Abrupt conclusion of the late Miocene-early Pliocene biogenic bloom at 4.6-4.4 Ma.

The late Miocene-early Pliocene biogenic bloom was an extended time interval characterised by elevated ocean export productivity at numerous locations. As primary productivity is nutrient-limited at low-to-mid latitudes, this bloom has been attributed to an increase or a redistribution of available nutrients, potentially involving ocean-gateway or monsoon-related mechanisms. While the exact causal feedbacks remain debated, there is even less consensus on what caused the end of the biogenic bloom. Here, we compile Mio-Pliocene paleoproductivity proxy data from all major ocean basins to evaluate the timing and pacing of this termination. This systematic analysis reveals an abrupt and sustained reduction in low-latitude ocean productivity at 4.6-4.4 Ma. The decline in productivity coincided with a prolonged period of low orbital eccentricity and a shift towards lower-amplitude obliquity, an astronomical configuration linked to reduced East Asian Monsoon intensity and decreased riverine nutrient supply.

Bacterial diversity across a highly stratified ecosystem: A salt-wedge Mediterranean estuary.

Highly stratified Mediterranean estuaries are unique environments where the tidal range is low and the tidal currents are almost negligible. The main characteristics of these environments are strong salinity gradients and other environmental parameters. In this study, 454 pyrosequencing of the 16S rRNA gene in combination with catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) was used to estimate the bacterial diversity across the Krka estuary in February and July 2013. The comparison of the data derived from these two techniques resulted in a significant but weak positive correlation (R=0.28) indicating a substantial difference in the bacterial community structure, depending on the applied method. The phytoplankton bloom observed in February was identified as one of the main factors shaping the bacterial community structure between the two environmentally contrasting sampling months. Roseobacter, Bacteroidetes and Gammaproteobacteria differed substantially between February and July. Typical freshwater bacterial classes (Actinobacteria and Betaproteobacteria) showed strong vertical distribution patterns depending on the salinity gradient. Cyanobacteria decreased in abundance in February due to competition with phytoplankton, while the SAR11 clade increased its abundance in July as a result of a better adaptation toward more oligotrophic conditions. The results provided the first detailed insight into the bacterial diversity in a highly stratified Mediterranean karstic estuary.