Accomplishments & Reports: Core Biogeochemical Measurements and Experimental Studies of the Microbial Loop

D. Karl, D. Hebel, T. Houlihan, D. Pence, R. Scharek, C. Carrillo, L. Fujieki, J. Christian, K. Björkman, A. Colman, R. Letelier, D. Jones, G. Tien and C. Moyer
University of Hawaii, SOEST, Department of Oceanography
Honolulu, HI 96822
(dkarl@soest.hawaii.edu)

Our current models of the trophic organization of Antarctic marine ecosystems have evolved considerably during the past decade. Prior to 1980, energy flow in Southern Ocean habitats was thought to be dominated by relatively short and, therefore, efficient transfers from large (>20 µm) phytoplankton cells to krill and, subsequently, to apex predators. More recently, our concept of the marine food web has been expanded to reflect the potential roles of heterotrophic microorganisms including bacteria, protozoans and small (<150 µm) non-krill crustaceans.

Heterotrophic microorganism-based food webs, also referred to as microbial loops (Azam et al. 1983) are present in all aquatic environments including Antarctic habitats. These detritus driven systems are fueled by non-respiratory community carbon losses including dissolved and particulate organic matter release by excretion, predation and mortality. Because microbial loops require several trophic levels to transfer carbon and energy to apex predators, most detritus based food webs are inherently inefficient and sometimes constitute major energy sinks.

It is important to emphasize that comprehensive, quantitative ecosystem studies of energy and carbon flow through the Antarctic food web do not exist. At best, only order of magnitude estimates for a few selected regions are available. A major, unexpected result of the field studies conducted to date is the apparent uncoupling of algal and bacterial metabolic processes (Cota et al. 1990; Karl et al. 1991; Karl and Bird 1993). The reasons for this uncoupling are not well understood at present but the potential implications are profound. Consequently, we must view the microbial loop models as hypotheses that deserve a thorough, quantitative field evaluation.

One of the major obligations for Project S-046 personnel, in the overall context of the PALMER LTER program, is to make repeat measurements of a variety of "core" biogeochemical measurements, including: inorganic carbon system parameters (alkalinity, total carbon dioxide and derived estimates of partial pressure of CO₂), dissolved oxygen, inorganic and organic nutrients, hydrogen peroxide, dissolved organic carbon, particulate ATP, chl a, bacterial cell numbers, bacterial productivity, and total and dissolved lipopolysaccharide (LPS). Collectively, these measurements will help describe the magnitude and intensity of autotrophic and microheterotrophic processes within the LTER study region. All measurements are made using JGOFS program standardized protocols which will allow more meaningful comparisons to be made between antarctic habitats and other regions of the world ocean. During SANTA CLAµS, we collected profile samples at approximately 30 stations in the LTER grid, including 9 in Crystal Sound and one near Hovgaard Island, 19 in Paradise Harbor, 2 in Andvord Fjord, 2 in Dallman Bay, 2 in Gerlache Strait and 7 at Deception Island. Inorganic nutrients (phosphate, nitrate+nitrite, nitrite and silicate), dissolved oxygen, chl a and hydrogen peroxide concentrations were measured at sea. All other samples were retrograded to the University of Hawaii for subsequent processing.

In addition to these sample collections and measurements, S-046 personnel conducted numerous experiments to evaluate and elucidate the carbon and energy pathways among microorganisms including, but not limited to, measurements and controls on rates of photosynthesis, photorespiration and dark respiration, seawater cultures to assess coupling between algae and bacteria, stoichiometric coupling between dissolved nutrients and dissolved biogenic gases. Comprehensive experiments on controls of primary production and particle export conducted under bloom conditions (chl a >15 µg l^-1) in Paradise Harbor should allow us to close the carbon cycle in at least one region of the Peninsula.

References

Azam, F., T. Fenchel, J. G. Field, J. S. Gray, L. A. Meyer-Reil and F. Thingstad. 1983. The ecological role of water-column microbes in the sea. Marine Ecology Progress Series, 10: 257-263.

Cota, G. F., S. T. Kottmeier, D. H. Robinson, W. O. Smith, Jr. and C. W. Sullivan. 1990. Bacterioplankton in the marginal ice zone of the Weddell Sea: Biomass, production and metabolic activities during austral autumn. Deep-Sea Research, 37: 1145-1167.

Karl, D. M., O. Holm-Hansen, G. T. Taylor, G. Tien and D. F. Bird. 1991. Microbial biomass and productivity in the western Bransfield Strait, Antarctica during the 1986-87 austral summer. Deep-Sea Research, 38: 1029-1055.

Karl, D. M. and D. F. Bird. 1993. Bacterial-algal interactions in antarctic coastal ecosystems. In: R. Guerrero & C. Pedros-Alio (eds.), Trends in Microbial Ecology, Spanish Society for Microbiology, pp. 37-40.