Fungal feeding preferences and molecular gut content analysis of two abundant oribatid mite species (Acari: Oribatida) under the canopy of Prosopis laevigata (Fabaceae) in a semi-arid land.

Prosopis laevigata (mesquite; Fabaceae) forms fertility islands in soils of semi-arid lands where microbial diversity concentrates in response to the accumulation of resources in the soil beneath individual plants, promoting organic matter decomposition and nutrient cycling. This phenomenon provides suitable conditions for the proliferation of key edaphic elements such as fungi and mites. Mite-fungal interactions are central for our understanding of nutrient cycling processes in resource-limited arid food webs; yet, no information is available about fertility islands in semi-arid lands. Thus, we aimed to determine in vitro fungal-based feeding preferences and molecular gut content of the oribatid mite species Zygoribatula cf. floridana and Scheloribates cf. laevigatus, which are abundant under the canopy of P. laevigata in an intertropical semi-arid zone in Central Mexico. Our results on the gut content analysis of these oribatid species resulted in the ITS-based identification of the following fungi: Aspergillus homomorphus, Beauveria bassiana, Filobasidium sp., Mortierella sp., Roussoella sp., Saccharomyces cerevisiae, Sclerotiniaceae sp. and Triparticalcar sp. Furthermore, under laboratory conditions both oribatid mite species exhibited feeding preferences on melanized fungi, such as Cladosporium spp., whereas A. homomorphus and Fusarium penzigi were avoided. Our findings indicated that the analyzed oribatid mite species have similar feeding preferences for melanized fungi, which might suggest resource partitioning and a degree of preference, explaining the coexistence of both oribatid species.

Short-Term Evaluation of the Spatial Distribution of Trophic Groups of Amoebae in the Rhizosphere of Zea mays Inoculated with Rhizophagus intraradices.

Primary production in terrestrial ecosystems is sustained by plants, microbiota, and fungi, which are the major organic matter providers in the root zone, setting in motion the soil food webs. Predators like soil amoebae voraciously feed on bacteria, fungi, and microbial eukaryotes releasing the nutrients sequestered in their biomass. Early food web setting up is crucial for seedling nutrition and its further development after establishment. Mycorrhizal fungi are more than phosphorus providers, and we wonder what their role is in structuring the predators' trophic groups in the root zone. We evaluated the effect of Rhizophagus intraradices inoculated in Zea mays (mycorrhizosphere), on the structuration of amoebae trophic groups along vertical and horizontal (3, 6, and 9 cm) soil distribution when compared to un-inoculated plants, after 20 days in microcosms. Amoebae species richness was highest in non-mycorrhizal seedlings in the root zone at 6- to 9-cm depth, and 3 cm away from plants. More bacterial species are needed when plants are devoid of mycorrhiza, and their influence is constrained 3 cm away from roots. Higher diversity of trophic groups was recorded at mycorrhizal seedlings and at the compartment influenced by the mycelium at 6- to 9-cm depth. The highest bacterivorous diversity, higher number of rare species and protozoa-eating amoebae, and the absence of fungivorous group recorded at the mycorrhizosphere of Z. mays, indicate that the community was very different from the non-mycorrhizal plants. We conclude that the arbuscular mycorrhizal fungus exerts significant changes on the community of trophic groups of amoebae.

Effect of Predation by Colpoda sp. in Nitrogen Fixation Rate of Two Free-Living Bacteria.

Biological nitrogen fixation is limited to several groups of prokaryotes, some of them reduce nitrogen as free-living nitrogen-fixing bacteria. Protozoa predation on these latter releases sequestered nitrogen that may enhance the formation of new bacterial biomass and possibly increase nitrogen fixation within soil microbial communities. We aim to evaluate the predation effect of Colpoda sp. on two nitrogen fixers: Azospirillum lipoferum and Stenotrophomonas sp. during their lag, early exponential, and exponential phases. The kinetics of bacterial population growth was determined in the predators' presence or absence and the effect of predation on the rate of N fixation was evaluated through the reduction of acetylene to ethylene technique. Colpoda sp. showed a non-significant difference in preferences between the two species offered as prey. Consequently, the abundance of A. lipoferum and Stenotrophomonas sp. decreased significantly due to predator's pressure and both species responded by increasing their specific growth rate. Likewise, predation promoted greater nitrogen fixation rate by CFU during the lag phase in A. lipoferum (0.20 nM/CFU with predation vs 0.09 nM/CFU without predation) and Stenotrophomonas sp. (0.22 nM/CFU vs 0.09 nM/CFU respectively). During early exponential phase (29 h), the rate diminished to 0.13 and 0.05 nM/CFU in A. lipoferum and to 0.09 nM/CFU and 0.05 nM/CFU in Stenotrophomonas sp. Finally, during the exponential phase (52 h), only A. lipoferum without predation produced 0.003 nM/CFU of ethylene. Thus, the nitrogenase activity was higher in the lag and the early exponential phases when predator activity was involved.

Balamuthia mandrillaris in South America: an emerging potential hidden pathogen in Perú.

Balamuthia mandrillaris is a free living amoeba that can be isolated from soil. It is an emerging pathogen causing skin lesions as well as CNS involvement with a fatal outcome if untreated. Further, infections can sometimes can also appear in peripheral areas such as extremities (usually knee), or trunk. Moreover, it often progresses to an infiltrative lesion that occasionally becomes ulcerated. In countries like Peru, a skin lesion will precede other symptoms. This primary cutaneous lesion can be present for weeks or even months. However, the appearance of neurological disease predicts a poor prognosis. Diagnosis requires a high level of suspicion.

Trophic structure of amoeba communities near roots of Medicago sativa after contamination with fuel oil no. 6.

Root exudation increases microbial activity, selecting bacterial and fungal communities that metabolize organic matter such as hydrocarbons. However, a strong contamination pulse of hydrocarbons around plant roots may reorganize the soil's microbial trophic structure toward amoebae feeding on bacteria. We conducted a microcosm experiment to elucidate the effect of Medicago sativa on the trophic structure of naked amoebae after a strong pulse of pollution (50,000 ppm of fuel oil no. 6, which is a mixture of long chains ranging from C10 to C28). Plants were seeded 24 h after contamination and species of amoebae in the microcosms were identified at 1, 30, and 60 days after pollution. Several species from three trophic groups of naked amoeba were still alive 24 h after the hydrocarbon pulse. Non-planted microcosms harbored three trophic groups after 60 days, while planted ones nourished four groups. The bacterivore group was the most diverse in all microcosms, followed by protist-eaters and omnivores. The quantity of amoebae was significantly higher (3.4×10(3) organisms/g soil) in the planted pots than in the non-planted ones (1.3×10(3) organisms/g soil after 30 days of pollution (P ≤ 0.01). The shortest hydrocarbon chains (C10-C14) disappeared or diminished in all microcosms, and the longest ones increased in the planted ones. M. sativa thus exerted a positive effect on species richness, quantity, and the composition of amoebae trophic groups in contaminated soil. This indirect effect on bacterial predators is another key factor underlying hydrocarbon assimilation by living organisms during phytoremediation.

Vertical distribution of the free-living amoeba population in soil under desert shrubs in the Negev desert, Israel.

A field study was designed to examine the effect of desert shrubs on the dynamics of free-living amoebae in arid soil. Soil samples from 0- to 50-cm depths were collected at 10-cm intervals in each of the four seasons. The vertical distributions of the four main morphological types of amoebae, grouped according to their mobility, and of small flagellate populations were measured under the canopies of Hammada scoparia and Atriplex halimus, shrubs belonging to the chloride-absorbing xerohalophytes. The result obtained from the field study demonstrated that the total number of protozoa was significantly higher during the wet seasons (winter and spring) than during the dry seasons. The protozoan population was more diverse under the canopy of H. scoparia during the wet seasons, reaching 8,000 individuals per 1 g of dry soil, whereas during the dry seasons, the populations were higher under the canopy of A. halimus, with a mean of 250 individuals. The protozoan population in the deeper layers (40 to 50 cm) was found to be as active as that in the upper layers, demonstrating that, in the desert, soil columns below 20 cm are fertile and worth studying. The type 1 amoebae (e.g., Acanthamoeba and Filamoeba spp.) were the most abundant throughout the study period, and their numbers were significantly higher than those of the other amoeba types.