Rodrigo Andrés Martínez

Ring1A is a transcriptional repressor that interacts with the Polycomb-M33 protein and is expressed at rhombomere boundaries in the mouse hindbrain

In Drosophila, the products of the Polycomb group (Pc‐G) of genes act as chromatin‐associated multimeric protein complexes that repress expression of homeotic genes. Vertebrate Pc‐G homologues have been identified, but the nature of the complexes they form and the mechanisms of their action are largely unknown. The Polycomb homologue M33 is implicated in mesoderm patterning in the mouse and here we show that it acts as a transcriptional repressor in transiently transfected cells. Furthermore, we have identified two murine proteins, Ring1A and Ring1B, that interact directly with the repressor domain of M33. Ring1A and Ring1B display blocks of similarity throughout their sequences, including an N‐terminal RING finger domain. However, the interaction with M33 occurs through a region at the C‐terminus. Ring1A represses transcription through sequences not involved in M33 binding. Ring1A protein co‐localizes in nuclear domains with M33 and other Pc‐G homologues, such as Bmi1. The expression of Ring1A at early stages of development is restricted to the neural tube, whereas M33 is expressed ubiquitously. Within the neural tube, Ring1A RNA is located at the rhombomere boundaries of the hindbrain. Taken together, these data suggest that Ring1A may contribute to a tissue‐specific function of Pc‐G–protein complexes during mammalian development.

Future Climate Scenarios for a Coastal Productive Planktonic Food Web Resulting in Microplankton Phenology Changes and Decreased Trophic Transfer Efficiency

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3°C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13–16 μg chlorophyll (chl) a l−1. In the control mesocosms, chl a remained below 1 μg l−1. Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures.

A light-induced shortcut in the planktonic microbial loop.

Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.

Distribution of microplankton and fish larvae related to sharp clines in a Patagonian fjord

Vertical distribution of microzooplankton and fish larvae was studied in a Patagonian fjord from Chile. Zooplankton collection, including larvae of the sprat...