Leho Ainsaar

Global assessment of arbuscular mycorrhizal fungus diversity reveals very low endemism

Cosmopolitan plant root symbionts The aboveground lives of plants are only sustainable because of the symbiotic soil fungi that encase their roots. These fungi swap nutrients with plants, defend them from attack, and help them withstand abrupt environmental changes. Out of necessity, fungal symbionts in the soil would appear to be restricted and local to certain plant species. Davison et al., however, discovered that some taxa are globally distributed. How these underground fungi have dispersed so widely remains a mystery; perhaps human farmers have had something to do with it. Science, this issue p. 970 The wide distribution of plant-root fungal symbionts seems to be driven by recent dispersal rather than ancient tectonics. The global biogeography of microorganisms remains largely unknown, in contrast to the well-studied diversity patterns of macroorganisms. We used arbuscular mycorrhizal (AM) fungus DNA from 1014 plant-root samples collected worldwide to determine the global distribution of these plant symbionts. We found that AM fungal communities reflected local environmental conditions and the spatial distance between sites. However, despite AM fungi apparently possessing limited dispersal ability, we found 93% of taxa on multiple continents and 34% on all six continents surveyed. This contrasts with the high spatial turnover of other fungal taxa and with the endemism displayed by plants at the global scale. We suggest that the biogeography of AM fungi is driven by unexpectedly efficient dispersal, probably via both abiotic and biotic vectors, including humans.

Evidence for a widespread carbon isotopic event associated with late Middle Ordovician sedimentological and faunal changes in Estonia

An episode of remarkable biotic, climatic, sea-level and facies changes took place during the late Viruan (Caradoc) epoch in the Baltoscandian area. We studied the carbon isotopic composition of carbonate sediments from this period. Data on the stable carbon isotopic composition of whole-rock carbonates from three south Estonian core sections, together with those on ostracode, distribution are presented. In two core sections, a positive δ 13 C shift of 2 ‰ was revealed in the upper part of the Keila Stage (mid-Caradoc). The comparison of isotope and ostracode data in the sections suggested the occurrence of a gap of late Keilan age in the marginal area of the basin equivalent to the North Estonian Confacies Belt. The beginning of the excursion approximately coincided with climatic and sea-level changes in the shallow shelf area. The peak of the late Keilan excursion preceded the biotic crises and maximum black shale accumulation in the Baltoscandian palaeobasin. An approximately synchronous carbon isotopic event has been reported from North America, referring to a possible global oceanographic event in the Caradoc epoch. Although the positive carbon isotopic excursion and related environmental events of the late Keilan age have some unique features, they show more similarities to the end-Ordovician and Silurian events, characterized by oceanic change from stratified state to thermohaline circulation state, than to warm anoxic events related to eustatic sea-level rise.

The Osmussaar Breccia in Northwestern Estonia — Evidence of a ~475 Ma Earthquake or an Impact?

The Osmussaar Breccia occurs in beds of the ~475 Ma basal Middle Ordovician (Arenig and Llanvirn series) siliclastic-carbonate rocks of northwestern Estonia. The Breccia consists of fragmented and slightly displaced (sandy) limestones, which are penetrated by veins and bodies of strongly cemented, breccia-like, lime-rich sandstone injections. The rocks above (horizontally-bedded, hard limestone) and below (weakly cemented silt and sandstone) are undisturbed and do not contain the sediment intrusions. Osmussaar Breccia is found over an area of more than 5000 km2 and is distributed in a west- east oriented elliptical half-circle centred approximately at Osmussaar Island (59°18′ N; 23°28′ E). The thickness of the brecciated unit ranges from 1-1.5 m on Osmussaar to a few (tens of) cm at ~70 km east of the island. Arenitic sandstone of the sediment injections contains quartz grains with planar deformation features (PDF). Several hypotheses concerning the origin of the Osmussaar event have been proposed: catastrophic earthquake, regional tectonic movements, tectonic movements occurring simultaneously with coastal processes, and an impact event. The latter hypothesis was suggested in connection with the discovery of the nearby-situated Neugrund impact structure. However, the sediment intrusions are stratigraphically ~60 Ma younger than the impact structure. Osmussaar Breccia does not correspond to any known impact structure of this age in Baltoscandia. Also, results of a seabed geophysical survey in the Baltic Sea for the search of a possible undiscovered feature did not identify any large structure in the area of the Osmussaar Breccia. Consequently, we suggest that a devastating ~475 Ma earthquake with an epicentre close to Osmussaar split the sea floor. It initiated underwater mud-flows eroding the primary Neugrund crater ejecta and/or crater rim walls, thus reworking the impact materials into the sedimentary injections, which is suggested by rounded morphology of the shocked quartz grains found in breccia matrix.

Rich and heterogeneous fossil ostracod fauna in the Ordovician sediment intrusions at Osmussaar Island, Estonia, reveals an ancient impact event

Carbonate quartz-sandstone intrusions, penetrating the brecciated limestone of the Volkhov and Kunda Stages (Dapingian–early Darriwilian) in the northwestern part of Estonia have been posing questions about their genesis for more than a century. The intrusions contain a rich, diverse and well preserved ostracod fauna, which is extremely heterogeneous and shows associations of species from different habitats or facies zones. The ostracod fauna in the intrusions refers to rapid horizontal mixing and displacement of a thick layer of bottom sediments, possibly resulting from a yet unknown impact event.

Chapter 15 – The Position of the Ordovician–Silurian Boundary in Estonia Tested by High-Resolution δ13C Chemostratigraphic Correlation

Abstract Correlation of the base of the Silurian System, which is defined by graptolite biostratigraphy, with the non-graptolitiferous carbonate shelf successions has been complicated due to rapid environmental changes and widespread unconformities of glacioeustatic origin in the end-Ordovician interval. The Hirnantian carbon isotope excursion (HICE) occurring in the Hirnantian Global Stage at a short distance below the Ordovician/Silurian boundary has been widely used in global and regional chemostratigraphic correlation of this boundary interval. In the present study we analyzed the δ 13 C trends in carbonates of the Ordovician/Silurian boundary interval from the drillcore sections of the Pandivere area in central Estonia. Comparison of the high-resolution Hirnantian and Rhuddanian δ 13 C curves suggests some changes in the traditional stratigraphic correlation scheme of Estonia. The new data demonstrate that the HICE crosses the traditional Ordovician/Silurian boundary, which should therefore be placed higher up in the Baltoscandian succession, in the lower part of the Juuru Regional Stage. The base of the Juuru Stage, traditionally correlated with the system boundary, has been defined by shelly fossil data. The second phase of the global Hirnantian extinction was followed by a slow faunal recovery with the appearance of postextinction biota in late Hirnantian that obviously included some “Silurian-type” shelly fossil groups. For that reason, the traditional position of the Ordovician/Silurian boundary in many regions with non-graptolitiferous successions, established by shelly fauna should be critically reevaluated.

Stratigraphy of the Ordovician–Silurian boundary interval in Östergötland, Sweden, based on ostracod distribution and stable carbon isotopic data

The Hirnantian glaciation event at the end of the Ordovician brought along major biological and oceanographic changes. The Harpabollia harparum ostracod assemblage characterized by its distinct species composition has been described only from localities where the presence of the Hirnantian strata is proven. This paper focuses on the distribution of this association from the Baltic area to Scandinavia (Sweden) and compares the faunal composition between these two areas. Altogether 4733 ostracods of 69 species were collected from the Ordovician–Silurian boundary interval in the Råssnäsudden section in Östergötland, SE Sweden. Pre-Hirnantian and Llandoverian assemblages are present, but there is no evidence of any H. harparum association or any HICE (Hirnantian carbon isotope excursion) peak in δ13C values. This evidence suggests a gap in the Råssnäsudden outcrop that corresponds to most of the Hirnantian.

Microbial island biogeography: isolation shapes the life history characteristics but not diversity of root-symbiotic fungal communities

Island biogeography theory is one of the most influential paradigms in ecology. That island characteristics, including remoteness, can profoundly modulate biological diversity has been borne out by studies of animals and plants. By contrast, the processes influencing microbial diversity in island systems remain largely undetermined. We sequenced arbuscular mycorrhizal (AM) fungal DNA from plant roots collected on 13 islands worldwide and compared AM fungal diversity on islands with existing data from mainland sites. AM fungal communities on islands (even those >6000 km from the closest mainland) comprised few endemic taxa and were as diverse as mainland communities. Thus, in contrast to patterns recorded among macro-organisms, efficient dispersal appears to outweigh the effects of taxogenesis and extinction in regulating AM fungal diversity on islands. Nonetheless, AM fungal communities on more distant islands comprised a higher proportion of previously cultured and large-spored taxa, indicating that dispersal may be human-mediated or require tolerance of significant environmental stress, such as exposure to sunlight or high salinity. The processes driving large-scale patterns of microbial diversity are a key consideration for attempts to conserve and restore functioning ecosystems in this era of rapid global change.

Asterozoan Pedicellariae and Ossicles Revealed from the Middle Ordovician of Baltica

Isolated asterozoan ossicles and pedicellariae occur in micropalaeontological samples from the Darriwilian (Middle Ordovician) of the Mishina Gora section, north-western Russia. The well-preserved Bursulella unicornis type fossils represent the oldest hitherto record of asterozoan pedicellariae. The accompanying ossicles are not diagnostic but allow their tentative placement within the asteroid stem group. The abundance of disarticulated ossicles in the samples proves that asterozoans were not as rare as they have been considered and were important members of Ordovician ecosystems in the Baltic Palaeobasin.