Quarterdeck Volume 5, Number 2, Summer 1997

Beggiatoa and hydrocarbon seeps
Unique bacteria thriving in a unique environment
Roxanne L. Nikolaus

Sedimentation and resuspension across the central Louisiana inner shelf
Youcheng Zhang


Beggiatoa and hydrocarbon seeps
Unique bacteria thriving in a unique environment

Roxanne L. Nikolaus

The term "hydrocarbon seep" refers to a site on the seafloor where oil and gas escape, or seep, from the sediment. Hydrocarbon seeps exist at numerous locations along the continental slope of the northern Gulf of Mexico, supporting specialized communities that derive energy from inorganic chemical compounds rather than light-similar to communities found at hydrothermal vent sites. Mats of pigmented and non-pigmented Beggiatoa bacteria are a prevalent feature of hydrocarbon seep communities.

The bacterial genus Beggiatoa is characterized by colorless cells which connect to create filaments that move by gliding and can be over one centimeter long and 100 micrometers wide. When hydrogen sulfide is present the bacteria convert it to solid sulfur through an oxidizing chemical reaction and store the resulting sulfur granules internally. The distribution of this genus of sulfur oxidizing bacteria ranges from coastal brackish marshes and inland sulfur springs to the more recently discovered marine hydrothermal vents and hydrocarbon seeps.

Some of the initial questions raised about hydrocarbon seep Beggiatoa revolved around how they fulfill their carbon and energy requirements and whether pigmented and non-pigmented Beggiatoa filaments fulfill those requirements in the same way. The oxygen-deprived sediment at seep sites is not only charged with hydrocarbons, but it is rich in hydrogen sulfide and carbon dioxide. This makes it capable of supporting three types of Beggiatoa: 1) Those that use only carbon dioxide for carbon and hydrogen sulfide for energy (chemoautotropic), 2) Those that use organic compounds for both carbon and energy (heterotrophic), and 3) Those that use organic compounds for carbon but hydrogen sulfide for energy (mixotrophic). The main objective of my study was to identify the trophic modes, or modes of nutrition, of pigmented and non-pigmented Beggiatoa found at hydrocarbon seeps.

I found that non-pigmented seep Beggiatoa are chemoautotrophic. Laboratory experiments showed that extract derived from non-pigmented bacteria had substantial levels of activity of the key enzyme autotrophs need to produce glucose. The presence of this enzyme, Ribulose-1,5-bisphosphate carboxylase (RuBisCo), is a strong indicator of autotrophic ability. Also, live non-pigmented cells were able to incorporate appreciable levels of carbon dioxide and oxidize hydrogen sulfide.

I judged that pigmented Beggiatoa are either heterotrophic or mixotrophic. Extract derived from pigmented cells had low levels of RuBisCo activity, and live pigmented cells could not incorporate substantial amounts of carbon dioxide. The difference between heterotrophic and mixotrophic modes of nutrition in seep Beggiatoa is whether their energy comes from organic compounds or hydrogen sulfide. The presence of sulfur granules in pigmented filaments indicated the potential ability of pigmented Beggiatoa to use hydrogen sulfide for energy.

Because both pigmented and non-pigmented filaments have access to hydrogen sulfide, carbon dioxide, and organic compounds in the hydrocarbon seep environment, it is possible that seep Beggiatoa can change their trophic mode to take advantage of the most abundant compounds available at a specific location. This would give them a competitive advantage in an environment such as seep sites where the supply of essential compounds can be quite variable.

By carrying out distinct yet inter-related processes of incorporating carbon dioxide, biodegrading hydrocarbons, and converting hydrogen sulfide, Beggiatoa play a vital role in hydrocarbon-seep community processes. These bacteria link and greatly influence the way carbon and sulfur are cycled through the environment not only at seep sites but ultimately throughout the ocean.

Editor's note: Roxanne Nikolaus currently works as an intern at the Consortium for Oceanographic Research and Education in Washington, D.C., in partial fulfillment of the requirements for a Master of Marine Affaris and Policy degree from the University of Miami. She expects to graduate in December 1997.

A non-pigmented Beggiatoa filament, approximately 35 micrometers wide.
(Photo by Roxanne Nikolaus)

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Sedimentation and resuspension across the central Louisiana inner shelf

Youcheng Zhang

The settling and resuspension of sediments are two important processes in continental shelf environments. They are controlled by river discharge (fresh water, nutrients, and sediments), hydrodynamics (wind, waves, tides, currents, circulation pattern), and production of particles by marine organisms on the shelf. Knowledge of sedimentation and resuspension on the shelf is essential to understand the transport of pollutants in the shelf environments.

Short-term sedimentation and resuspension are evaluated by measuring the concentration of particulate matter in shelf waters, sediment fluxes (the product of particle mass and settling or resuspension velocity), and shear stress near the seabed caused by waves, tides, and currents. The concentration of particulate matter can be determined optically with a transmissometer, which measures the effects of particles in the water on a beam of light (See "Tools of the Trade" in Quarterdeck Vol. 5, No. 1). Settling and resuspension fluxes can be measured directly using sediment traps. It is important to remember that the traps primarily collect the large, rapidly settling particles such as fecal pellets of marine organisms and aggregates of particles rather than the total concentration of particles. The average profiles of sediment fluxes over time are likely to differ significantly from the instant-aneous profiles of particle concentrations determined optically or by water filtration.

For my dissertation I studied sediment flux, composition, and resuspension across the inner part of the central Louisiana continental shelf using particle settling traps, optical sensors, bottom current measurements, and geochemical analyses.

Transmissometer data were collected during 10 LATEX (Louisiana-Texas Shelf Physical Oceanography Program) hydrographic cruises during 1992-1994. I determined the cross-shelf distribution of particulate matter during each cruise. The optical sensing revealed that the particulate matter distribution was closely related to the shelf hydrography and that the bottom nepheloid layer (a cloudy layer formed by a high concentration of particulate matter) was ubiquitous on the central Louisiana shelf.

Sediment traps were attached to LATEX current meter moorings at six sites along 90.5°W and 92°W, west of the Mississippi Delta, to form two cross-shore sections from a depth of eight to fifty meters. Traps were generally deployed and recovered every six to ten weeks from April 1992 to July 1994. I determined the spatial and temporal variability in settling and resuspension sediment fluxes, and also in the percentages of organic carbon and carbonate and grain size distributions of trap collections.

The sediment trap data strongly suggested that resuspension was a common and often dramatic feature on the central Louisiana shelf during this study. Vertical profiles of flux and particle composition clearly indicate local resuspension in the region. This is further evidenced by organic analyses that show compositional similarity between the samples from near-bottom traps and those from the top of gravity cores.

I calculated the shear stress on the seabed based on bottom current meter data from these sites. Analysis of bed shear stress indicates that currents were sufficient on many occasions to resuspend bottom sediments. A quantitative relationship between the total flux near the seabed and bed shear stress was established using a statistical technique.

This study suggests that the total fluxes near the bottom at a given site can be estimated based on an analysis of the bed shear stress using the statistical model I established.

Editor's note: Youcheng Zhang is currently employed as a post-doctoral researcher by Dr. Wilford Gardner and Dr. Mary Jo Richardson at Texas A&M University.

Graph showing the relationship between observed flux measured by the sediment traps one meter above the seafloor and the total flux predicted by the statistical model I proposed.

 

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Last updated June 7, 1997