Revista Chilena de Historia Natural 79 (4): 505-516, 2006
RESEARCH ARTICLE
Enzyme activities of phytoplankton in the South Shetland Islands (Antarctica) in relation to
nutrients and primary production
Given the potential significance of enzyme activities as a link between internal metabolic
pathways and environmental nutrients, we investigated the relationships of nitrate reductase (NR) and alkaline phosphatase (AP) with
primary production and inorganic nutrients in South Shetland Islands, Antarctica. Enzymatic activities of the phytoplankton (0.7-210 ìm),
primary productivity, autotrophic biomass and inorganic nutrients were studied in the upper 100 m depth at nine stations during a
cruise in the northwestern area of South Shetland Islands (Antarctica), during late austral spring (December 2000). NR activities
fluctuated between 0 and 42.8 nmol L<sup>-1</sup> h<sup>-1</sup> (mean = 10.08 nmol L<sup>-
1</sup> h<sup>-1</sup>, SD = 10.42 nmol L<sup>-1</sup> h<sup>-1</sup>), AP activities between
0.81 and 5.67 nmol L<sup>-1</sup> h<sup>-1</sup> (mean = 2.68 nmol L<sup>-1</sup> h<sup>-
1</sup>, SD = 0.95 nmol L<sup>-1</sup> h<sup>-1</sup>). Stations with primary productivity (PP) and
chlorophyll a greater than 2 mg C m<sup>-3</sup> h<sup>-1</sup> and 0.75 µg chlorophyll a
L<sup>-1</sup>, respectively, presented higher enzymatic activities of nitrate reductase, alkaline phosphatase than
those stations characterized by primary productivity and chlorophyll a less than 2 mg C m<sup>-3</sup>
h<sup>-1</sup> and 0.17 µg chlorophyll a L<sup>-1</sup>, respectively. The AP specific activity was
negatively correlated with orthophosphate concentrations lower than 2.0 µM, which indicates that the microplankton were under
phosphate deficient environment condition. Our results indicated that NR specific activity was positively associated with autotrophic
biomass and primary productivity estimates, giving evidence of the use of nitrate by phytoplankton as external nitrogen source in
surface waters. In addition, high NR activities were positively correlated with NO<sub>3</sub><sup>-</sup>,
suggesting the occurrence of nitrate respiration in the well oxygenated surface waters of Antarctica.
Key words:
nitrate reductase,
alkaline phosphatase, primary productivity, chlorophyll, Antarctica
