We used the instantaneous growth rate method to determine the effects of food, temperature, krill length, sex, and maturity stage on in situ summer growth of krill across the southwest Atlantic sector of the Southern Ocean. The main aims were to examine the separate effects of each variable and to generate a predictive model of growth based on satellite-derivable environmental data. Both growth increments in length on moulting (GIs) and daily growth rates (DGRs, min d(-1)) ranged greatly among the 59 swarms, from 0.58-15% and 0.013-0.32 mm d(-1). However, all swarms maintained positive mean growth, even those in the low chlorophyll a (Chl a) zone of the central Scotia Sea. Among a suite of indices of food quantity and quality, large-scale monthly Chl a values from SeaWiFS predicted krill growth the best. Across Our Study area, the great contrast between bloom and nonbloom regions was a major factor driving variation in growth rates, obscuring more subtle effects of food quality. GIs and DGRs decreased with increasing krill length and decreased above a temperature optimum of 0.5 degrees C. This probably reflects the onset of thermal stress at the northern limit of krill's range. Thus, growth rates were fastest in the ice edge blooms of the southern Scotia Sea and not at South Georgia as previously suggested. This reflects both the smaller size of the krill and the colder water in the south being optimum for growth. Males tended to have higher GIs than females but longer intermoult periods, leading to similar DGRs between sexes. DGRs of equivalent-size krill tended to decrease with maturity stage, suggesting the progressive allocation of energy toward reproduction rather than somatic growth. Our maximum DGRs are higher than most literature values, equating to a 5.7% increase in mass per day. This value fits within a realistic energy budget, Suggesting a maximum carbon ration of similar to 20% d(-1). Over the whole Scotia Sea/South Georgia area, the gross turnover of krill biomass was similar to 1% d(-1).

Proteinaceous cytoplasmic inclusions with characteristic anatomical localization patterns are common in marine algae, but the function of these cell structures has not been demonstrated. Thalli of the endemic Arctic kelp Laminaria solidungula J. Agardh, cultured under N-replete conditions and examined by light and electron microscopy, were found to have a variety of cellular inclusions. One type of inclusion was shown by cytochemical methods to be proteinaceous. The protein bodies were 1 to 10 mum in diameter, had a single bounding membrane, and occurred singly or in large clusters in cells of the cortex, Intact proteinaceous inclusions were absent from N-starved thalli, although structures presumed to be depleted protein bodies had an anatomical distribution comparable to that of the protein bodies in the N-replete algae. These findings on Laminaria solidungula provide the first experimental evidence in support of the hypothesis that proteinaceous cellular inclusions in marine macroalgae constitute a nitrogen store that, like the stores of nitrate and amino acids accumulated by many algae, could be utilized during seasons when nitrate concentrations in the water column are low but when photosynthetic carbon fixation must be maximized.

Antarctic and arctic marine waters have similar near-freezing temperatures, but differ greatly in dissolved inorganic nitrogen (DIN) availability. Antarctic algae have high DIN supply year-round; arctic algae are N-limited during the summer. Temperate algae experience low temperatures and low DIN supply on a seasonal basis, but never concurrently. Nitrogen supply influences the ability of algae to achieve the high enzyme activities necessary for cold acclimation. The present study compared N-allocation strategies of antarctic, arctic, and temperate seaweeds grown under N-replete and N-limited conditions at near-freezing temperature. Sporophytes of the antarctic endemic, Niman-tothallus grandifolius, did not store NO3-, had small pools of nitrogenous compounds, and were unable to sustain growth for longer than 1 mo under N-limitation. However, N-starved plants with negative growth rates exhibited chlorophyll fluorescence ratios (F-v/F-m) similar to those of N-replete plants, and photosynthetic rates remained positive, suggesting that PSII reaction centres (RCII) were functioning efficiently. In contrast, the arctic endemic, Laminaria solidungula, maintained relatively high growth rates during 9 mo of N-starvation. The arctic kelp utilised both internal NO3- pools and organic nitrogenous components, such as protein and chlorophyll, to support growth. Despite declines in the density of RCII and photosynthetic capacity, N-limited L. solidungula continued to accumulate carbon reserves. Like the arctic plants, temperate L. saccharina from the Atlantic coast of Maine had internal reserves of NO3- and organic compounds that provided the initial N-source for growth under low external N-supply. The internal N-reserves were depleted fairly rapidly, however, and the temperate kelp showed simultaneous reductions in growth rate, photosynthetic capacity, and F-v/F-m after only 3 mo under low N-supply. Overall, the arctic species alone has an N-allocation strategy for surviving long periods of concurrent low temperature and low N-supply, The antarctic species appears to be primarily adapted to maintaining photosynthesis and growth under low light and low temperature, rather than low DIN supply. The temperate species is poorly adapted to survive prolonged periods of both low N and low temperature, even though ecotypes of this species extend into the Arctic.

Nitrogen uptake and assimilation strategies were compared in polar macroalgae from differing dissolved inorganic nitrogen (DIN) regimes. The antarctic endemic, Himantothallus grandifolius, experiences high nitrate concentrations year-round and occasionally high, but variable, ammonium levels. The arctic endemic, Laminaria solidungula, is exposed to seasonal fluctuations in DIN, with N-limitation occurring during the summer. Both species demonstrated saturation kinetics for nitrate and ammonium uptake. L. solidungula showed 'storate-speccalist' characteristics of nitrate uptake, with high V-max allowing this species to take advantage of seasonally elevated nitrate concentrations. H. grandifolius had a high V-max for ammonium, allowing the alga to utilise pulses of this nutrient. In the presence of both DIN forms, nitrate uptake was significantly reduced in both species. Furthermore, H. grandifolius and L. solidungula demonstrated significantly reduced uptake and assimilation of nitrate during short-term and prolonged periods of darkness, while ammonium uptake and assimilation were relatively unaffected by light. Although preferential uptake of ammonium, particularly in the dark, allows both species to conserve energy in their cold, low-light environments, the antarctic species, which does not have the additional problem of N-limitation, showed stronger energy-conserving traits. Nitrogen assimilation characteristics of the arctic species appeared to balance energy conservation with the need to minimise N-limitation in an environment that alternates between low light and low N-availability.

Recent work has focused on a possible relationship between marine organic delta(13)C and the concentration of carbon dioxide dissolved in seawater [CO2(aq)]. This relationship is based on the general assumption that diffusive CO2 uptake is the main pathway of photosynthetic carbon acquisition by phytoplankton. This study found an inverse linear relationship between [CO2(aq)] and organic delta(13)C in the marine diatoms Chaetoceros calcitrans and Ditylum bright-wellii. However, the relationship was not a function of diffusive CO2 use by these diatoms, which have previously been shown to be HCO3- users. Our findings underline the importance of understanding the mechanisms of phytoplankton carbon acquisition in interpreting carbon isotope data.

ABSTRACTThe utilization of inorganic carbon by three species of marine diatom, Skeletonema costatum (Grev.) Cleve. Ditylum brightwellii (West) Grun., and Chaetoceros calcitrans Paulsen was investigated using an inorganic carbon isotopic disequilibnum technique and inorganic carbon dose‐response curves. Stable carbon isotope data of the diatoms are also presented. Observed rates of photosynthetic oxygen evolution were greater than could be accounted for by the theoretical rate of CO2 supply from the uncatalyzed dehydration of HCO3− in the external medium, suggesting use of HCO3− as an inorganic carbon source. Data from the isotopic disequilibrium experiment demonstrate the use of both HCO3− and CO2 for photosynthesis. Carbon isotope discrimination values support the use of HCO3− by the diatoms.

Effects of cell size and/or specific growth rate were studied in 2 species of marine diatom, the large-celled Ditylum brightwellii and the smaller Chaetoceros calcitrans. Cells were grown as light-limited continuous cultures to produce a wide range of specific growth rates from 0.12 d(-1) in D. brightwellii to 1.01 d(-1) in C. calcitrans. Carbon isotope discrimination (Delta) values, relative to source delta(13)C Of dissolved inorganic carbon (DIG), showed no relationship to specific growth rate within species. When examined interspecifically there was some evidence that growth rate or cell size affected the C-13/C-12 ratios of the diatoms. At each photon flux density (PFD) used for growth, the specific growth rate of C. calcitrans was at least twice that of D. brightwellii. Values of Delta were greater in D. brightwellii at PFDs of 5, 20 and 40 pmol photon m(-2) s(-1). These data are in agreement with a hypothesis stating that faster-growing diatoms should be enriched in C-13. However, at the highest growth irradiance of 60 mu mol m(-2) s(-1), Delta values were higher in C. calcitrans than in D. brightwellii. Source delta(13)C values varied between individual cultures and demonstrated the importance of directly measuring the delta(13)C of DIC. The value of physiological data in fully interpreting the stable carbon isotope ratios of diatoms is also discussed.