The dependence of growth and photosythesis on cell size in diatoms is evaluated in terms of an energy-balance model of microalgal physiology. Based on a review of available observations it appears that cell size can account for much of the interspecific variability in maximum growth rate ( p, , , ) and in optical absorption cross-section (ach,). Both CL, and a,,, decrease with increases in cell size. Cell size does not appear to influence the chlorophyll a:carbon ratio, the quantum efficiency of photosynthesis nor the photon flux density (PFD) at which growth rate is llght-saturated. Available observations do not allow an evaluation of the size dependence of maintenance metabolic rates or the PFD at which light compensation of growth occurs. This analysis confirms the competitive advantage of small cell size of microalgae under nutrient-sufficient conditions at both light-saturating and light-limiting growth rates to the extent that small-celled diatoms have enhanced catalytic efficiencies of growth and light absorption.

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Marine macroalgae and seagrasses collected from Penguin Island, Western Australia, and red marine macroalgae from Australasia and Europe were analysed for the natural abundance of stable isotopes in their organic matter. These measurements revealed 1 specles of green macroalgae and 9 species of red macroalgae with a 6I3C below -30%. These new observations bring the total of reports of wild-collected manne macrophytes with 613C values below -30% to 3 species of ulvophycean chlorophytes and 22 species of florideophycean rhodophytes The 22 rhodophyte species are in 18 genera in 10 famllies of 4 orders. Marine algae with very negative S13C values have been shown in other work to be unable to use HC03-and rely on CO2 diffusion Into the thallus for photosynthesis. The quantitative implications of the low I3C/l2C ratio and the inability to use HCOf were analysed to predict the maximum in situ rate of photosynthesis. The relatively low predicted rates agree with measured rates (by other workers) of C assimilation in photosynthesis and growth. These low potential rates of C acquisition can be related to the low mean photon flux density required for growth in 17 of the 22 species of red algae and all 3 species of green algae. Low mean photon flux densities are characteristic of habitats in the subtidal (18 species of red algae, 3 species of green algae) and shaded microhabitats in the intertidal (1 species of red algae). Even the 5 other algae, littoral and infralittoral, seem from the literature to have low photosynthetic and relative growth rates, although it is not clear whether the low metabolic rates result from dependence on CO;, diffusion or whether diffusive CO2 entry as the sole means of inorganic C supply is permitted by a low metabolic rate imposed by some other cause.

We investigated the consequences of growth at Limiting photon flux density (PFD) for the uptake of inorganic carbon by 2 species of red seaweed which are able to use H C 0 3 for photosynthesis [Palmaria palmata (Huds.) Larnour and Laurencia pinnatifida (L.) Kuntze] and one which is dependent on diffusive uptake of bulk CO, [Lomentaria articulata (Huds.) Lyngb.] We attempted to induce energetic limitation of plants and/or reduce their demand for inorganic carbon by growing them at a low PFD and comparing their carbon use characteristics with those of plants grown at high PFD. There were significant increases in the carbon isotope discrimination (more negative delta values relative to the PDB standard) when the 3 species were grown at 25 relative to 75 pm01 photons m-2 S-'. In the 2 species which are able to use bicarbonate, the decreased SI3c was associated with lower initial slopes of inorganic carbon uptake curves. Taken together, the changes in 6I3C and dissolved organic carbon uptake indicated that light-limited P. palmata and L. pinnatifjda were not using low concentrations of inorganic carbon as efficiently as were plants grown at high PFD. This hypothesis was further supported by evidence that light limitation also caused a reduction in C:N ratio and in the % dry matter made up of carbon in those species. In contrast, growth of L. articulata, which is dependent on diffusive uptake of CO, for photosynthesis, was inhibited by the higher PFD and there were no significant effects of growth PFD on inorganic carbon uptake efficiency, C:N ratio or % C in this species. KEY WORDS: Laurencja pinnatifida-Lomentaria articulata. Palmaria palmata. CO2-HCO,-. Light limitation. Photosynthesis. Stable carbon isotope ratios 0 Inter-Research 1994 Resale of full article not permitted

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 lightlimlted continuous cultures to produce a wide range of specific growth rates from 0.12 d-' in D. brightwellii to 1.01 d-' in C. calcitrans. Carbon isotope discrimination (A) values, relative to source 8':'C of dissolved inorganic carbon (DIC), 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 ratios of the diatoms. At each photon flux density (PFD) used for growth, the specific growth rate of C. of physiological data in fully interpreting the stable carbon isotope ratios of diatoms is also discussed.