How many ciliates are there in the dried out sediments of a natural weathering pit?

The natural granite basins/pits, pilas o pilancones of La Pedriza, are eroded cavities where rainwater collects and constitute some of the minor structures or microforms in the granite. The genesis of the basins are usually caused by the dissolution and mechanical erosion of the rock, aided by its capacity to retain water. Evapotranspiration exposes sediment or debris in the cavity bed. This detritus may contain a highly varied size of granite particles, organic crust of varying thickness and, in many cases, well developed soil growing bryophytes and vascular plants that shape authentic ecosystems for colonization by species.

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The frequent alternation of states between desiccation and inundation that occurs in natural basins, makes the microbial communities that may appear there, are subject to drastic diurnal and seasonal changes in temperature and humidity. Specifically, in some of the dry pits studied by Microepics we observed daily variations in temperature of 37°C, with surface temperatures reaching up to 59°C and 90% relative humidity.

Would these be suitable ecological niches for the development of protists? How many of them would appear per gram of dry sediment in the basin? And how many species? Would there be differences in the composition of the protist community of various basins? All these questions and many more set us collecting sediment samples from the basins in September 2013, after the long La Pedriza summer. The initial microscopic observation of hydrated sediments revealed a variety of protists (amoebae, flagellates and ciliates). Among the ciliates, the presence of a large number of bacterivorous species encouraged us to embark on a larger research project. And so Microepics was born. We stored these dry sediments in the laboratory and a year later conducted a quantitative study of the ciliate species in the sediments of three of the granitic basins. These sediments in particular are heterogeneous in composition, generally with a firm soil / earth foundation, which adds large siliceous grains to some of the basins, and different types of bryophytes to others. In this study we determined the ecological succession of ciliate species in a 14 day microscopic follow up from the initial hydration of sediments, with the aim of revealing the potential cryptic biodiversity (hidden biodiversity) of species with low abundance and/or slower growth or development. We include some micrographs of representative species.

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ciliadosMicrographs of representative ciliate protists in granitic basins (Author:Microepics project)

Overall, in the granitic basins analyzed so far we have identified 55 ciliate morphotypes (morphological species). Up to 12 species can coexist in one gram of dry sediment (DW). The distribution of ciliates varies in each basin, both in species richness (the number of species) and abundance (the number of individuals), and the succession (how number of species and number of individuals changes over time) of populations is very complex. Only three species are common to all the basins we have examined: Cyclidium glaucoma, Halteria grandinella and Vorticella aquadulcis.

The population succession study showed that after 14 days, the cumulative number of species have appeared in all three basins tended to stabilize, with a maximum of 25 species found in the same basin. Thus, two weeks may be the minimum time needed to reveal the cryptic diversity of ciliates found in these types of sediments. The most abundant ciliates in these basins, regardless of the sediment type, were bacterivorous ciliates: the species Colpoda steinii reached an average density of 2.4 x104 per day individuals g-1 DW  sediment, Homalogastra serosa 4.6 x103 individuals g-1 DW, and 3.8 x103 Cyclidium glaucoma individuals g-1 DW.

But, how can ciliates and other protists survive for long periods in dry sediments and sudden changes in temperature? Many have developed resistance structures as part of their life cycle, called cysts. Cysts are metabolically inactive states that allow these unicellular microorganisms to withstand adverse environmental conditions such as the lack of nutrients and desiccation. Ciliated cysts vary in size, composition and shape, and are often used as a character for identification to the species level. Not all ciliates have cysts, but they are very important for the survival and dispersal of species in certain environments, because they can be transported on the wings / legs of birds or hair and limbs of various animals. When the conditions are right, generally when you add enough water to their medium, as in this case, the cyst wall breaks and emergent vegetative cell can continue the life cycle of the species. There are cysts that allow species to survive desiccation for years.

As far as we know, this is the first detailed identification study of protist ciliates in granitic basins of this type in the world. Our next steps will be (on the cataloging the biodiversity of protists) to continue the study of rare or little-known species, investigating what their resistance strategies to widely varying environmental conditions can reveal about the ciliate life cycles and their adaptations to these environments. On the other side, we will isolate and cultivate the dominant bacterivorous species in granitic basins. These will be used for predation studies with bacteria which are indicators of fecal contamination, in search of protists with the potential for bioremediation.

We will keep you informed on how Microepics is progressing with these lines of research.

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