The decline and fate of an iron-induced subarctic phytoplankton bloom

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.     

Mucosal mast cells are functionally active during spontaneous expulsion of intestinal nematode infections in rat

Infestation of the gastrointestinal tract by parasitic nematodes is invariably associated with mucosal mastocytosis, which is a thymus-dependent phenomenon in parasitized rats, and is adoptively transferable with a T cell-enriched population of thoracic duct lymphocytes. When derived by in vitro culture, mucosal mast cells (MMC) arise from a bone marrow precursor after stimulation by T cell-derived factors. In rats infected with the nematode Trichinella spiralis, mucosal mastocytosis is temporally associated with the immune expulsion of the adult worms whereas in the case of Nippostrongylus brasiliensis, mastocytosis is frequently observed to occur after worm expulsion has been completed. Consequently, there has been doubt as to whether MMC are active and serve a functional role in the expulsion of rat intestinal nematodes. MMC contain and secrete a neutral proteinase, rat mast cell protease II (RMCP II); detection and assay of secreted RMCP II therefore provides a direct measurement of MMC activity. Here we describe the release of this enzyme into the blood of rats infected with N. brasiliensis or T. spiralis. Our results show that the systemic secretion of RMCP II coincides with the immune expulsion of these nematodes, demonstrating clearly for the first time that rat MMC are functionally active during the immune elimination of primary nematode infections.     

Identification and comparison of two sequence elements that confer cell-type specific transcription in yeast

The MAT alpha 2 protein of budding yeast represses a set of genes; if the MATa1 protein is also present, a further set of genes is repressed. DNA sequence comparisons reveal a 20-base pair 'operator' sequence that is present in genes repressed by a1/alpha 2. A related, but distinct, sequence is found in genes repressed by alpha 2 alone.