichens from the lower Devonian of the Welsh borderlands. New Phytologist 197: 264-275

Rettalack, GJ. 1996. Were the Ediacaran fossils lichens? Paleobiology 20: 523-544.

Xiao S, Knauth, LP. 2012. Paleontology: fossils come in to land. Nature doi:10.1038/nature11765

Retallack, G. (2012). Ediacaran life on land Nature DOI: 10.1038/nature11777

Australian multicellular fossils point to life on land, not at sea, geologist proposes

I'm going to have to find the time to write about this in a bit more detail, but here's the press version.

Australian multicellular fossils point to life on land, not at sea, geologist proposes

In a nutshell, this paper interprets the ediacaran fossil marine "animals" as organisms of biological soil crusts. The argument that these famous fossils are terrestrial has been around a long time. I think the new element is evidence that these are paleosols, specifically with evidence of a fossil crust. I don't know if I believe it...but more on this later.

Did crusts kill the dinosaurs? (just kidding, but check out this Cretaceous biocrust)

Crusts are old. Maybe sometime soon I’ll write about the idea that the first terrestrial communities were cyanobacterial biocrusts, rather than true land plants…or I’ll ask someone else to do it who knows a lot more. Recently in a geology journal that few biologists are likely to wander to, Simpson et al. (2010) report on a fossilized crust from the Cretaceous. Now, the Cretaceous isn’t that old, it’s noted as the last stand of the dinosaurs and the coming out party of the angiosperms…but the amazing thing is that their fossil looks very much like a living modern day crust.

It was found in my old stomping grounds of Grand Staircase-Escalante National Monument near the contact between the Wahweap formation and the Kaiparowits formation. The fossil was located near Cottonwood Canyon where I would often camp in the shade after long hot days of hiking to remote sampling sites, and attempt to remove the multi-day sunscreen & soil crust that had accumulated on me in the nearby creek. The sandstone portions of the Wahweap formation can support some very nice biocrust today if you work harder than a cow and walk far away from water sources. So needless to say this stirs some nostalgia in me. Little did I know there were fossil crusts in the rocks, it really makes you wonder what other formations have them.

As you can see from the photos, they have a pinnacled structure like most of the Colorado Plateau crusts of today. This puzzles me a bit, we have long thought that frost heaving plays a large part in generating uplift in these pinnacled crusts. Naturally, erosion sculpts things that stick up, but eventually some equilibrium is found where the sculpted pinnacled form is well-preserved by a late successional biocrust. But the Cretaceous was a much warmer time than today, the areas where the rocks were formed would not likely have experienced frost, correct me if I’m wrong. I was similarly puzzled a few years ago when I saw a fantastic crust on the Oregon Dunes while cycle touring. It had the exact same pinnacled structure as Colorado Plateau crusts, despite the maritime climate and lack of frost. Maybe we need to rethink the origin of this structure.



Read more about the fossil crust here. All photos are from Simpson et al. 2010.

Simpson, W.S., Simpson, E.L., Wizevich, M.C., Malenda, H.F., Hillbert-Wolf, A.L., Tindall, S.E. 2010. A preserved Late Cretaceous biological soil crust in the capping sandstone member,Wahweap Formation, Grand Staircase-Escalante National Monument, Utah: Paleoclimatic implications. Sedimentary Geology 230: 139-145.