1994 Arctic Ocean Section
Roles of Heterotrophic Bacteria and Protists
in the Arctic Ocean Carbon Cycle
Barry F. Sherr, Evelyn B. Sherr and David Kirchman
Our role in the AOS-94 expedition was to determine the distribution and
activity of heterotrophic microbes (bacteria and phagotrophic protists) in the
context of the overall food web of the upper column. Prevailing theory sug-
gests that bacteria and heterotrophic protists have a minor role in the food
webs of polar ecosystems compared to those of temperate and tropical oceanic
regions, because low water temperatures inhibit microbial growth. According
to this concept, the lack of heterotrophic microbial activity results in a greater
fraction of plant production being available for larger consumers in polar food
Biomass per unit volume and per square meter for bacteria and for three
size classes of protists;
Bacterial production and growth rate; and
Rates of bacterivory.
We also investigated the hypothesis that marine bacterioplankton are meta-
bolically suppressed by extremely low water temperature, by investigating stand-
ing stocks and turnover of labile sugars and amino acids and the response of
bacterioplankton to the addition of substrates and to temperature.
The preliminary results indicate an active microbial community. Bacterial
abundances were high (0.51.5 106 cells/mL) in the upper 50 m of the water
column, declining to < 0.2 106 cells/mL below 100 m. Both bacterial and
heterotrophic protist biomass was in the range of 0.41.0 g C/m3; the total
biomass of heterotrophic microbes often exceeded the phytoplankton biomass.
Measured phytoplankton production was insufficient to support the rates of
bacterial production; extracellular polysaccharides produced by bottom-ice dia-
toms are a probable additional source of organic substrate for pelagic bacteria.
Bacterial activity had Q10 values of 23 over a temperature range of 1C
to +15C. The maximum growth rate was observed at a temperature of about
Barry Sherr and Evelyn Sherr are with the College of Oceanic and Atmospheric Sciences at Oregon State Univer-
sity in Corvallis, Oregon, U.S.A. David Kirchman is with the Graduate College of Marine Studies, University of
Delaware, Lewes, Delaware, U.S.A.