High-latitude controls of thermocline nutrients and low latitude biological productivity.

The ocean's biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. If there were no return path of nutrients from deep waters, the biological pump would eventually deplete the surface waters and thermocline of nutrients; surface biological productivity would plummet. Here we make use of the combined distributions of silicic acid and nitrate to trace the main nutrient return path from deep waters by upwelling in the Southern Ocean and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere and North Atlantic Ocean, is the main source of nutrients for the thermocline. We also find that an additional return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical mixing, perhaps driven by tides, brings abyssal nutrients to the surface and supplies them to the thermocline of the North Pacific. Our analysis has important implications for our understanding of large-scale controls on the nature and magnitude of low-latitude biological productivity and its sensitivity to climate change.

Atomic force microscopy study of living diatoms in ambient conditions.

We present the first in vivo study of diatoms using atomic force microscopy (AFM). Three chain-forming, benthic freshwater species -Eunotia sudetica, Navicula seminulum and a yet unidentified species - are directly imaged while growing on glass slides. Using the AFM, we imaged the topography of the diatom frustules at the nanometre range scale and we determined the thickness of the organic case enveloping the siliceous skeleton of the cell (10 nm). Imaging proved to be stable for several hours, thereby offering the possibility to study long-term dynamic changes, such as biomineralization or cell movement, as they occur. We also focused on the natural adhesives produced by these unicellular organisms to adhere to other cells or the substratum. Most man-made adhesives fail in wet conditions, owing to chemical modification of the adhesive or its substrate. Diatoms produce adhesives that are extremely strong and robust both in fresh- and in seawater environments. Our phase-imaging and force-pulling experiments reveal the characteristics of these natural adhesives that might be of use in designing man-made analogues that function in wet environments. Engineering stable underwater adhesives currently poses a major technical challenge.

A novel fluorescent silica tracer for biological silicification studies.

BACKGROUND: Biological silica production has drawn intense attention and several molecules involved in biosilicification have been identified. Cellular mechanisms, however, remain unknown mainly due to the lack of probes required for obtaining information on live specimens. RESULTS: The fluorescence spectra of the compound 2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole (PDMPO) are affected by the presence of >3.2 mM silicic acid. Increase in intensity and shift in the fluorescence coincide with the polymerization of Si. The unique PDMPO-silica fluorescence is explored here to visualize Si deposition in living diatoms. The fluorophore is selectively incorporated and co-deposited with Si into the newly synthesized frustules (the outer silica shells) showing an intense green fluorescence. CONCLUSIONS: We suggest that a fluorescence shift is due to an interaction between PDMPO and polymeric silicic acid. PDMPO is an excellent probe for imaging newly deposited silica in living cells and has also a potential for a wide range of applications in various Si-related disciplines, including biology of living organisms as diatoms, sponges, and higher plants, clinical research (e.g. lung fibrosis and cancer, bone development, artificial bone implantation), and chemistry and physics of materials research.

Evaluation of sequence variation and selection in the bindin locus of the red sea urchin, Strongylocentrotus franciscanus.

Recent evidence suggests that gamete recognition proteins may be subjected to directed evolutionary pressure that enhances sequence variability. We evaluated whether diversity enhancing selection is operating on a marine invertebrate fertilization protein by examining the intraspecific DNA sequence variation of a 273-base pair region located at the 5' end of the sperm bindin locus in 134 adult red sea urchins (Strongylocentrotus franciscanus). Bindin is a sperm recognition protein that mediates species-specific gamete interactions in sea urchins. The region of the bindin locus examined was found to be polymorphic with 14 alleles. Mean pairwise comparison of the 14 alleles indicates moderate sequence diversity (p-distance = 1.06). No evidence of diversity enhancing selection was found. It was not possible to reject the null hypothesis that the sequence variation observed in S. franciscanus bindin is a result of neutral evolution. Statistical evaluation of expected proportions of replacement and silent nucleotide substitutions, observed versus expected proportions of radical replacement substitutions, and conformance to the McDonald and Kreitman test of neutral evolution all indicate that random mutation followed by genetic drift created the polymorphisms observed in bindin. Observed frequencies were also highly similar to results expected for a neutrally evolving locus, suggesting that the polymorphism observed in the 5' region of S. franciscanus bindin is a result of neutral evolution.

Purification, recovery, and laser-driven fluorination of silicon from dissolved and particulate silica for the measurement of natural stable isotope abundances.

A procedure for the purification, recovery, and determination of isotopic abundances of silicon from biogenic and lithogenic particulate matter and dissolved silicic acid is reported. Purification involves the reaction of acid molybdate with dissolved silicon in natural waters or that produced by the dissolution of particulate silica by hydrofluoric acid. The resulting silicomolybdic acid is then quantitatively precipitated by reaction with triethylamine hydrochloride. The silicon is recovered as silicon dioxide through stepwise combustion of the dried precipitate. Fluorination of the product for isotopic analysis is accomplished by laser heating under pure fluorine generated by the decomposition of a fluorine-based salt. The resulting silicon tetrafluoride is separated from hydrogen fluoride and other fluorination byproducts cryogenically using a variable-temperature cold trap. Yields for silicon recovery are 99.9% for precipitation and greater than 95% for the purification/fluorination procedure. Reproducibility of the isotopic composition for pure quartz granules processed through the procedure is ±0.1‰ for δ(30)Si.