We assembled a dataset of 14C-based productivity measurements to understand the critical variables required for accurate assessment of daily depth-integrated phytoplankton carbon fixation (PP(PPeu)u) from measurements of sea surface pigment concentrations (Csat)(Csat). From this dataset, we developed a light-dependent, depth-resolved model for carbon fixation (VGPM) that partitions environmental factors affecting primary production into those that influence the relative vertical distribution of primary production (Pz)z) and those that control the optimal assimilation efficiency of the productivity profile (P(PBopt). The VGPM accounted for 79% of the observed variability in Pz and 86% of the variability in PPeu by using measured values of PBopt. Our results indicate that the accuracy of productivity algorithms in estimating PPeu is dependent primarily upon the ability to accurately represent variability in Pbopt. We developed a temperature-dependent Pbopt model that was used in conjunction with monthly climatological images of Csat sea surface temperature, and cloud-corrected estimates of surface irradiance to calculate a global annual phytoplankton carbon fixation (PPannu) rate of 43.5 Pg C yr‒1. The geographical distribution of PPannu was distinctly different than results from previous models. Our results illustrate the importance of focusing Pbopt model development on temporal and spatial, rather than the vertical, variability.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.1997.42.1.0001

Photosynthetic rates derived from satellite‐based chlorophyll concentration

The variation in growth rate with temperature of unicellular algae suggests that an equation can be written to describe the maximum expected growth rate for temperatures less than 40°C. Measured rates of phytoplankton growth in the sea and in lakes are reviewed and compared with maximum expected rates. The assimilation number (i.e., rate of photosynthetic carbon assimilation per weight of chlorophyll a) for phytoplankton photosynthesis is related to the growth rate and the carbon/chlorophyll a ratio in the phytoplankton. Since maximum expected growth rate can be estimated from tempera­ ture, the maximum expected assimilation number can also be estimated if the carbon/ chlorophyll a ratio in the phytoplankton crop is known. Many investigations of phytoplankton photosynthesis in the ocean have included measures of the assimilation number, while fewer data are available on growth rate. Assimilation numbers for Antarctic seas are low as would be expected from the low ambient temperatures. Tropical seas and temperate waters in summer often show low assimilation numbers as a result of low ambient nutrient concentrations. However, coastal estuaries with rapid nutrient regeneration processes show seasonal variations in the assimilation number with temperature which agree well with expectation. The variation in maximum expected growth rate with temperature seems a logical starting point for modeling phytoplankton growth and photosynthesis in the sea.

Temperature and phytoplankton growth in the sea

Equations for estimating the organic carbon in phytoplankton from cell volume or plasma volume are derived from original data on five species of diatoms and from data in the literature. Differences among species are important sources of error in such estimates. Plasma volume provides a more precise estimate of cell carbon in diatoms than does cell volume. Diatoms, because of their lower carbon per cell volume, should probably be treated separately from other phytoplankton in such computations.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1967.12.3.0411

Estimating the organic carbon content of phytoplankton from cell volume or plasma volume1

The photosynthesis-irradiance response (PE) curve, in which mass-specific photosynthetic rates are plotted versus irradiance, is commonly used to characterize photoacclimation. The interpretation of PE curves depends critically on the currency in which mass is expressed. Normalizing the light-limited rate to chl a yields the chl a-specific initial slope (α c h l ). This is proportional to the light absorption coefficient (a c h l ), the proportionality factor being the photon efficiency of photosynthesis (Φ m ). Thus, α c h l is the product of a c h l and Φ m . In microalgae α c h l typically shows little (<20%) phenotypic variability because declines of Φ m under conditions of high-light stress are accompanied by increases of a c h l , The variation of α c h l among species is dominated by changes in a c h l due to differences in pigment complement and pigment packaging. In contrast to the microalgae, α c h l declines as irradiance increases in the cyanobacteria where phycobiliproteins dominate light absorption because of plasticity in the phycobiliprotein:chl a ratio. By definition, light-saturated photosynthesis (P m ) is limited by a factor other than the rate of light absorption. Normalizing P m to organic carbon concentration to obtain P m C allows a direct comparison with growth rates. Within species, P m C is independent of growth irradiance. Among species, P m C covaries with the resource-saturated growth rate. The chl a:C ratio is a key physiological variable because the appropriate currencies for normalizing light-limited and light-saturated photosynthetic rates are, respectively, chl a and carbon. Typically, chl a:C is reduced to about 40% of its maximum value at an irradiance that supports 50% of the species-specific maximum growth rate and light-harvesting accessory pigments show similar or greater declines. In the steady state, this down-regulation of pigment content prevents microalgae and cyanobacteria from maximizing photosynthetic rates throughout the light-limited region for growth. The reason for down-regulation of light harvesting, and therefore loss of potential photosynthetic gain at moderately limiting irradiances, is unknown. However, it is clear that maximizing the rate of photosynthetic carbon assimilation is not the only criterion governing photoacclimation.

Photoacclimation of photosynthesis irradiance response curves and photosynthetic pigments in microalgae and cyanobacteria1

Bacteria and algae are the numerically dominant organisms in the plankton of lakes and the ocean and their metabolism largely controls pelagic energy flow and nutrient cycling. The purpose of this review is to examine the means by which algae and heterotrophic bacteria interact in planktonic systems. In terrestrial systems, bacterial-plant interactions have been well studied in the rhizosphere, the zone in the soil where microorganisms are influenced by plant roots (108). The concept of the "phycosphere" was introduced as a pelagic analogy to the rhizosphere (7). The phycosphere would be the zone surrounding an algal cell within which microorganisms are influenced by algal products. In considering bacterial-algal interactions, then, we should ask ourselves whether a phycosphere exists. I cite specific evidence from the literature for various types of interactions. In doing this I rely heavily on laboratory studies because relatively little field oriented work is available. I include neither the photosynthetic bacteria (e.g. Chlorobiaceae, Rhodospirillaceae) nor the large macroalgae (seaweeds) in this review. The cyanobacteria have both algal and bacterial characteristics, being prokaryotes and capable of oxygenic photosynthesis. As planktonic, aerobic photo-autotrophs the cyanobacteria clearly belong with the phytoplankton in terms of this review.

Interactions between bacteria and algae in aquatic ecosystems

The various aspects of the adaptation of plankton algae lo light and temperature are discussed. The shape of a light intensity-photosynthesis curve is shown to be an important means of describing the physiological adjustment of an algal population.



If the algae are not exposed to adverse influences such as poisons, pronounced nutrient deficiency or light shocks, the rate of real photosynthesis per mg chlorophyll a at 1 Klux (incandescent light) should be about 0.4–0.6 mg C/hour. Hence this rate presents an excellent means of judging the quality of experiments.



Experiments are presented where Chlorella pyrenoidosa was adapted to light intensities between 0.32 klux and 21 Klux. This alga adapts to different light intensities by varying the amount of pigments per cell. Algae grown at 1 Klux have about 10 times more chlorophyll per cell than those grown at 21 klux. Other species of algae—but by no means all—are shown to behave in the same way.



The problem of algal resistance to photo-oxidation at high light intensities is discussed. Adaplation is shown to he one of the mechanisms which make the algae resistent. “Chlorophyll inactivation” is another.



Experiments with the diatom Skeletonema costatum concerning adaptation to different temperatures have been performed. The fact that the alga has essentially the same rate of photosynthesis per cell at all light intensities at 20°C and 7°C, may be attributed to an increase of all the enzymes at the low temperature. The amount of protein per cell was twice as high at 7°C as at 20°C.

The Adaptation of Plankton Algae

The Adaptation of Plankton Algae. II. Aspects of the Temperature Adaptation of Skeletonema costatum

Antibiotic Substances from Cells and Culture Solutions of Unicellular Algae with Special Reference to some Chlorophyll Derivatives

Growth Inhibiting Substances Formed by Algae

A culture of Skeletonema costatum grown at a light intensity of 3 klux and at 20°C was synchronized in diurnally intermittent illumination of 12 hour light and 12 hour dark. The culture was hardly fully synchronous as the cell division period lasted about 9 hours. The cell division started in the middle of the light period. The concentration of the pigments: chlorophyll a, chlorophyll 6 and fucoxanthin and the rate of light-saturated photosynthesis were followed every hour during the 24 hour period. Both the concentration of pigments and the photosynthetic activity showed a rhythmical variation. The concentration per cell of all three pigments examined increased during the development of the cells and decreased automatically during the period of cell division. An increase in the pigment concentration was found only in the light period. The rate of light-saturated photosynthesis calculated per unit of cell number increased during the cell development and decreased during the division period. The increase in the photosynthetic activity at light-saturation started about 4 hours after the end of cell division, which was 4 hours before the light was turned on while the increase in the concentration of chlorophyll a first started 1–2 hours after this moment.



The variation in photosynthetic activity was compared with that found by other workers. The results found with Chlorella ellipsoidea by Japanese scientists (Nihci et al.) was explained as an inhibition phenomenon because the cells were not adapted to the experimental conditions.

Erik G. Jørgensen

Papers

The Adaptation of Plankton Algae

The Adaptation of Plankton Algae. II. Aspects of the Temperature Adaptation of Skeletonema costatum

Antibiotic Substances from Cells and Culture Solutions of Unicellular Algae with Special Reference to some Chlorophyll Derivatives

Growth Inhibiting Substances Formed by Algae

Photosynthetic Activity during the Life Cycle of Synchronous Skeletonema Cells