Heavy metals, depending on their oxidation states, can be highly reactive and, as a consequence, toxic to most organisms. They are produced by an expanding variety of anthropogenic sources suggesting an increasingly important role for this form of pollution. The toxic effect of heavy metals appears to be related to production of reactive oxygen species (ROS) and the resulting unbalanced cellular redox status. Algae respond to heavy metals by induction of several antioxidants, including diverse enzymes such as superoxide dismutase, catalase, glutathione peroxidase and ascorbate peroxidase, and the synthesis of low molecular weight compounds such as carotenoids and glutathione. At high, or acute, levels of metal pollutants, damage to algal cells occurs because ROS levels exceed the capacity of the cell to cope. At lower, or chronic, levels algae accumulate heavy metals and can pass them on to organisms of other trophic levels such as mollusks, crustaceans, and fishes. We review here the evidence linking metal accumulation, cellular toxicity, and the generation of ROS in aquatic environments.

Heavy metal–induced oxidative stress in algae†

A new regulatory model can describe acclimation of phytoplankton growth rate (p), chlorophyll a : carbon ratio and nitrogen: carbon ratio to irradiance, temperature and nutrient availability. The model uses three indices of phytoplankton biomass-phytoplankton carbon (C), phytoplankton nitrogen (N), and chlorophyll a (Chl). The model links the light-saturated rate of photosynthesis to N: C, requires that Chl a synthesis be coupled to nitrogen assimilation, and includes several regulatory features. These include feedback inhibition of the nitrogen assimilation rate by increases in the N: C ratio, as well as regulation of Chl a synthesis by the balance between light absorption and photosynthetic carbon fixation. The model treats respiration as the sum of the maintenance metabolic requirement and the cost of biosynthesis. In addition, the model can account for accumulation and mobilization of energy reserves (i.e. variability of N: C) and photoacclimation (i.e. variability of Chl : N and Chl : C) in response to variations in irradiance and nutrient availability. The assumptions of the model are shown to be in agreement with experimental observations and the model output compares favorably with data for cultures in balanced and unbalanced growth. The light-, nutrient-, and temperature-dependencies of phytoplankton growth rate have often been modeled separately. Phytoplankton growth rate has been treated as a function of the dissolved nutrient concentration using the Monod equation or as a function of the cellular content (or quota) of limiting nutrient using the Droop equation (McCarthy 1980; Droop 1983; Morel 1987). Under conditions of balanced growth, the Droop and Monod equations are consistent with each other and with Michaelis-Menten nutrient uptake kinetics (Morel 1987). The temperature-dependence of growth rate has been treated as an exponential dependence or an Arrhenius equation, although other functions have also been used (Eppley 1972; Li 1980; Ahlgren 1987). The lightdependencies of growth and photosynthesis rates have been treated by a number of equations, including a modification of the Monod equation, a hyperbolic tangent, and a Poisson function

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1998.43.4.0679

A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature

Growth physiology and fate of diatoms in the ocean: a review

SUMMARY 157INTRODUCTION 158AN.^LYTICAL METHODS 159LIGHT HARVESTING BY MICROALGAE 160RANGE OF PHOTON FLUX DENSITIES ALLOWING GROWTH AND PHOTOSYNTHESIS INPHOTOTHOPHIC MICROALGAE (GENOTYPIC ADAPTATION) 163 Growth 163Photosynthesis 165Photoinhibition 165 PHENOTYPIC ADAPTATION 168 Changes in amounts of pigments 168Interpretation of the effects of pigment changes: models 169Observed changes in P vs I curves 170 ENERGETIC CONSIDERATIONS 174 General 174Reduction of capital costs 175Reduction of maintenance costs 175Energetic costs of changing the photosyothetic apparatus 177S2-S3 decay 177Proton leakage due to passive uniport 178 PHYLOGENETIC ASPECTS OF DIFFERENCES IN LIGHT RESPONSES OF MICROALGALPHOTOSYNTHESIS AND PHOTOLITHOTROPHIC GROWTH 178 Phylogenetic diflerences in photosynthetic structures 178Comparison of tbe photosynthetic characteristics of green algae and otherpbototrophs 180 ECOLOGICAL CONSIDERATIONS 182ACKNOWLEDGEMENTS 185REFERENCES 185 SUMMARY Analysis of data in the literature relating to micrcalgal adaptations to different photon fluxdensities indicates that different algal classes have significantly different ligbt requirennents forgrowth and photosynthesis. Although there is some variability within each class, dinoflagellatesand blue-green algae generally photosynthesize and grow best at low photon flux densities.Diatoms also tend to be able to grow at very low photon flux densities (growth for some specieshas been reported at less than 1 fi.E m"' s~'). Comparison of the photon flux densities at whichphotoinhibition occurs in dinoflagellates and diatoms suggests that the former often experiencephotoinhibition at comparatively low irradiances. In contrast, diatoms often can toleraterelatively high light environments. This tolerance of a large absolute range of photon fluxdensities may, in part, explain why diatoms are often associated with spring blooms. Green algae* New address; Department of Botany, La Trobe University, Bundoora, Victoria, 3083 Australia.0028-646X/83/020157 + 35 S03.00/0 © 1983 The New Phytologist

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.1983.tb03422.x

Adaptation of unicellular algae to irradiance: an analysis of strategies

Summary
The carbon:chlorophyll a ratio (C:chl a or θ) is a sensitive indicator of physiological state in microalgae. The dependence of θ on photon flux density (PFD or I) and temperature in exponentially growing nutrient-sufficient microalgae can be described by an empirical equation with four coefficients. C:chl a increases linearly with increased light level at constant temperature and decreases exponentially with increased temperature at constant light level. Both the slope (ɛ) and intercept (θ0) of linear regressions of θ on photon flux density increase at low temperature. The intercept, θ0, increases from 6 to 40 g of carbon per gram of chlorophyll a (g C g chl a−1) between 30 and 0 °C and ɛ increases by over an order of magnitude from 004 to 1·9 g C g chl a−l m2 s μol photon−1 over the same temperature range.

Low-temperature chlorosis can be interpreted as an adaptive response in the allocation of cell resources between temperature-independent biophysical reactions involved in light-harvesting and temperature-dependent biochemical reactions. This response also reduces the potential for photoinhibitory damage at high light levels which can be exacerbated by low temperatures.

The range of values for θ in nature has not been adequately determined because of difficulty in separating phytoplankton from detritus, bacteria and microzooplankton. Based on the laboratory observations summarized in this paper, it would appear that use of a single value of θ for phytoplankton is inappropriate for ecological studies. For example, at a PFD of 50 μmol m−2 s−1, θ increases from 10 to 130 g C g chl a−1 between 30 and 0 °C under nutrient-sufficient conditions.

These conclusions are based on observations for eight diatoms, two green algae, one euglenoid and two cyanobacteria for which the appropriate data are available. In contrast to these groups, the dinofiagellates contain substantially less chlorophyll a The available data indicate that θ is about three times larger in dinoflagellates than in other algae under comparable PFDs at 20 °C. There are insufficient data available, however, to evaluate the light and temperature dependence of θ in dinoflagellates.

LIGHT AND TEMPERATURE DEPENDENCE OF THE CARBON TO CHLOROPHYLL a RATIO IN MICROALGAE AND CYANOBACTERIA: IMPLICATIONS FOR PHYSIOLOGY AND GROWTH OF PHYTOPLANKTON

Photosynthetic rates, growth rates, cell carbon, cell protein, and chlorophyll a content of two diatom and two dinoflagellate species were measured. The microalgae were chosen to have one small and one large species from each phylogenetic group; the two size categories differed from each other by 1.5 orders of magnitude in terms of cell carbon or cell protein. The cultures for the experiments were grown under continuous light at an irradiance high enough for the light-saturation of growth for all four species. The four species were found to have similar maximum photosynthetic rates per unit chlorophyll a. The diatom species showed lower carbon/chlorophyll a ratios and higher photosynthetic rates per unit carbon than the dinoflagellates. The higher growth rates of the diatoms were shown to be related to their higher photosynthetic rates per unit carbon. The ecological significance of the physiological difference between these two groups of microalgae is discussed.

COMPARATIVE PHYSIOLOGICAL STUDY OF MARINE DIATOMS AND DINOFLAGELLATES IN RELATION TO IRRADIANCE AND CELL SIZE. II. RELATIONSHIP BETWEEN PHOTOSYNTHESIS, GROWTH, AND CARBON/CHLOROPHYLL a RATIO1,2

Cell division rates and chlorophyll a and protein contents for ten diatom and dinoflagellate species were measured. Species were chosen to include a wide range of cell size in terms of both cell volume and cell protein: from 0.004 ng protein/cell for a small Chaetoceros sp. to 2.2 ng protein/cell for Prorocentrum micans Ehrenberg. Experiments were conducted in batch or semi-continuous cultures at 21 C under continuous illumination from 8–256 μEin .m-2'.s-1. Light saturation of cell division occurred at 32–80 μEin m-1 s-1 for all species, with no observable difference between the two phylogenetic groups. When the light-saturated cell division rates were plotted against cell size as protein/cell, the diatoms and dinoflagellates fell on two separate lines with the diatoms having higher rates. Chl a /protein ratios (μg/μg) decreased with increasing irradiance. The diatoms had higher chl a per unit protein. The relationship between cell division rate and the chl a/protein ratio is discussed.

Augustine T. Chan

Papers

COMPARATIVE PHYSIOLOGICAL STUDY OF MARINE DIATOMS AND DINOFLAGELLATES IN RELATION TO IRRADIANCE AND CELL SIZE. II. RELATIONSHIP BETWEEN PHOTOSYNTHESIS, GROWTH, AND CARBON/CHLOROPHYLL a RATIO1,2

Comparative physiological study of marine diatoms and dinoflagellates in relation to irradiance and cell size. i. growth under continuous light1,2