Microbial activity is a fundamental component of oceanic nutrient cycles. Photosynthetic microbes, collectively termed phytoplankton, are responsible for the vast majority of primary production in marine waters. The availability of nutrients in the upper ocean frequently limits the activity and abundance of these organisms. Experimental data have revealed two broad regimes of phytoplankton nutrient limitation in the modern upper ocean. Nitrogen availability tends to limit productivity throughout much of the surface low-latitude ocean, where the supply of nutrients from the subsurface is relatively slow. In contrast, iron often limits productivity where subsurface nutrient supply is enhanced, including within the main oceanic upwelling regions of the Southern Ocean and the eastern equatorial Pacific. Phosphorus, vitamins and micronutrients other than iron may also (co-)limit marine phytoplankton. The spatial patterns and importance of co-limitation, however, remain unclear. Variability in the stoichiometries of nutrient supply and biological demand are key determinants of oceanic nutrient limitation. Deciphering the mechanisms that underpin this variability, and the consequences for marine microbes, will be a challenge. But such knowledge will be crucial for accurately predicting the consequences of ongoing anthropogenic perturbations to oceanic nutrient biogeochemistry.

https://escholarship.org/content/qt21r294gk/qt21r294gk.pdf

Processes and patterns of oceanic nutrient limitation

CONTENTS INTRODUCTION ..... .... .... .... .... .... .... .... .... .... .... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .... .... .... 633 MECHANISMS ..... .... .... .... .... .... .... .... .... .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... .... ........ ........ .... 636 Photosystem Il Inacti vaJion, Damage and Recovery: The DI Story 637 A voidance of PSll Damage: The Xanthophyll Cycle Story .... 640 Damage rmd A voidance: A Blurring of the Division . . . . . . ...... . . . . . . . . . . . . . . . . . . 643 PHOTOINHIBITION IN THE FIELD AND OPEN OCEAN . . . .. ... .. . .. .... .... . . . . . . . . .... 644 Terrestrial VegetaJion .. .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... .... . . . . . . . . .... .... .... . . . . . . . . . . . . . . . . . . . . . . . . 644 Phytoplankton 646 SIGNIFICANCE TO PRODUCTION: A MODELING APPROACH 648 OBSERVED CHANGES IN PRODUCTION ....... . . . .... ........ 653 PHOTO INHIBITION AND PLANT DISTRIBUTIONS .... .... .... .... .... .... . . . . . . . . . . . . . . . . . . . . 654 CONCLUSIONS .... .... .... .... .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ .... .... .... .... ........ .... ........ 655

Photoinhibition of Photosynthesis in Nature

The evolution of organisms capable of oxygenic photosynthesis paralleled a long-term reduction in atmospheric CO2 and the increase in O2. Consequently, the competition between O2 and CO2 for the active sites of RUBISCO became more and more restrictive to the rate of photosynthesis. In coping with this situation, many algae and some higher plants acquired mechanisms that use energy to increase the CO2 concentrations (CO2 concentrating mechanisms, CCMs) in the proximity of RUBISCO. A number of CCM variants are now found among the different groups of algae. Modulating the CCMs may be crucial in the energetic and nutritional budgets of a cell, and a multitude of environmental factors can exert regulatory effects on the expression of the CCM components. We discuss the diversity of CCMs, their evolutionary origins, and the role of the environment in CCM modulation.

CO2 CONCENTRATING MECHANISMS IN ALGAE: Mechanisms, Environmental Modulation, and Evolution

https://moritz.botany.ut.ee/%7Eolli/b/Giordano05.pdf

Mechanisms in Algae: Mechanisms, Environmental Modulation, and Evolution

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments1 and subsequently confirmed by large-scale iron fertilization experiments2. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales3, providing an alternative to phosphorus as the ultimate limiting nutrient4. Oceanographic observations have been interpreted both to confirm and refute this hypothesis5, 6, but direct experimental evidence is lacking7. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs8 and strongly impacted by Saharan dust input9. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus10, 11. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.

Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic

The effects of nitrate, phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP 1327). Although cell chlorophyll a (chl a) content decreased in nutrient-starved cells, the ratios of light-harvesting accessory pigments (chl c and fucoxanthin) to chl a were unaffected by nutrient starvation. The chl a-specific light absorpition coefficient (a*) and the functional absorption cross-section of PSII (σ) increased during nutrient starvation, consistent with reduction of intracellular self-shading (i.e. a reduction of the “package effect”) as cells became chlorotic. The light-harvesting complex proteins remained a constant proportion of total cell protein during nutrient starvation, indicating that chlorosis mirrored a general reduction in cell protein content. The ratio of the xanthophylls cycle pigments diatoxanthin and diadinoxanthin to chl a increased during nutrient starvation. These pigments are thought to play a photo-protective role by increasing dissipation of excitation energy in the pigment bed upstream from the reaction centers. Despite the increase in diatoxanthin and diadinoxanthin, the efficiency of PSII photochemistry, as measured by the ration of variable to maximum fluorescence (Fv/Fm) of dark-adapted cells, declined markedly under nitrate and iron starvation and moderately under phosphate starvation. Parallel to changes in Fv/Fm were decreases in abundance of the reaction center protein D1 consistent with damage of PSII reaction centers in nutrient-starved cells. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. Most marked was the decline in the abundance of the small subunit of RUBISCO in nitrate-starved cells. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient.

Response of the photosynthetic apparatus of Phaeodactylum tricornutum (Bacillariophyceae) to nitrate, phosphate, or iron starvation

The relationship between the diadinoxanthin cycle and changes in fluorescence yield in the diatom Chaetoceros muelleri Lemm. (clone CH10, Amorient Aquafarm, Inc., Hawaii) was investigated. High-light-induced changes in fluorescence yield and xanthophyll de-epoxidation occurred very rapidly (first order rate constant 1.60 min−1). The observed light-induced changes in diatoxanthin and diadinoxanthin concentration were consistent with a two-pool scheme for diadinoxanthin, one of which does not undergo de-epoxidation. Changes in xanthophyll concentration correlated with changes in in vivo absorbance indicating that diadinoxanthin cycle activity in vivo can be monitored spectrophotometrically. However, changes in cell absorbance were small relative to total optical absorption cross section. Increases in the concentration of diatoxanthin were linearly correlated with increases in the rate constant for thermal de-excitation in the antenna of photosystem II (PSII). Antenna quenching produced or mediated by diatoxanthin may, thus, protect the PSII reaction center in diatoms. Changes in the maximum fluorescence yield suggested that changes in the reaction center also contributed to nonphotochemical quenching of fluorescence. Thus, reaction center quenching affected the relationship between antenna quenching and changes in photochemical efficiency producing the effect of a decrease in fluorescence yield without a decrease in photochemical efficiency.

Short-term response of the diadinoxanthin cycle and fluorescence yield to high irradiance in chaetoceros muelleri (bacillariophyceae)1

Nutrient enrichment experiments were conducted m May and June of 1993 at 8 stations along a North Atlantlc transect, from Morocco to Nova Scotia, Canada. Variable fluorescence (F,/F,,) was measured in order to estimate the health or physiological state of the population as a whole. Low values across the transect indicated nutrient limited photosynthetic efficiency and probable growth rates ranging from 10 to about 50% of p,,. Where the lowest value was measured, over the Grand Banks of Newfoundland, Canada, nitrogen addition to incubated samples resulted in large, significant increases in photochemical efficiency. Numbers and cell-specific fluorescence of 3 major groups of picophytoplankton were studied using flow cytometry, in order to further quantlfy the physiological response to nutrient additions. Results ind~cated nitrogen linxtation of physiology and/or abundance of small eukaryotes, cyanobacteria, and prochlorophytes. Abundance (cell numbers) and cellular fluorescence of the 3 groups responded differently to nutrient additions. Prochlorophytes showed the greatest response to incubation in terms of cell numbers, responding especially to nitrogen addition. By contrast, cyanobacterial numbers did not change from initial values or with treatment, although cell pigment content did. Cellular fluorescence as measured by the flow cytometer reflected cell pigment content in most experiments. Increased cellular fluorescence of all groups in nitrogen-amended treatments relat~ve to unamended controls lnd~cated physiological limitation by nitrogen.

/pdf/nitrogen-limitation-of-north-atlantic-phytoplankton-analysis-2rhr01uz6v.pdf

Nitrogen limitation of North Atlantic phytoplankton: analysis of physiological condition in nutrient enrichment experiments

Using ship-based phytoplankton fluorescence techniques on a 9-d transect of the North Atlantic Ocean, we have produced the first synoptic view of variations in the maximum quantum yield of photosynthetic energy conversion (4,) on an ocean-wide basis. Based on our 4, measurements, the North Atlantic can be divided into two basins. The southeastern basin (from the African coast to the midocean ridge) is characterized by relatively little horizontal variation in 4, over vast stretches of the ocean. In the northwestern basin (from the midocean ridge to the Canadian coast), we measured lower values of 4, than in the eastern basin, particularly in the upper water column. Lowest surface values were found in an area of strong horizontal flow. Our results suggest that nutrient supply limits phytoplankton productivity over vast stretches of the North Atlantic in this period (early summer). However, surface values of 4, across the North Atlantic appear to be independent of nutrient concentration and distance to nitrate supply. We propose that horizontal advection may play an important role in determining the local conditions to which phytoplankton productivity parameters respond. Our results also suggest that an apparent relationship between remotely sensed variables (ocean color, sea surface height) and phytoplankton photosynthetic parameters exists that may provide the basis for the development of new algorithms used to estimate phytoplankton productivity from satellites.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1996.41.4.0755

Miguel Olaizola

Papers

Response of the photosynthetic apparatus of Phaeodactylum tricornutum (Bacillariophyceae) to nitrate, phosphate, or iron starvation

Short-term response of the diadinoxanthin cycle and fluorescence yield to high irradiance in chaetoceros muelleri (bacillariophyceae)1

Nitrogen limitation of North Atlantic phytoplankton: analysis of physiological condition in nutrient enrichment experiments

Synoptic study of variations in the fluorescence based maximum quantum efficiency of photosynthesis across the North Atlantic Ocean