No difference found in ribosomal DNA sequences from physiologically diverse clones of Karenia brevis (Dinophyceae) from the Gulf of Mexico

Maximum growth rate and toxin content were significantly differentamong five strains of Karenia brevis isolated from Texas andFlorida when grown under identical culture conditions. Sequenceanalysis of the 18S rRNA gene and internal transcribed spacer(ITS) regions revealed, however, that all five strains wereidentical. Consequently, a clear genetic basis for physiologicalvariability among various geographical isolates of K. brevisfrom the Gulf of Mexico could not be assessed using these geneticmarkers. Both the ITS and 18S rRNA regions may be useful inspecies-specific probe selection. At the intra-specific level,however, an alternative marker will be needed to assess thediversity among K. brevis populations in the Gulf of Mexico.     

Nitrogen inputs into the euphotic zone by vertically migrating Rhizosolenia mats

Rhizosolenia mats conduct extensive vertical migrations in theoligotrophic central North Pacific (cNP) gyre that permit thesediatoms to acquire nitrate at depth and return to the surfacefor photosynthesis. The ultimate fate of this N within the ecosystemis unknown, but may include remineralization by grazing, lossto depth by sinking biomass, or N excretion by Rhizosoleniamats. Direct release of N by mats into the mixed layer wouldrepresent an upward biological pump that circumvents the diffusionbarriers and nutrient sinks at the base of the oceanic euphoticzone. We examined Rhizosolenia mat N release along a transect(28–31° N) in the summer of 2002 (Hawaii to California)and 2003 (Hawaii to west of Midway Island) using sensitive fluorometricand chemiluminescence methods. Nitrate, NO₂^– and NH₄⁺release was determined. Nitrate and NH₄⁺ release by the matsoccurred in both 2002 (22.84 ± 6.04 and 3.69 ±1.74 nmol N µg^–1 Chl a h^–1, respectively)and 2003 (23.74 ± 3.54 and 3.60 ± 0.74 nmol Nµg^–1 Chl a h^–1, respectively). Nitrite releaseonly occurred in the 2003 summer period but occurred in bothyears when Fe chelators were added. Fv/Fm values decreased westwardin 2003 suggesting a gradient of increasing physiological stresstowards the west. The various physiological measures are consistentwith concurrent Fe stress; however, other possibilities exist.Nitrate excretion was the dominant form of N release in bothyears and provided a substantial addition to the ambient nitratepool in the mixed layer. Rhizosolenia mat nitrate release suppliesat least 4–7% of the nitrate pool on daily basis, andpossibly as much as 27%. Rhizosolenia mats are part of a largephytoplankton community that appears to migrate, and rates couldbe significantly higher. Literature reports suggest little orno nitrification in the upper euphotic zone, and thus biologicaltransport and release of nitrate may be a major source to thisregion. This N release is uncoupled from upward CO₂ transportand, like N₂ fixation, provides a component of the N pool availablefor net carbon removal.     

Facies variability in mixed carbonate–siliciclastic platform slopes (Miocene)

Miocene tropical carbonate platform slopes in southern Spain contain classical reef-slope facies distribution but also an unexpected abundance of serpulid-rich facies, locally forming build-ups. Two sections from the Miocene Sorbas and Níjar Basins were mapped and analyzed petrographically in order to identify the factors determining this facies variability. Reef-slope facies is intercalated with serpulid-rich facies and siliciclastic bodies. Serpulids are the pioneers colonizing the substrate in zones of quiet hydrodynamic conditions after hydrographical changes such as eventual river discharge. The interplay of sea-level changes and hydrographical conditions, together with episodic terrestrial influx, control lateral and along-slope facies variability as well as the facies distribution across the carbonate platform. Neither a deterministic distribution of facies belts nor a stochastic partitioning of facies in mosaics can accurately explain the facies distribution. A new model is proposed to explain facies variability in the context of intrinsic and extrinsic factors.