Susan L Brown

Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawai`i. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.

Phenotypic and genotypic characterization of multiple strains of the diazotrophic cyanobacterium, Crocosphaera watsonii, isolated from the open ocean

Diazotrophic cyanobacteria have long been recognized as important sources of reduced nitrogen (N) and therefore are important ecosystem components. Until recently, species of the filamentous cyanobacterium Trichodesmium were thought to be the primary sources of fixed N to the open ocean euphotic zone. It is now recognized that unicellular cyanobacteria are also important contributors, with members of the oligotrophic genus Crocosphaera being the only cultured examples. Herein we genetically and phenotypically characterize 10 strains isolated from the tropical Atlantic and North Pacific Oceans, and show that although all of the strains are highly similar at the genetic level, with the internal transcribed sequence (ITS) region sequence varying by approximately 2 bp on average, there are many unexpected phenotypic differences between the isolates (e.g. cell size, temperature optima and range, extracellular material excretion and variability in rates of nitrogen fixation). However based on the observed sequence similarity, we propose that all of these isolates are members of the genus Crocosphaera (type strain Crocosphaera watsonii WH8501), and that the phenotypic diversity we see may reflect ecologically important variation relevant for modelling N(2) fixation in the oligotrophic ocean.

Seasonal dynamics of phytoplankton in the Antarctic Polar Front region at 170°W

Phytoplankton dynamics in the region of 55-70degreesS, 170degreesW were investigated using Sea-viewing Wide Field-of-View Sensor satellite imagery, shipboard sampling and experimental rate assessments during austral spring and summer, 1997-1998. We used image-analysis microscopy to characterize community biomass and composition, and dilution experiments to estimate growth and microzooplankton grazing rates. Iron concentrations were determined by flow-injection analysis. The phytoplankton increase began slowly with the onset of stratification at the Polar Front (PF) (60-61degreesS) in early November. Seasonally enhanced levels of chlorophyll were found as far north as 58degreesS, but mixed-layer phytoplankton standing stock was highest, approaching 200 mg C m(-3), in the region between the receding ice edge and a strong silicate gradient, which migrated from similar to62degreesS to 65degreesS during the study period. The most southern stations sampled on four cruises were characterized by small pennate diatoms and Phaeocystis. From the PF to the Southern Antarctic circumpolar current front (similar to65degreesS), this ice margin assemblage was seasonally replaced by a community dominated by large diatoms. The large diatom community developed only in waters where measured iron concentrations were initially high (greater than or equal to0.2 nM), and crashed when dissolved silicate was depleted to low levels. Phytoplankton growth rates were highest (0.5-0.6 d(-1)) between the PF and silicate front (60degreesS and 63degreesS) in December. In January, growth rates were lowest (0.1 d(-1)) near the PF, and the highest rates (0.34.4 d(-1)) were found in experiments between 64.8degreesS and 67.8degreesS. Phytoplankton production estimates were highest south of the PF through December and January, averaging 2.2-2.4 mmol C m(-3) d(-1) and reaching levels of 5 mmol cm(-3) d(-1) (64.8degreesS and 67.8degreesS in January). Microzooplankton grazers consumed 54-95% of production for experiments conducted on four AESOPS cruises. They were less efficient in balancing growth rates during the time of highest phytoplankton growth and increase in December, and most efficient in February-March, after the large diatom bloom had collapsed. The diatom bloom region in the present study is in an upwelling zone for Antarctic circumpolar deep water with high iron content. This may explain why this marginal ice zone differs from others where blooms have not been observed

Primary production, new production, and growth rate in the equatorial Pacific: Changes from mesotrophic to oligotrophic regime

[1] Under an apparent monotony characterized by low phytoplankton biomass and production, the Pacific equatorial system may hide great latitudinal differences in plankton dynamics. On the basis of 13 experiments conducted along the 180° meridian (8°S-8°N) from upwelled to oligotrophic waters, primary production was strongly correlated to chlorophyll a (chl a), and the productivity index PI (chl a-normalized production rate) varied independently of macronutrient concentrations. Rates of total ( 14 C uptake) and new ( 15 N-NO 3 uptake) primary production were measured in situ at 3°S in nutrient-rich advected waters and at 0° where the upwelling velocity was expected to be maximal. Primary production was slightly higher at the equator, but productivity index profiles were identical. Despite similar NO 3 concentrations, new production rates were 2.6 times higher at 0° than at 3°S, in agreement with much higher concentrations of biogenic particulate silica and silicic acid uptake rates ( 32 Si method) at the equator. Furthermore, phytoplankton carbon concentrations from flow cytometric and microscopical analyses were used with pigment and production values to assess C:chl a ratios and instantaneous growth rates (μ). Growth rates in the water column were significantly higher, and C:chl a ratios lower at 0° than at 3°S, which is consistent with the more proximate position ofthe equatorial station to the source of new iron upwelling into the euphotic zone. For the transect as a whole, compensatory (inverse) changes of C:chl a and μ in response to varying growth conditions appear to maintain a high and relatively invariant PI throughout the equatorial region, from high-nutrient to oligotrophic waters.

Mesoscale variability in biological community structure and biomass in the Antarctic Polar Front region at 170°W during austral spring 1997

The influence of the Antarctic Polar Front (PF) on microbial biomass and community structure was investigated during late spring, October-November 1997, as part of the U.S. Joint Global Ocean Flux Study Antarctic Environment and Southern Ocean Process Study. In conjunction with SeaSoar sampling, samples for flow cytometry and epifluorescence image analysis were collected across the PF region along a 170°W transect and in two maps involving repeated crossings of the front. Phytoplankton abundance and size estimates clearly showed the influence of the front, with smaller, more numerous cells to the north and larger, less abundant cells to the south. Autotrophic biomass varied substantially across the region, ranging from 8 to 102 μg C L−1. Biomass accumulation, dominated by Phaeocystis spp. and Chaetoceros spp., was particularly apparent in discrete areas downstream of a frontal meander feature. Grazer biomass, ranging from 1 to 31 μg C L−1, was usually much less than 50% of phytoplankton biomass and did not show any spatial trends with regard to the PF. The distribution of heterotrophic bacteria was clearly influenced by the PF, with larger, less abundant cells south of the frontal zone. The developing assemblage of phytoplankton in the frontal meander was biologically distinct and spatially separated from the community sampled at the marginal ice zone. Analysis of phytoplankton biomass increases along PF current streamlines yielded net growth rates of ∼0.08 d−1, pointing to in situ growth, rather than transport, as the primary mechanism for chlorophyll accumulation. The significance of the front on the development of the seasonal phytoplankton increase is evident, yet the spatial heterogeneity of the microbial assemblage indicates a complex physical environment with multiple mesoscale influences.

Carbon allocation under light and nitrogen resource gradients in two model marine phytoplankton1

Marine phytoplankton have conserved elemental stoichiometry, but there can be significant deviations from this Redfield ratio. Moreover, phytoplankton allocate reduced carbon (C) to different biochemical pools based on nutritional status and light availability, adding complexity to this relationship. This allocation influences physiology, ecology, and biogeochemistry. Here, we present results on the physiological and biochemical properties of two evolutionarily distinct model marine phytoplankton, a diatom (cf. Staurosira sp. Ehrenberg) and a chlorophyte (Chlorella sp. M. Beijerinck) grown under light and nitrogen resource gradients to characterize how carbon is allocated under different energy and substrate conditions. We found that nitrogen (N)‐replete growth rate increased monotonically with light until it reached a threshold intensity (~200 μmol photons · m−2 · s−1). For Chlorella sp., the nitrogen quota (pg · μm−3) was greatest below this threshold, beyond which it was reduced by the effect of N‐stress, while for Staurosira sp. there was no trend. Both species maintained constant maximum quantum yield of photosynthesis (mol C · mol photons−1) over the range of light and N‐gradients studied (although each species used different photophysiological strategies). In both species, C:chl a (g · g−1) increased as a function of light and N‐stress, while C:N (mol · mol−1) and relative neutral lipid:C (rel. lipid · g−1) were most strongly influenced by N‐stress above the threshold light intensity. These results demonstrated that the interaction of substrate (N‐availability) and energy gradients influenced C‐allocation, and that general patterns of biochemical responses may be conserved among phytoplankton; they provided a framework for predicting phytoplankton biochemical composition in ecological, biogeochemical, or biotechnological applications.

Evolutionary and Biotechnological Implications of Robust Hydrogenase Activity in Halophilic Strains of Tetraselmis

Although significant advances in H2 photoproduction have recently been realized in fresh water algae (e.g. Chlamydomonas reinhardtii), relatively few studies have focused on H2 production and hydrogenase adaptations in marine or halophilic algae. Salt water organisms likely offer several advantages for biotechnological H2 production due to the global abundance of salt water, decreased H2 and O2 solubility in saline and hypersaline systems, and the ability of extracellular NaCl levels to influence metabolism. We screened unialgal isolates obtained from hypersaline ecosystems in the southwest United States and identified two distinct halophilic strains of the genus Tetraselmis (GSL1 and QNM1) that exhibit both robust fermentative and photo H2-production activities. The influence of salinity (3.5%, 5.5% and 7.0% w/v NaCl) on H2 production was examined during anoxic acclimation, with the greatest in vivo H2-production rates observed at 7.0% NaCl. These Tetraselmis strains maintain robust hydrogenase activity even after 24 h of anoxic acclimation and show increased hydrogenase activity relative to C. reinhardtii after extended anoxia. Transcriptional analysis of Tetraselmis GSL1 enabled sequencing of the cDNA encoding the FeFe-hydrogenase structural enzyme (HYDA) and its maturation proteins (HYDE, HYDEF and HYDG). In contrast to freshwater Chlorophyceae, the halophilic Tetraselmis GSL1 strain likely encodes a single HYDA and two copies of HYDE, one of which is fused to HYDF. Phylogenetic analyses of HYDA and concatenated HYDA, HYDE, HYDF and HYDG in Tetraselmis GSL1 fill existing knowledge gaps in the evolution of algal hydrogenases and indicate that the algal hydrogenases sequenced to date are derived from a common ancestor. This is consistent with recent hypotheses that suggest fermentative metabolism in the majority of eukaryotes is derived from a common base set of enzymes that emerged early in eukaryotic evolution with subsequent losses in some organisms.