Robert R. Bidigare

Effect of Phytoplankton Cell Geometry on Carbon Isotopic Fractionation

The carbon isotopic compositions of the marine diatom Porosira glacialis and the marine cyanobacterium Synechococcus sp. were measured over a series of growth rates (μ) in a continuous culture system in which the concentration and carbon isotopic composition of CO2(aq) were determined. These data were compared with previously published isotopic results of growth rate experiments using the marine diatom Phaeodactylum tricornutum and the marine haptophyte Emiliania huxleyi. Systematic relationships were found to exist between μ/[CO2(aq)] and carbon isotopic fractionation (ϵP) for each species. Maximum isotopic fractionation (ϵf) for P. glacialis, E. huxleyi, and P. tricornutum was ∼25‰, suggesting that this value may be typical for maximum fractionation associated with Rubisco and β-carboxylases for marine eukaryotic algae. By contrast, ϵf determined for Synechococcus clone CCMP838 was ∼7‰ lower. The slopes of the lines describing the relationship between ϵP and μ/[CO2(aq)] for eukaryotic algal species were different by a factor of more than 20. This result can be accounted for by differences in the surface area and cellular carbon content of the cells. Comparison of chemostat experimental results with calculated results using a diffusion based model imply that the algae in the experiments were actively transporting inorganic carbon across the cell membrane. Our results suggest that accurate estimates of paleo-[CO2(aq)] from ϵP measured in sediments will require knowledge of growth rate as well as cell surface area and either cell carbon quota or cell volume. Given growth rate estimates, our empirical relationship permits reliable calculations of paleo-[CO2(aq)] using compound-specific isotopic analyses of C37 alkadienones (select haptophytes) or fossilized frustules (diatoms).

Consistent fractionation of 13C in nature and in the laboratory: Growth‐rate effects in some haptophyte algae

The carbon isotopic fractionation accompanying formation of biomass by alkenone-producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by epsilon p approximately delta e - delta p, where delta e and delta p are the delta 13C values for dissolved CO2 and for algal biomass (determined by isotopic analysis of C37 alkadienones), respectively, and if Ce is the concentration of dissolved CO2, micromole kg-1, then b = 38 + 160*[PO4], where [PO4] is the concentration of dissolved phosphate, microM, and b = (25 - epsilon p)Ce. The correlation found between b and [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone-containing algae, most likely by trace-metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r2 = 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fall cruises as well as during an iron-enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO4] < or = 0.1 microM) fall above the b = f[PO4] line. Analysis of correlations expected between mu (growth rate), epsilon p, and Ce shows that, for our entire data set, most variations in epsilon p result from variations in mu rather than Ce. Accordingly, before concentrations of dissolved CO2 can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO4] and Cd/Ca ratios in shells of planktonic foraminifera.

Long-term changes in plankton community structure and productivity in the North Pacific Subtropical Gyre : The domain shift hypothesis

Oceanic productivity, fishery yields and the net marine sequestration of atmospheric greenhouse gases are all controlled by the structure and function of planktonic communities. Detailed paleoceanographic studies have documented abrupt changes in these processes over timescales ranging from centuries to millennia. Most of these major shifts in oceanic productivity and biodiversity are attributable to changes in Earths climate, manifested through large-scale ocean–atmosphere interactions. By comparison, contemporary biodiversity and plankton community dynamics are generally considered to be “static”, in part due to the lack of a suitable time frame of reference, and the absence of oceanic data to document ecosystem change over relatively short timescales (decades to centuries). Here we show that the average concentrations of chlorophyll a (chl a) and the estimated rates of primary production in the surface waters of the North Pacific Subtropical Gyre (NPSG) off Hawaii have more than doubled while the concentrations of dissolved silicate and phosphate have decreased during the past three decades. These changes are accompanied by an increase in the concentration of chl b, suggesting a shift in phytoplankton community structure. We hypothesize that these observed ecosystem trends and other related biogeochemical processes in the upper portion of the NPSG are manifestations of plankton community succession in response to climate variations. The hypothesized photosynthetic population “domain shift” toward an ecosystem dominated by prokaryotes has altered nutrient flux pathways and affected food web structure, new and export production processes, and fishery yields. Further stratification of the surface ocean resulting from global warming could lead to even more enhanced selection pressures and additional changes in biogeochemical dynamics.

A Methanotrophic Marine Molluscan (Bivalvia, Mytilidae) Symbiosis: Mussels Fueled by Gas

An undescribed mussel (family Mytilidae), which lives in the vicinity of hydrocarbon seeps in the Gulf of Mexico, consumes methane (the principal component of natural gas) at a high rate. The methane consumption is limited to the gills of these animals and is apparently due to the abundant intracellular bacteria found there. This demonstrates a methane-based symbiosis between an animal and intracellular bacteria. Methane consumption is dependent on the availability of oxygen and is inhibited by acetylene. The consumption of methane by these mussels is associated with a dramatic increase in oxygen consumption and carbon dioxide production. As the methane consumption of the bivalve can exceed its carbon dioxide production, the symbiosis may be able to entirely satisfy its carbon needs from methane uptake. The very light (δ13C = -51 to -57 per mil) stable carbon isotope ratios found in this animal support methane (δ13C = -45 per mil at this site) as the primary carbon source for both the mussels and their symbionts.

Seasonal patterns of ocean biogeochemistry at the U.S. JGOFS Bermuda Atlantic time-series study site

Seasonal patterns in hydrography, oxygen, nutrients, particulate carbon and nitrogen and pigments were measured on monthly cruises at the Bermuda Atlantic Time-series Study site, 80 km southeast of Bermuda. Between October 1988 and September 1990, the annual cycle was defined by the creation of 160–230 m-deep mixed layers in February of each year and a transition to strong thermal stratification in summer and fall. The 230 m mixed layer in February 1989 resulted in mixed-layer nitrate concentrations of 0.5–1.0 μmole kg−1, carbon fixation rates over 800 mg C m−2 day−1, and a phytoplankton bloom with chlorophyll concentrations over 0.4 mg m−3. Chlorophyll a, particulate organic matter, inorganic nutrients and primary production had returned to prebloom levels the following month with the exception of a chlorophyll maximum layer at 100 m. Particle fluxes at 150 m in February 1989 reached 56 mg C m−2 day−1 and 11 mg N m−2 day−1 (0.77 mmole N m−2 day−1). Estimates of new production during the bloom period calculated from changes in oxygen and nitrate profiles ranged from 100 to 240 mmoles N m−2, significantly higher than the sediment trap fluxes and approaching the measured total production rates. In spring of 1990, mixed layer depths did not exceed 160 m, nitrate was rarely detectable in the upper euphotic zone, chlorophyll a concentrations were similar to 1989, and particulate organic matter concentrations were lower. The period of elevated biomass lasted for 3 months in 1990, and phytoplankton pigment composition varied between cruises. The average rates of primary production and particle flux were higher in 1990 than those measured in the spring of 1989, despite the differences in mixed layer depth. Throughout both years, NO3 : PO4 ratios in the upper thermocline exceeded Redfield ratios. The maintenance of this pattern requires a net uptake of PO4 between 150 and 250 m, a depth range usually associated with net remineralization. The exact mechanism that maintains elevated PO4 uptake and its implication for the nutrient supply to the euphotic zone remain unknown.

Spatial and temporal variability of phytoplankton pigment distributions in the central equatorial Pacific Ocean

Abstract During the 1992 U.S. JGOFS Equatorial Pacific (EqPac) study, we participated in survey (12°N-12°S, along 135°–140°W) and time-series (0°, 140°W) cruises to identify the factors that control spatial and temporal variations in phytoplankton pigment biomass, taxonomic composition and size structure. To achieve this goal, we determined pigment marker distributions in conjunction with collaborative measurements performed by EqPac investigators. Distributions of phytoplankton pigments measured during early 1992 (El Nino conditions) were different from those measured during late 1992 (“normal” conditions). Most notably, the accessory pigment distributions revealed that the 1991–1992 El Nino event produced a significant reduction in eukaryotic phytoplankton biomass, especially that contributed by prymnesiophytes, pelagophytes and diatoms. This variability was most likely caused by a combination of physical (Kelvin waves, tropical instability waves and advection), chemical (iron limitation) and biological (growth-grazing imbalances) processes. The results of this pigment study underscore the need for sampling biological properties on the appropriate time and space scales, and the necessity of physical measurements for interpreting their distributions.

Gulf of Mexico hydrocarbon seep communities

Sediment and water samples were collected by submersible in September 1986 at 16 locations on the carbonate cap overlying a conical diapir, which was formed by the upward migration of oil and gas through a subsurface fault on the continental slope off Louisiana, USA (27°47′N; 91°30.4′W). The biological community at the site was photographed quantitatively with still and video cameras. Rigorous spatial sampling indices were maintained so that variation in chemical parameters and in the abundance of photographed organisms could be estimated within the bounds of the study site. Concentrations of extractable organic material (EOM) ranged from 0.24 to 119.26‰ in the sediment samples, while methane concentrations in the water samples were from 0.037 to 66.474 μM. The visible biological community was predominantly composed of the chemosynthetic tube worms (Vestimentifera) Lamellibrachia sp. and Escarpia sp., and an undescribed, methane-oxidizing mussel (Mytilidae: Bathymodiolus-like), as well as diverse non-chemosynthetic organisms. The ranked abundance of tube worms was significantly correlated (p<0.05) with the concentration of EOM in the sediment samples, while the abundance of mussels was significantly correlated (p<0.05) with the concentration of methane in the water samples. Tube worms and mussels both occurred in dense clusters; however, the clusters of mussels had a more restricted distribution within the study site than did clusters of tube worms. Both organisms were most abundant in the vicinity of the subsurface fault.

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.

A comparison of HPLC pigment signatures and electron microscopic observations for oligotrophic waters of the North Atlantic and Pacific Oceans

Abstract The use of HPLC pigment analysis has become a primary tool for investigating the taxonomic composition of natural phytoplankton populations. In this study, we compare, for the first time, the taxonomic composition based upon HPLC pigment signatures with direct electron microscopic taxonomic identifications from two sets of open ocean oligotrophic field samples. Electron microscopic observations at sites in the Atlantic and Pacific Oceans (Hydrostation S and Station ALOHA, respectively) agree with taxonomic partitioning based upon HPLC algorithms in the, upper water-column samples, but there is increasing disagreement between the two methods in deeper water samples. This disparity probably results from depth-dependent changes in cellular pigment content and accessory pigment-to-chlorophyll ratios. At both locations, the eukaryotic ultraplankton was similar in taxonomic composition, at least at the class level, and the Prymnesiophyceae and the newly described Pelagophyceae were the two most abundant groups of eukaryotes.

EVIDENCE A PHOTOPROTECTIVE FOR SECONDARY CAROTENOIDS OF SNOW ALGAE1

Snow algae occupy a unique habitat in high altitude and polar environments. These algae are often subject to extremes in nutrient availability, acidity, solar irradiance, desiccation, and ambient temperature. This report documents the accumulation of secondary carotenoids by snow algae in response to the availability of nitrogenous nutrients. Unusually large accumulations of astaxanthin esters in extra‐chloroplastic lipid globules produce the characteristics red pigmentation typical of some snow algae (e.g. Chlamydomonas nivalis (Bauer) Wille). Consequently these compounds greatly reduce the amount of light available for absorption by the light‐harvesting pigment‐protein complexes, thus potentially limiting photoinhibition and photodamage caused by intense solar radiation. The esterification of astaxnthin with fatty acids represents a possible mechanism by which this chromophore can be concentrated within cytoplasmic globules to maximize its photoprotective efficiency.