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Phytoplankton are responsible for nearly half of global primary productivity and play crucial roles in the Earth's biogeochemical cycles. However, the long-term adaptive responses of phytoplankton to rising CO 2 remains unknown. Here we examine the physiological and proteomics responses of a marine diatom, Phaeodactylum tricornutum, following long-term (c. 900 generations) selection to high CO 2 conditions. Our results show that this diatom responds to long-term high CO 2 selection by downregulating proteins involved in energy production (Calvin cycle, tricarboxylic acid cycle, glycolysis, oxidative pentose phosphate pathway), with a subsequent decrease in photosynthesis and respiration. Nearly similar extents of downregulation of photosynthesis and respiration allow the high CO 2 -adapted populations to allocate the same fraction of carbon to growth, thereby maintaining their fitness during the long-term high CO 2 selection. These results indicate an important role of metabolism reduction under high CO 2 and shed new light on the adaptive mechanisms of phytoplankton in response to climate change., (© 2021 The Authors. New Phytologist © 2021 New Phytologist Foundation.)

Many marine organisms are exposed to decreasing O 2 levels due to warming-induced expansion of hypoxic zones and ocean deoxygenation (DeO 2 ). Nevertheless, effects of DeO 2 on phytoplankton have been neglected due to technical bottlenecks on examining O 2 effects on O 2 -producing organisms. Here we show that lowered O 2 levels increased primary productivity of a coastal phytoplankton assemblage, and enhanced photosynthesis and growth in the coastal diatom Thalassiosira weissflogii. Mechanistically, reduced O 2 suppressed mitochondrial respiration and photorespiration of T. weissflogii, but increased the efficiency of their CO 2 concentrating mechanisms (CCMs), effective quantum yield and improved light use efficiency, which was apparent under both ambient and elevated CO 2 concentrations leading to ocean acidification (OA). While the elevated CO 2 treatment partially counteracted the effect of low O 2 in terms of CCMs activity, reduced levels of O 2 still strongly enhanced phytoplankton primary productivity. This implies that decreased availability of O 2 with progressive DeO 2 could boost re-oxygenation by diatom-dominated phytoplankton communities, especially in hypoxic areas, with potentially profound consequences for marine ecosystem services in coastal and pelagic oceans., (© 2022. The Author(s).)

Cell size influences the rate at which phytoplankton assimilate dissolved inorganic carbon (DIC), but it is unclear whether volume-specific carbon uptake should be greater in smaller or larger cells. On the one hand, Fick's Law predicts smaller cells to have a superior diffusive CO 2 supply. On the other, larger cells may have greater scope to invest metabolic energy to upregulate active transport per unit area through CO 2 -concentrating mechanisms (CCMs). Previous studies have focused on among-species comparisons, which complicates disentangling the role of cell size from other covarying traits. In this study, we investigated the DIC assimilation of the green alga Dunaliella tertiolecta after using artificial selection to evolve a 9.3-fold difference in cell volume. We compared CO 2 affinity, external carbonic anhydrase (CA ext ), isotopic signatures (δ 13 C) and growth among size-selected lineages. Evolving cells to larger sizes led to an upregulation of CCMs that improved the DIC uptake of this species, with higher CO 2 affinity, higher CA ext and higher δ 13 C. Larger cells also achieved faster growth and higher maximum biovolume densities. We showed that evolutionary shifts in cell size can alter the efficiency of DIC uptake systems to influence the fitness of a phytoplankton species., (© 2020 The Authors New Phytologist © 2020 New Phytologist Foundation.)

Rising atmospheric CO 2 concentrations are predicted to have a significant impact on global phytoplankton populations. Of particular interest in freshwater systems are those species that produce toxins or impact water quality, though evidence for how these species, and many others, will respond is limited. This study investigated the effects of elevated CO 2 (1,000 ppm) relative to current atmospheric CO 2 partial pressures (400 ppm), on growth, cell size, carbon acquisition, and photophysiology of five freshwater phytoplankton species including a toxic cyanophyte, Raphidiopsis raciborskii, from Lake Wivenhoe, Australia. Effects of elevated CO 2 on growth rate varied between species; notably growth rate was considerably higher for Staurastrum sp. and significantly lower for Stichococcus sp. with a trend to lower growth rate for R. raciborskii. Surface area to volume ratio was significantly lower with elevated CO 2 , for all species except Cyclotella sp. Timing of maximum cell concentrations of those genera studied in monoculture occurred in the lake in order of CO 2 affinity when free CO 2 concentrations dropped below air equilibrium. The results presented here suggest that as atmospheric levels of CO 2 rise, R. raciborskii may become less of a problem to water quality, while some species of chlorophytes may become more dominant. This has implications for stakeholders of many freshwater systems., (© 2020 Phycological Society of America.)

A mesocosm experiment was conducted in Wuyuan Bay (Xiamen), China, to investigate the effects of elevated pCO 2 on bloom formation by phytoplankton species previously studied in laboratory-based ocean acidification experiments, to determine if the indoor-grown species performed similarly in mesocosms under more realistic environmental conditions. We measured biomass, primary productivity and particulate organic carbon (POC) as well as particulate organic nitrogen (PON). Phaeodactylum tricornutum outcompeted Thalassiosira weissflogii and Emiliania huxleyi, comprising more than 99% of the final biomass. Mainly through a capacity to tolerate nutrient-limited situations, P. tricornutum showed a powerful sustained presence during the plateau phase of growth. Significant differences between high and low CO 2 treatments were found in cell concentration, cumulative primary productivity and POC in the plateau phase but not during the exponential phase of growth. Compared to the low pCO 2 (LC) treatment, POC increased by 45.8-101.9% in the high pCO 2 (HC) treated cells during the bloom period. Furthermore, respiratory carbon losses of gross primary productivity were found to comprise 39-64% for the LC and 31-41% for the HC mesocosms (daytime C fixation) in phase II. Our results suggest that the duration and characteristics of a diatom bloom can be affected by elevated pCO 2 . Effects of elevated pCO 2 observed in the laboratory cannot be reliably extrapolated to large scale mesocosms with multiple influencing factors, especially during intense algal blooms., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Diatoms, an important group of phytoplankton, bloom annually in the Southern Ocean, covering thousands of square kilometers and dominating the region's phytoplankton communities. In their role as the major food source to marine grazers, diatoms supply carbon, nutrients and energy to the Southern Ocean food web. Prevailing environmental conditions influence diatom phenotypic traits (for example, photophysiology, macromolecular composition and morphology), which in turn affect the transfer of energy, carbon and nutrients to grazers and higher trophic levels, as well as oceanic biogeochemical cycles. The paucity of phenotypic data on Southern Ocean phytoplankton limits our understanding of the ecosystem and how it may respond to future environmental change. Here we used a novel approach to create a 'snapshot' of cell phenotype. Using mass spectrometry, we measured nitrogen (a proxy for protein), total carbon and carbon-13 enrichment (carbon productivity), then used this data to build spectroscopy-based predictive models. The models were used to provide phenotypic data for samples from a third sample set. Importantly, this approach enabled the first ever rate determination of carbon productivity from a single time point, circumventing the need for time-series measurements. This study showed that Chaetoceros simplex was less productive and had lower protein and carbon content during short-term periods of high salinity. Applying this new phenomics approach to natural phytoplankton samples could provide valuable insight into understanding phytoplankton productivity and function in the marine system.

Diatoms and other phytoplankton in coastal waters experience rapid pH changes in milieu due to high biological activities and/or upwelled CO2-rich waters. While CO2 concentrating mechanisms (CCMs) are employed by all diatoms tested to counter low CO2 availability in seawater, little is known how this mechanism responds to fast pH changes. In the present study, the model diatom Thalassiosira pseudonana was acclimated for 20 generations to low pH (7.81) at an elevated CO2 of 1000 μatm (HC) or to high pH (8.18) at ambient CO2 levels of 390 μatm (LC), then its physiological characteristics were investigated as cells were shifted from HC to LC or vice versa. The maximal electron transport rate (ETRmax) in the HC-acclimated cells was immediately reduced by decreased CO2 availability, showing much lower values compared to that of the LC-acclimated cells. However, the cells showed a high capacity to regain their photochemical performance regardless of the growth CO2 levels, with their ETRmax values recovering to initial levels in about 100 min. This result indicates that this diatom might modulate its CCMs quickly to maintain a steady state supply of CO2, which is required for sustaining photosynthesis. In addition, active uptake of CO2 could play a fundamental role during the induction of CCMs under CO2 limitation, since the cells maintained high ETR even when both intracellular and periplasmic carbonic anhydrases were inhibited. It is concluded that efficient regulation of the CCM is one of the key strategies for diatoms to survive in fast changing pH environment, e.g. for the tested species, which is a dominant species in coastal waters where highly fluctuating pH is observed.

Carbon dioxide concentrating mechanisms (also known as inorganic carbon concentrating mechanisms; both abbreviated as CCMs) presumably evolved under conditions of low CO(2) availability. However, the timing of their origin is unclear since there are no sound estimates from molecular clocks, and even if there were, there are no proxies for the functioning of CCMs. Accordingly, we cannot use previous episodes of high CO(2) (e.g. the Palaeocene-Eocene Thermal Maximum) to indicate how organisms with CCMs responded. Present and predicted environmental change in terms of increased CO(2) and temperature are leading to increased CO(2) and HCO(3)(-) and decreased CO(3)(2-) and pH in surface seawater, as well as decreasing the depth of the upper mixed layer and increasing the degree of isolation of this layer with respect to nutrient flux from deeper waters. The outcome of these forcing factors is to increase the availability of inorganic carbon, photosynthetic active radiation (PAR) and ultraviolet B radiation (UVB) to aquatic photolithotrophs and to decrease the supply of the nutrients (combined) nitrogen and phosphorus and of any non-aeolian iron. The influence of these variations on CCM expression has been examined to varying degrees as acclimation by extant organisms. Increased PAR increases CCM expression in terms of CO(2) affinity, whilst increased UVB has a range of effects in the organisms examined; little relevant information is available on increased temperature. Decreased combined nitrogen supply generally increases CO(2) affinity, decreased iron availability increases CO(2) affinity, and decreased phosphorus supply has varying effects on the organisms examined. There are few data sets showing interactions amongst the observed changes, and even less information on genetic (adaptation) changes in response to the forcing factors. In freshwaters, changes in phytoplankton species composition may alter with environmental change with consequences for frequency of species with or without CCMs. The information available permits less predictive power as to the effect of the forcing factors on CCM expression than for their overall effects on growth. CCMs are currently not part of models as to how global environmental change has altered, and is likely to further alter, algal and aquatic plant primary productivity.

Phytoplankton life forms, including unicells, colonies, pseudocolonies, and multicellular organisms, span a huge size range. The smallest unicells are less than 1 microm3 (e.g. cyanobacteria), while large unicellular diatoms may attain 10(9) microm3, being visible to the naked eye. Phytoplankton includes chemo-organotrophic unicells, colonies and multicellular organisms that depend on symbionts or kleptoplastids for their capacity to photosynthesize. Analyses of physical (transport within cells, diffusion boundary layers, package effect, turgor, and vertical movements) and biotic (grazing, viruses and other parasitoids) factors indicate potential ecological constraints and opportunities that differ among the life forms. There are also variations among life forms in elemental stoichiometry and in allometric relations between biovolume and specific growth. While many of these factors probably have ecological and evolutionary significance, work is needed to establish those that are most important, warranting explicit description in models. Other factors setting limitations on growth rate (selecting slow-growing species) await elucidation.

The diversity of CCMs among aquatic oxygenic photolithotrophs causes difficulties in generalising about their responses to environmental change. A frequent response of organisms with CCMs to increasing CO 2 is increased organic carbon content, even if (as is often the case) the growth rate is not increased. In the few studies performed, the internal carbon dioxide concentration relative to the growth carbon dioxide concentration is decreased for organisms grown at elevated CO 2 , but not sufficiently so to alter the degree of Rubisco saturation with CO 2 . The more commonly measured affinity for external CO 2 generally also decreases with increased CO 2 concentration for growth. The present global distribution of cyanobacteria and algae with CCMs over a wide temperature range contrasts with terrestrial C 4 plants which mainly grow in warmer habitats. This suggests that algae and cyanobacteria will benefit less from increasing global temperatures in terms of geographical range than will terrestrial C 4 plants. The general correlate of increased temperature and a shallowing (shoaling) of the thermocline means that phytoplankton will experience a higher mean PAR and UV-B flux, and a decreased availability of combined nitrogen, phosphorus and iron. The general conclusion is that this combination of environmental changes will in part offset the decreased CCM activity in response to increased CO 2 . The few data available on responses to environmental change of the minority of aquatic oxygenic photolithotrophs lacking CCMs, suggest some similarities of responses of these organisms and those with CCMs. Organic carbon burial (“blue carbon”) by undisturbed seagrass beds will probably increase under environmental change. [ABSTRACT FROM AUTHOR]

Using cultures of a number of different marine algae (diatoms Skeletonema constatum (Grev.) Cleve and Phaeodactylum tricornutum Bohlin, chrysophyte Isochrysis galbana Parke, green flagellate Dunaliella tertiolecta Butcher, dinoflagellate Gonyaulax tamarensis Lebour) the short-term pattern of [SUP14]CO[SUB2] assimilation has been investigated. In all except D. tertiolecta the labelling of amina acids and intermediates of the tricarboxylic acid (Krebs) cycle was significantly heavier than that of sugar phosphates. Over periods of 30-120 s labelling in amino acids and Krebs cycle intermeiates accounted for 41-95% of the [SUP14]C fixed (depending on the alga). Over shorter tiems (< 10 s) the pattern in the 2 diatoms showed significant labelling of C[SUP4] acids (and related compounds) and little labelling of sugar phosphates. The reverse was seen with D. tertiolecta. Also, in the 2 diatoms and in G. tamarensis significant inhibition of photosynthesis by oxygen could only be achieved with 100 % oxygen; atmospheric levels having little effect. Parallel measurements of 2 carboxylating enzymes showed that ribulose-1,5-diphosphate carboxylase (RuDPCase) was significantly greater than phospho(enol)pyruvate carboxylase (PEPCase) activity only in the green flagellate. It is suggested that photosynthesis in marine diatoms depends on an active PEPCase utilizing bicarbonate as a substrate and that a less active RuDPCase utilizes CO[SUB2] . In D. tertiolecta the pattern more closely resembles that of a "Calvin (C[SUB3])" plant. The dinoflagellate and the chrysophyte appeared to show a mixed C[SUB3] and C[SUB4] photosynthesis. [ABSTRACT FROM AUTHOR]