Tracy A. Villareal

High rates of N2 fixation by unicellular diazotrophs in the oligotrophic Pacific ocean

The availability of nitrogen is important in regulating biological productivity in marine environments. Deepwater nitrate has long been considered the major source of new nitrogen supporting primary production in oligotrophic regions of the open ocean, but recent studies have showed that biological N2 fixation has a critical role in supporting oceanic new production. Large colonial cyanobacteria in the genus Trichodesmium and the heterocystous endosymbiont Richelia have traditionally been considered the dominant marine N2 fixers, but unicellular diazotrophic cyanobacteria and bacterioplankton have recently been found in the picoplankton and nanoplankton community of the North Pacific central gyre, and a variety of molecular and isotopic evidence suggests that these unicells could make a major contribution to the oceanic N budget. Here we report rates of N2 fixation by these small, previously overlooked diazotrophs that, although spatially variable, can equal or exceed the rate of N2 fixation reported for larger, more obvious organisms. Direct measurements of 15N2 fixation by small diazotrophs in various parts of the Pacific Ocean, including the waters off Hawaii where the unicellular diazotrophs were first characterized, show that N2 fixation by unicellular diazotrophs can support a significant fraction of total new production in oligotrophic waters.

Red tides in the Gulf of Mexico: Where, when, and why?

[1] Independent data from the Gulf of Mexico are used to develop and test the hypothesis that the same sequence of physical and ecological events each year allows the toxic dinoflagellate Karenia brevis to become dominant. A phosphorus-rich nutrient supply initiates phytoplankton succession, once deposition events of Saharan iron-rich dust allow Trichodesmium blooms to utilize ubiquitous dissolved nitrogen gas within otherwise nitrogen-poor sea water. They and the co-occurring K. brevis are positioned within the bottom Ekman layers, as a consequence of their similar diel vertical migration patterns on the middle shelf. Upon onshore upwelling of these near-bottom seed populations to CDOM-rich surface waters of coastal regions, light-inhibition of the small red tide of ~1 ug chl l(-1) of ichthytoxic K. brevis is alleviated. Thence, dead fish serve as a supplementary nutrient source, yielding large, self-shaded red tides of ~10 ug chl l(-1). The source of phosphorus is mainly of fossil origin off west Florida, where past nutrient additions from the eutrophied Lake Okeechobee had minimal impact. In contrast, the P-sources are of mainly anthropogenic origin off Texas, since both the nutrient loadings of Mississippi River and the spatial extent of the downstream red tides have increased over the last 100 years. During the past century and particularly within the last decade, previously cryptic Karenia spp. have caused toxic red tides in similar coastal habitats of other western boundary currents off Japan, China, New Zealand, Australia, and South Africa, downstream of the Gobi, Simpson, Great Western, and Kalahari Deserts, in a global response to both desertification and eutrophication.

Upward transport of oceanic nitrate by migrating diatom mats

The oligotrophic gyres of the open sea are home to a flora that includes the largest known phytoplankton. These rare species migrate as solitary cells or aggregations (mats) between deep nutrient pools (below 80–100 m) and the surface. This migration contributes to new production because of the concomitant upward transport of nitrate. But just how significant this contribution is remains uncertain because of the difficulty of making quantitative measurements of these rare cells. Here we report remote video observations of a previously undersampled class of diatom (Rhizosolenia) mats throughout the upper 150 m of the central North Pacific Ocean. These mats are virtually invisible to divers, and their presence increases the calculated phytoplankton-mediated nitrate transport into the surface ocean by up to a factor of eight. Cruise averages indicate that Rhizosolenia mats transport 18–97 µmol N m−2 d−1; however, this value reached 171 μmol N m−2 d−1 at individual stations, a value equivalent to 59% of the export production. Although considerable temporal and spatial variability occurs, this means of upward nutrient transport appears to be an important source of new nitrogen to the surface ocean, and may contribute to other regional elemental cycles as well.

New perspectives on nitrogen-fixing microorganisms in tropical and subtropical oceans

New molecular and microscopic evidence indicates that the open ocean harbors a diverse range of novel free-living and symbiotic nitrogen-fixing microorganisms. Although the extent to which these microorganisms fix nitrogen is currently unclear, ongoing research indicates that they might make a substantial contribution to the open ocean nitrogen budget.

Buoyancy regulation and the potential for vertical migration in the oceanic cyanobacterium trichodesmium.

Diel protein and carbohydrate content in Trichodesmium thiebautii was measured to evaluate the relationship to buoyancy status. Carbohydrate:protein ratio was the best predictor of buoyancy and fit a cosine curve with increasing values during the day and decreasing values at night in cycles that paralleled observed diel buoyancy patterns. This ratio also increased in short-term experiments as a function of light and increased in parallel with decreasing positive buoyancy. We used changes in this ratio to estimate the potential for vertical migration. Whereas limited vertical excursions in the upper 70 m are possible, deeper migrations appear unlikely unless respiration rates decrease significantly. N:P ratios in sinking and floating colonies were used to test for the P acquisition at depth (vertical migration). We noted that pooled N:P ratios were not significantly different between sinking and ascending colonies (N:P = 65.6 and 66.3, respectively) collected along the northern Australian coast, much like published results from north of Hawaii. Highly significant differences (p <0.0001) were observed in the western Gulf of Mexico between sinking and ascending colonies (N:P = 87.0 and 43.5, respectively) and provide the best direct evidence to date of vertical migration for P acquisition. Our physiological data on compositional changes during buoyancy reversals suggest a complex relationship between light and nutrients. It appears likely that light and P metabolism interact to regulate the vertical extent of migrations, with deep vertical migration regulated by P metabolism superimposed on a mixed-layer light-driven migration. The variability in N:P ratios suggests that care should be taken in assuming buoyancy reversals always result in P acquisition in this oceanic cyanobacterium.

A historical assessment of Karenia brevis in the western Gulf of Mexico

This work examines the historical records of red tides in the western Gulf of Mexico (GOM) as they pertain to the toxic dinoflagellate Karenia brevis(Davis) G. Hansen and Moestrup. K. brevis commonly causes major fish kills, human respiratory distress, and significant economic disruption in the Gulf of Mexico. It can also lead to illness by consumption of contaminated shellfish. The nearly annual blooms that have occurred in Florida in the past several decades have focused most attention on the eastern Gulf of Mexico. There are few published chronological accounts of red tides in the western Gulf of Mexico despite a wealth of information on probable red tide blooms in Mexico during the 17th–19th century. Using these data and more modern records, we present a chronology of K. brevis in the western Gulf of Mexico. A 1879 report of red tide blooms in Veracruz, Mexico in the period 1853–1871 provides a clear description of concurrent fish kills and respiratory irritation, and provides the earliest verifiable account of K. brevis in the Gulf of Mexico. An analysis of the records suggests that Texas has experienced a notable increase in red tide frequency in the years 1996–2000. However, the record is too limited to assign any causes.

The Relative Food Value of Diatoms, Dinoflagellates, Flagellates, and Cyanobacteria for Tintinnid Ciliates

Summary Growth rates of two coastal tintinnid ciliates, Tintinnopsis acuminata Daday and Tintinnopsis vasculum Meunier , were measured in batch cultures of 10 diatom clones, 10 clones of dinoflagellates and flagellates, and 2 clones of chroococcoid cyanobacteria. Diatom clones represented species lacking significant external processes ( Minutocellus ), species possessing siliceous setae ( Chaetoceros ), and species possessing β-chitin threads ( Thalassiosira, Cyclotella ). Both tintinnids grew rapidly when fed diatoms lacking threads or setae; neither species grew when fed setae-bearing diatoms. The smaller tintinnid, Tintinnopsis acuminata , exhibited extensive mortality when fed thread-bearing diatoms; the larger tintinnid, Tintinnopsis vasculum , grew only when fed the two smallest thread-bearing diatoms. Both tintinnids grew rapidly on diatoms with threads reduced by culture on a shaker table: growth rates were inversely related to diatom cross-sectional diameter (= cell + threads). Tintinnids feeding on the prymnesiophytes, Dicrateria, Isochrysis , and Pavlova , grew at rates similar to those feeding on diatoms lacking significant external processes. T. vasculum showed moderate growth when fed dinoflagellates, while T. acuminata grew well when fed chlorophytes and prasinophytes. Both tintinnids exhibited extensive mortality when fed the chroococcoid eyanobaeteria, Synechococcus .

INTERNAL NITRATE CONCENTRATIONS IN SINGLE CELLS OF LARGE PHYTOPLANKTON FROM THE SARGASSO SEA1

Nitrate concentrations within individual cells of Ethmodiscus, Pyrocystis, and Halosphaera and chains of Rhizosolenia were determined from samples collected in the Sargasso Sea. In all cases, field populations exhibited a wide range of internal nitrate concentrations (INCs) within a single sampling date. Halosphaera INCs reached 100 mM, in contrast to diatom and dinoflagellate INCs, which did not exceed 22 mM. Sinking Rhizosolenia, Ethmodiscus and Pyrocystis had significantly lower internal NO3‐ pools than did floating cells (P< 0.05). Ethmodiscus incubations in surface seawater resulted in a dramatic reduction in the proportion of high INC cells concurrent with decreases in average INCs and an increased proportion of sinking cells. Population buoyancy was inversely related to INC, and negatively buoyant cells rarely exceeded I mM INC, suggesting that a critical INC threshold may exist. The photosynthetic parameters Pmax and α decreased with time as internal NO3‐‐ Pools were depleted. Internal nitrate depletion rates were consistent with oxygen production rates during this time. Based on the known characteristics of Pyrocystis and Ethmodiscus, we conclude that virtually all of the > 100 μm‐sized phytoplankton present in the Sargasso Sea can vertically migrate. However, the appropriate time scale for migrators such as Halosphaera that reproduce by swarmer formation is unclear and may be significantly different than the other taxa studied. Changes in the frequency distributions, buoyancy‐internal pool relationships, and general P‐1 photosynthesis‐irradiance time series data in Ethmodiscus suggest that nutrient limitation is related to these migrations. High INC appears to be a fundamental property of the largest microalgal cells present in oligotrophic seas and suggests that nitrate transport by these nonmotile cells is widespread.

Marine Nitrogen-Fixing Diatom-Cyanobacteria Symbioses

This review summarizes growth and nitrogen-fixation data on diatom-cyanobacteria symbioses. Symbiotic associations between diatoms and filamentous cyanobacteria are frequently noted in tropical and subtropical waters, and direct evidence indicates that at least two associations are diazotrophic. Abundance data is limited, but Rhizosolenia-Richelia blooms are recorded from the central Pacific gyre at up to 104 cells L−1. In-situ growth rates of these associations are not known; however, laboratory data on the Rhizosolenia-Richelia symbiosis suggests the host-symbiont association can reproduce at approximately 0.8–1.0 div day−1. Growth kinetics in nitrogen-poor medium suggest that transfer of fixed nitrogen from host to symbiont occurs and that this process can support the host’s growth under N-limiting conditions. Additional symbioses, particularly in Hemiaulus, may be more widespread than previously realized due to the difficulty in identifying the symbiont in standard light microscopy. Few field measurements of N2-fixation are available for these associations. Rhizosolenia-Richelia appears to be locally important in the central Pacific gyre. Hemiaulus spp. are a common and abundant diatom in oligotrophic seas, and if 80–100% of the cells are symbiotic as reported, then they represent an unquantified, and potentially substantial, source of nitrogen-fixation.

BIOLOGICAL AND CHEMICAL CHARACTERISTICS OF THE GIANT DIATOM ETHMODISCUS (BACILLARIOPHYCEAE) IN THE CENTRAL NORTH PACIFIC GYRE

Cells of the giant diatom Ethmodiscus Castr. gathered from the upper 15 m were examined for O2 evolution, nitrate reductase activity (NRA), C and N composition, internal NO concentrations, , and 15NO, 15NH , and 32Si uptake in a series of cruises in the central N. Pacific gyre. The δ15N (2.56–5.09 ‰), internal NO concentrations (0.0– 11.5 mM NO−), and NRA (6.7 ± 4.7 × 10−4μM NO cell −1·h−1) were consistent with recent exposure to elevated nitrate concentrations and utilization of deep NO as a primary N source. These results are similar to other diatoms that migrate vertically to the nutricline as part of their life cycle. Rate measures (Si[OH]4 uptake, NRA, and O2 evolution) indicated surface doubling times from 45 h to 75 h. Both NO and NH uptake in surface waters were low and inadequate to supply N needs at surface NO and NH concentrations. Our results suggest a partitioning in nutrient acquisition, with N acquired at depth and C and Si acquired at the surface. Doubling rates were two to three times higher than predicted from cell volume and C content models. These data are consistent with the observed elemental content being lower than expected because of the dominance of cell volume by the vacuole. Our calculations suggest that Ethmodiscus contributes little to the biogeochemistry of the upper water column via upward nutrient transport. Although reported as a paleo‐upwelling indicator, thisevidence suggests that Ethmodiscus has adapted to the nutrient‐poor open ocean by a vertical migration strategy and has biological characteristics inconsistent with a upwelling indicator.