STUART P. DONACHIE, LUIS M. TUPAS, CHRISTOPHER J. CARRILLO, and DAVID M. KARL, Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822
JAMES R. CHRISTIAN, Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada
The microbial loop is ubiquitous in marine and freshwater ecosystems (Hobbie 1994), but dissimilar biotic and abiotic factors regulate its components' activities in different habitats. We have commenced a cross-site project to investigate regulation of microbial ectoenzyme expression and bacterial processes in polar and subtropical marine habitats, sites represented, respectively, by the Palmer Long-Term Ecological Research (LTER) grid (Waters and Smith 1992), and the Hawaii Ocean Time-series (HOT) station ALOHA (A Long-Term Oligotrophic Habitat Assessment) (Karl and Lukas 1996).
Working from R/V Polar Duke in the Palmer LTER region, Marguerite Bay, and Tickle Channel from 11 January to 7 February 1997, we first described potential activities of the ectoenzymes a-glucosidase (AGase), ß-glucosidase (BGase), and leucine aminopeptidase (LAPase) in seawater from various depths at in situ temperatures. Fluorogenic substrate analogs were applied after Hoppe (1983), Somville and Billen (1983), and Christian and Karl (1995a), and fluorescence was determined in a Perkin-Elmer LS-5B spectrofluorometer. Activities are potential because substrate analogs were applied at saturating rather than trace concentrations.
Within a region of the LTER grid bounded by stations 200.000 to 600.200, surface AGase activities (figure 1 A ) were lowest at the center and seaward of a line approximately 75 kilometers (km) off the peninsula (mean 0.190 nanomoles per liter per day, SD=0.189, n=46). Higher activities at each end of the grid may reflect topographically steered upwelling. Enzyme activity peaked in Marguerite Bay where a phytoplankton bloom (diatoms and Phaeocystis ) and highest oxygen (O2) and lowest carbon dioxide (CO2) levels were encountered (Carrillo and Karl, Antarctic Journal , in this issue). The pattern of BGase activities (figure 1 B ) across the grid was similar to that of BGase, except BGase activities were undetectable (<0.1 nanomoles per liter per day) in 29 of 53 surface samples. Across the grid, surface water BGase activities averaged 0.097 nanomoles per liter per day (SD=0.181, n=46). These data support the view that activities in the Antarctic Peninsula coastal zone may represent global minima (Christian and Karl 1995b). Christian and Karl (1994) also noted high BGase activities near sea ice in Marguerite Bay in 1991-1992. During the LTER PD 97-01 cruise, activities were highest in Marguerite Bay and Tickle Channel; the latter was blocked by sea-ice.
Proteolytic activity is common in polar marine bacteria (Kriss 1963), and Christian and Karl (1995b) described high LAPase activities in the LTER grid. In the southern oceans, this may reflect a bacterial requirement for more dissolved organic matter (DOM) for growth at low temperatures ( sensu Wiebe, Sheldon, and Pomeroy 1993). Activities peaked (>2,000 nanomoles per liter per day) along the 600 line (figure 2) and decreased with increasing latitude; elevated levels accompanied the bloom in Marguerite Bay and Tickle Passage (approximately 600 to approximately 1,800 nanomoles per liter per day, respectively). LAPase activity generally peaked at the surface and decreased rapidly below the 13 percent light level (approximately 50 meters) (figure 3).
LAPase activities in 0.8-micrometer Nuclepore-filtered seawater with organic or inorganic nitrogen (N) [histidine, leucine, proline, tryptophan, phenylalanine, tyrosine, glycine, imidazole, ammonium (NH4), nitrate (NO3)] nutrient additions applied separately at 1 micromolar (µ M )—N were generally repressed only by phenylalanine and tyrosine through 48-hour incubations. That no other N-source consistently affected the activity of any of these enzymes or increased bacterial numbers, the latter determined through flow cytometry (Monger and Landry 1993) may in part result from low organic nutrient diversity (Griffiths, Caldwell, and Morita 1984); during RACER II, a mixture of 18 amino acids (Sigma AA-S-18) enhanced bacterial numbers by 8.5-fold over a 72-hour incubation (Bird personal communication). Inorganic N availability rarely limits bacterial production in the southern oceans, and concentrations applied here were below those generally found in this area.
Our cross-site study has so far shown that microbial populations at these sites respond differently to tyrosine. At station ALOHA, AGase and LAPase activities are strongly enhanced. Furthermore, although LAPase activities at ALOHA are considerably lower, greatest reductions in activity with depth at that site occur below approximately 125 meters.
We gratefully acknowledge Capt. Karl Sanden, the crew of R/V Polar Duke , the Antarctic Support Associates staff, and our LTER program colleagues for assistance during PD 97-01. This work was supported by National Science Foundation grant DEB 95-26986 and OPP 96-32763 awarded to D.M. Karl. (SOEST contribution number 4566).
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