Ralph E. H. Smith

INTRACELLULAR PHOTOSYNTHATE ALLOCATION AND THE CONTROL OF ARCTIC MARINE ICE ALGAL PRODUCTION1

Ice algae are a case study in photo‐autotrophic growth and metabolism under chronically low temperature and irradiance. We measured the allocation of 14C‐labelled photosynthate among major classes of intracellular carbon (low molecular weight compounds, or LMW; lipid; protein; and polysaccharide) and found light‐dependent changes in allocation broadly similar to photo‐adaptive responses known in phytoplankton at higher temperatures; average relative allocation to protein varied inversely (10–37%) and allocation to lipids and polysaccharides directly (10–23%, and 16–21%, respectively) with the sub‐inhibiting irradiance levels we employed (3.5–33.0 μE·M−2·s−1). Unlike many observations at higher temperatures, ours indicated (on average) a large and light‐insensitive allocation to LMW (ca. 40%) and a greater light‐sensitivity in lipid than in polysaccharide allocation. At the higher incubation irradiances, resembling in situ levels typical of areas with little (0–5 cm) snow cover, allocation to protein was often low (10–13%) compared to many observations of nutrient‐sufficient or light‐limited phytoplankton. Allocation to protein increased substantially (to ca. 40%) during a period of intensified tidal mixing, and assimilation numbers also attained a maximum at about the same time. Suck dynamics show that the ice algae are not constrained to their often protein‐poor allocation by the constantly low ambient temperature. Rather, they display marked shifts in metabolism consistent with major changes in light and inorganic nutrient supply, driven in part by the physical process of tidal mixing.

Population dynamics of bacteria in Arctic sea ice

The dynamics of bacterial populations in annual sea ice were measured throughout the vernal bloom of ice algae near Resolute in the Canadian Arctic. The maximum concentration of bacteria was 6.0·1011 cells·m−2 (about 2.0·1010 cells·l−1) and average cell volume was 0.473 μm3 in the lower 4 cm of the ice sheet. On average, 37% of the bacteria were epiphytic and were most commonly attached (70%) to the dominant alga,Nitzschia frigida (58% of total algal numbers). Bacterial population dynamics appeared exponential, and specific growth rates were higher in the early season (0.058 day−1), when algal biomass was increasing, than in the later season (0.0247 day−1), when algal biomass was declining. The proportion of epiphytes and the average number of epiphytes per alga increased significantly (P<0.05) through the course of the algal bloom. The net production of bacteria was 67.1 mgC·m−2 throughout the algal bloom period, of which 45.5 mgC·m−2 occurred during the phase of declining algal biomass. Net algal production was 1942 mgC·m−2. Sea ice bacteria (both arctic and antarctic) are more abundant than expected on the basis of relationships between bacterioplankton and chlorophyll concentrations in temperate waters, but ice bacteria biomass and net production are nonetheless small compared with the ice algal blooms that presumably support them.

Heterotrophic activity and bacterial productivity in assemblages of microbes from sea ice in the high Arctic

SummaryHeterotrophic activity in the bottom few cm of annual sea ice in the Canadian Arctic was measured throughout the spring bloom of ice algae, using tritium-labelled thymidine and glucose. Experiments with chloramphenicol and cyclohexamide indicated that thymidine assimilation was due to procaryotic microbes but that about half of the glucose assimilation was due to eucaryotic organisms. Glucose and thymidine assimilation rates increased with salinity, from 10 ppt to 30 ppt. Thymidine assimilation rates increased from 1.16 to 4.94·10−21mol·cell−1·h−1 during the latter half of the algal bloom, while the exponential growth rate of the in situ populations decreased from 0.058 to 0.025 d−1. Bacterial production and specific growth rates calculated from thymidine assimilation were 149mgC·m−2 and 0.25 d−1 or less respectively over the 50 day observation period, compared with net primary production of 5,500 mgC·m−2. Thymidine assimilation rates suggested that about half of the bacterial production may be consumed or lost from the ice during the bloom.

Kinetics of intracellular carbon allocation in a marine diatom

Abstract To test models of intracellular carbon flow we measured the labelling kinetics (from 14 CO 2 ) of major classes of cell polymers (carbohydrate, protein, lipid) and of dissolved organic carbon produced by the marine diatom Thalassiosira pseudonana Hustedt, grown at rates of 0.2 to 2.0·day −1 under nitrogen or light limitation. Compartmental analysis indicated that tracer carbon quickly entered respiratory and excretory streams, accumulating in the cells at the rate of net production after only 25–50% of cell generation (doubling) time. Respiration rates were low (≤ 0.1 · day −1 ) and suggested that illuminated cells in steady-state growth made only minor use of oxidative respiration to support cell synthesis. The tracer was quick to enter all polymers; compartmental analysis indicated that polymer labelling rates were close to the rates of mass synthesis after several hours of incubation with 14 C. Polymer labelling also showed a reallocation of photosynthate from protein to carbohydrate within a few hours of perturbation (shift-down) of nutrient supply in a N-limited chemostat. In steady-state growth, the protein: carbohydrate ratio increased directly with N-limited growth rate but attained its maximum under extreme light-limitation. Carbon flow into the metabolic processes of respiration, excretion and polymer synthesis appeared to be mediated by a small and rapidly cycled pool of substrates under all steady-state growth conditions.

Irradiance and Lipid Production in Natural Algal Populations

Lipids are important to aquatic ecosystems, as essential dietary components for animals (including some of economic importance in both wild and cultured food production) (Olsen, this volume), as vectors for movement of hydrophobic materials (including many important contaminants), and as the proximate agents of toxicity in a variety of organisms (Landrum and Fisher, this volume). Microalgae, including phytoplankton and attached forms such as ice algae and periphyton, are major producers of aquatic lipids. A substantial body of measurements of lipid synthesis by natural populations of microalgae has developed, thanks largely to the relative ease with which 14C and simple chemical extraction protocols can be applied to measure the intracellular allocation of recent photosynthate (Morris et al., 1981,Morris et al., 1974; ). In practice, the term photosynthate here refers to carbon incorporated (and therefore labeled) within the span of typical primary production experiments (usually 4-24 hours). Such measurements have revealed substantial variation in the synthesis and relative allocation of photosynthate to lipids, which may be related to environmental and taxonomic factors (Madariaga, 1992; Wainman and Lean, 1992).

Implications of irradiance exposure and non-photochemical quenching for multi-wavelength (bbe FluoroProbe) fluorometry

Multi-wavelength fluorometers, such as the bbe FluoroProbe (FP), measure excitation spectra of chlorophyll a (Chl-a) fluorescence to infer the abundance and composition of phytoplankton communities as well as the concentration of chromophoric dissolved organic matter (CDOM). Experiments were conducted on laboratory cultures and on natural communities of freshwater phytoplankton to determine how the response of phytoplankton to high irradiance might affect fluorometric estimates of community composition and concentrations of Chl-a and CDOM. Cultures of a representative cyanobacterium, bacillariophyte, synurophyte, cryptophyte, and chlorophyte revealed changes in Chl-a excitation spectra as irradiance was increased to saturating levels and non-photochemical quenching (NPQ) increased. The degree of change and resulting classification error varied among taxa, being strong for the synurophyte and cryptophyte but minimal for the cyanobacterium. Acute-exposure experiments on phytoplankton communities of varying taxonomic composition from five lakes yielded variable results on apparent community composition. There was a consistent decrease in CDOM estimates, whereas Chl-a estimates were generally increased. Subsequent exposure to low PAR relaxed NPQ and tended to reverse the effects of high irradiance on composition, total Chl-a, and CDOM estimates. Relaxation experiments on near-surface communities in a sixth, large lake, Georgian Bay, showed that total Chl-a estimates increased by 44% on average when dark treatments were used to relax NPQ, though, in contrast to the findings from the small lakes, there was little effect on CDOM estimates. We observed a statistically-significant, negative linear relationship between the photon flux density of in situ irradiance and the accuracy of taxonomic assignment by FP in Georgian Bay. Not discounting the correlations between light intensity and the accuracy of the FP that were observed in this study, we conclude that the applicability of the reference spectra to the system under investigation is a more important consideration than variability in natural irradiance conditions.