Beschreibung: The facultative red algal epiphyte Acrochaetium sp. liberated spores preferentially and recruited more successfully in laboratory cultures when its host Gracilaria chilensis C. J. Bird, McLachlan et E. C. Oliveira was present. The same effect was also induced by cell-free medium from G. chilensis, suggesting it contained a molecular signal. Antibiotics prevented spore release in Acrochaetium sp., even when G. chilensis was present, suggesting a prokaryotic origin of the signal. Simultaneous application of N-butyl-homoserine-lactone (BHL) restored the spore-release capacity, which demonstrated that spore release was not directly inhibited by the antibiotics and indicated that bacterially generated N-acyl-homoserine-lactones (AHLs) regulate spore release. An involvement of AHL was further indicated by the fact that two different halofuranone inhibitors of AHL receptors also inhibited spore release when they were applied at relatively low concentrations. Of seven different AHLs tested, only BHL induced the effect. However, BHL was only active at relatively high concentrations (100 μM), and it was not detected in spore-release-inducing medium of G. chilensis. Another water-soluble AHL or an AHL structure analog is therefore probably the active compound in G. chilensis cultures. The data presented demonstrate that life cycle completion in Acrochaetium sp. strongly depends on bacteria, which are not always present in sufficient numbers on the alga itself. Exogenous bacteria that are associated with G. chilensis or with other potential substrates may therefore trigger timely spore liberation in Acrochaetium sp., provided that the necessary concentration of AHL is reached. This first finding of AHL perception in a red alga confirms that AHL signalling is more widespread among eukaryotes than was thought until recently. However, spore release of a second red alga, Sahlingia subintegra (Rosenv.) Kornmann, was unaffected by AHL, and the reaction observed is therefore not universal.

Beschreibung: The earthquake swarm that took place in 2012–2015 in the Upper Ubaye Valley (the most active seismic zone in the French Alps) is peculiar for two reasons: (1) it occurred a few kilometers from a previous swarm (active in 2003–2004); (2) it was initiated by an M L  4.3 shock and reactivated, more than two years later, by another M L  4.8 shock with an identical epicenter but a deeper focus. We present here the corresponding data set of ~13,000 events, of which ~3000 were relocated using a double-difference algorithm. The swarm extends north-northwest–south-southeast (N165°E) over a distance of 11 km, but daily snapshots along a 2.5 yr period allow us to identify transverse faults whose activity was often ephemeral. Focal mechanisms for 13 M L ≥3 events confirm the complexity of the swarm geometry, although the fault plane for the two "mainshocks" is very consistent (N156°E–N160°E strike, 52°–55°SW dip), with clear normal faulting and a slight dextral strike-slip component. Most foci were located in the 4–11-km depth range, within the crystalline basement. Taking into account the source sizes for the two "mainshocks", the hydraulic diffusivity of 0.05 m 2 s –1 found for the 2003–2004 swarm is shown to fit reasonably well the 2012 data, but not the 2014 reactivation sequence. Throughout the article, we discuss the difficult issue of the identification of "foreshocks" and "aftershocks" within such a sequence, even though a swarm, per se, makes this terminology inadequate. As previously suggested by other authors, between a foreshock–mainshock–aftershock sequence and an earthquake swarm also exists a whole gamut of seismic activity which makes this dichotomy much more complex than anticipated.

Beschreibung: Seismicity along subduction interfaces is usually dominated by large main-shock–aftershock sequences indicative of a continuum distribution of highly coupled large asperities. In the past decades, however, the increased resolution of seismic catalogues at some subduction zone seems to indicate instead a more complex rheological segmentation of the interface. Large and megathrust earthquake ruptures seem interspersed among regions of low seismic coupling and less stress buildup. In this weaker zone, the strain is primarily released via a combination of moderate-size swarm-like seismicity and aseismic slip. Along the Chilean subduction zone, the densification of the seismic network allowed for the identification of localized seismic clusters, some of them appearing in the form of swarms before megathrust earthquakes. The origin and driving processes of this seismic activity have not yet been identified. In this study, we follow a systematic approach to characterize the seismicity at two persistent clusters in Central Chile, one located offshore Navidad and one inland, at ∼40 km depth beneath Vichuquén, which occurred throughout ∼20 yr. We investigated these clusters, by deriving high-resolution hypocentral locations and moment tensors and performing a detailed analysis of spatio-temporal patterns, magnitude and interevent time distributions of the clustered earthquakes. Both clusters are characterized by weak to moderate seismicity (below Mw 6) and stand out as clear seismicity rate and Benioff strain anomalies. At the Navidad cluster, seismicity occurs in the form of swarms, with a characteristic duration of 2–7 d and location and thrust mechanisms compatible with activity on the slab interface. Conversely, we find at Vichuquén activity dominated by thrust earthquakes occurring as repeaters on the slab interface, with a slip rate of approximately ∼5.0 cm yr−1. We attribute these clusters to local features of the subducting plate: the Navidad swarms are likely driven by repeated high pore pressure transients along a pre-fractured patch of the slab, while the seismicity at the Vichuquén cluster is interpreted as the result of a subducting seamount. Both clusters have been active before and after the Mw 8.8 Maule earthquake and persisted afterwards with the seismicity decay following the Omori law. These interactions are especially evident for the Vichuquén cluster, where the seismicity rate increased considerably after the Maule earthquake and continues to be an area of clearly elevated seismicity rate compared to its surroundings.