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Despite seminal papers that stress the significance of silicon (Si) in plant biology and ecology, most studies focus on manipulations of Si supply and mitigation of stresses. The ecological significance of Si varies with different levels of biological organization, and remains hard to capture. We show that the costs of Si accumulation are greater than is currently acknowledged, and discuss potential links between Si and fitness components (growth, survival, reproduction), environment, and ecosystem functioning. We suggest that Si is more important in trait-based ecology than is currently recognized. Si potentially plays a significant role in many aspects of plant ecology, but knowledge gaps prevent us from understanding its possible contribution to the success of some clades and the expansion of specific biomes.

Genetic changes together with epigenetic modifications such as DNA methylation have been demonstrated to regulate many biological processes and thereby govern the response of organisms to environmental changes. However, how DNA methylation might act cooperatively with gene transcription and thereby mediate the long-term adaptive responses of marine microalgae to global change is virtually unknown. Here we performed a transcriptomic analysis, and a whole-genome bisulfite sequencing, along with phenotypic analysis of a model marine diatom Phaeodactylum tricornutum adapted for 2 years to high CO2 and/or warming conditions. Our results show that the methylated islands (peaks of methylation) mCHH were positively correlated with expression of genes in the subregion of the gene body when the populations were grown under high CO2 or its combination with warming for ~2 years. We further identified the differentially expressed genes (DEGs), and hence the metabolic pathways in which they function, at the transcriptomics level in differentially methylated regions (DMRs). Although DEGs in DMRs contributed only 18–24% of the total DEGs, we found that those DEGs acted cooperatively with DNA methylation and then regulated key processes such as central carbon metabolism, amino acid metabolism, ribosome biogenesis, terpenoid backbone biosynthesis, and degradation of misfolded proteins. Taken together, by integrating transcriptomic, epigenetic, and phenotypic analysis, our study provides evidence for DNA methylation acting cooperatively with gene transcription to contribute to the adaptation of microalgae to global changes.

Les auteurs exposent les principales ressources nécessaires pour les photoautotrophes photosynthétiques, les mécanismes impliqués dans leur acquisition et leur assimilation, ainsi que la manière dont la variation de ces facteurs contrôlent la productivité du phytoplancton.

L’auteur retrace l’évolution des organismes aquatiques photoautotrophes, ainsi que l’héritage et l’évolution des différents aspects de la machinerie photosynthétique des procaryotes aux eucaryotes, incluant les plantes d’eau douce et les herbiers marins.

Our scientific understanding of climate change makes clear the necessity for both emission reduction and carbon dioxide removal CDR The ocean with its large surface area great depths and long coastlines is central to developing CDR approaches commensurate with the scale needed to limit warming to below 2 C Many proposed marine CDR approaches rely on spatial upscaling along with enhancement and or acceleration of the rates of naturally occurring processes One such approach is ocean afforestation which involves offshore transport and concurrent growth of nearshore macroalgae seaweed followed by their export into the deep ocean The purposeful occupation for months of open ocean waters by macroalgae which do not naturally occur there will probably affect offshore ecosystems through a range of biological threats including altered ocean chemistry and changed microbial physiology and ecology Here we present model simulations of ocean afforestation and link these to lessons from other examples of offshore dispersal including rafting plastic debris and discuss the ramifications for offshore ecosystems We explore what additional metrics are required to assess the ecological implications of this proposed CDR In our opinion these ecological metrics must have equal weight to CDR capacity in the development of initial trials pilot studies and potential licensing 2022 Springer Nature Limited

It is commonly known that phytoplankton have a pivotal role in marine biogeochemistry and ecosystems as carbon fixers and oxygen producers, but their response to deoxygenation has scarcely been studied. Nonetheless, in the major oceanic oxygen minimum zones (OMZs), all surface phytoplankton groups, regardless of size, disappear and are replaced by unique cyanobacteria lineages below the oxycline. To develop reasonable hypotheses to explain this pattern, we conduct a review of available information on OMZ phytoplankton, and we re-analyze previously published data (flow cytometric and hydrographic) on vertical structure of phytoplankton communities in relation to light and O-2 levels. We also review the physical constraints on O-2 acquisition as well as O-2-dependent metabolisms in phototrophs. These considerations, along with estimates of the photosynthetic capacity of phytoplankton along OMZ depth profiles using published data, suggest that top-down grazing, respiratory demand, and irradiance are insufficient to fully explain the vertical structure observed in the upper, more sunlit portions of OMZs. Photorespiration and water-water cycles are O-2-dependent pathways with low O-2 affinities. Although their metabolic roles are still poorly understood, a hypothetical dependence on such pathways by the phytoplankton adapted to the oxic ocean might explain vertical patterns in OMZs and results of laboratory experiments. This can be represented in a simple model in which the requirement for photorespiration in surface phytoplankton and O-2-inhibition of OMZ lineages reproduces the observed vertical fluorescence profiles and the replacement of phytoplankton adapted to O-2 by lineages restricted to the most O-2-deficient waters. A high O-2 requirement by modern phytoplankton would suggest a positive feedback that intensifies trends in OMZ extent and ocean oxygenation or deoxygenation, both in Earth's past and in response to current climate change.

Much has been published on the effects of ocean acidification on plankton since the original Royal Society 2005 report. In addition to direct effects on primary production, it is clear that ocean acidification also has profound consequences for biogeochemistry. Furthermore, although ocean acidification can have direct effects of on grazers such as copepods, acidification induces changes in nutritional value of phytoplankton which can be passed on up the food chain. There has also been recognition of the complexity of the interactions between elevated CO2 and other environmental factors and this has seen an upsurge in climate change research involving multifactorial experiments. In particular, the interaction of ocean acidification with global warming resulting from the increasing greenhouse effect has been investigated. There has also been research on acidification and warming effects in inland water plankton. These, combined with novel experimental techniques and long term studies of genetic adaptation, are providing better insights to plankton biology and communities in a future world.

The photon costs of photoreduction/assimilation of nitrate (NO3−) into organic nitrogen in shoots and respiratory driven NO3− and NH4+ assimilation in roots are compared for terrestrial vascular plants, considering associated pH regulation, osmotic and ontogenetic effects. Different mechanisms of neutralisation of the hydroxyl (OH−) ion necessarily generated in shoot NO3− assimilation are considered. Photoreduction/assimilation of NO3− in shoots with malic acid synthesis and either accumulation of malate in leaf vacuoles or transport of malate to roots and catabolism there have a similar cost which is around 35% less than that for root NO3− assimilation and around 20% less than that for photoreduction/assimilation of NO3−, oxalate production and storage of Ca oxalate in leaf vacuoles. The photon cost of root NH4+ assimilation with H+ efflux to the root medium is around 70% less than that of root NO3− assimilation. These differences in photon cost must be considered in the context of the use of a combination of locations of NO3− assimilation and mechanisms of acid–base regulation, and a maximum of 4.9–9.1% of total photon absorption needed for growth and maintenance that is devoted to NO3− assimilation and acid–base regulation.

Ensuring that global warming remains, Ocean afforestation is considered as an important method to remove gigatons of CO2 from the atmosphere. Here the authors use the Great Atlantic Sargassum Belt as a natural analogue to show that the efficacy of ocean afforestation is determined by complicated feedbacks with the Earth system.

In most algae, NO3− assimilation is tightly controlled and is often inhibited by the presence of NH4+. In the marine, non-colonial, non-diazotrophic cyanobacterium Synechococcus UTEX 2380, NO3− assimilation is sensitive to NH4+ only when N does not limit growth. We sequenced the genome of Synechococcus UTEX 2380, studied the genetic organization of the nitrate assimilation related (NAR) genes, and investigated expression and kinetics of the main NAR enzymes, under N or light limitation. We found that Synechococcus UTEX 2380 is a β-cyanobacterium with a full complement of N uptake and assimilation genes and NAR regulatory elements. The nitrate reductase of our strain showed biphasic kinetics, previously observed only in freshwater or soil diazotrophic Synechococcus strains. Nitrite reductase and glutamine synthetase showed little response to our growth treatments, and their activity was usually much higher than that of nitrate reductase. NH4+ insensitivity of NAR genes may be associated with the stimulation of the binding of the regulator NtcA to NAR gene promoters by the high 2-oxoglutarate concentrations produced under N limitation. NH4+ sensitivity in energy-limited cells fits with the fact that, under these conditions, the use of NH4+ rather than NO3− decreases N-assimilation cost, whereas it would exacerbate N shortage under N limitation.

Summary The (maximum) growth rate (µmax) hypothesis predicts that cellular and tissue phosphorus (P) concentrations should increase with increasing growth rate, and RNA should also increase as most of the P is required to make ribosomes.Using published data, we show that though there is a strong positive relationship between the µmax of all photosynthetic organisms and their P content (% dry weight), leading to a relatively constant P productivity, the relationship with RNA content is more complex.In eukaryotes there is a strong positive relationship between µmax and RNA content expressed as % dry weight, and RNA constitutes a relatively constant 25% of total P. In prokaryotes the rRNA operon copy number is the important determinant of the amount of RNA present in the cell. The amount of phospholipid expressed as % dry weight increases with increasing µmax in microalgae. The relative proportions of each of the five major P‐containing constituents is remarkably constant, except that the proportion of RNA is greater and phospholipids smaller in prokaryotic than eukaryotic photosynthetic organisms. The effect of temperature differences between studies was minor.The evidence for and against P‐containing constituents other than RNA being involved with ribosome synthesis and functioning is discussed.

The photon costs of photoreduction/assimilation of nitrate (NO

BACKGROUND AND AIMS: Phosphorus (P) and nitrogen (N) are essential nutrients that frequently limit primary productivity in terrestrial ecosystems. Efficient use of these nutrients is important for plants growing in nutrient-poor environments. Plants generally reduce foliar P concentration in response to low soil P availability. We aimed to assess ecophysiological mechanisms and adaptive strategies for efficient use of P in Banksia attenuata (Proteaceae), naturally occurring on deep sand, and B. sessilis, occurring on shallow sand over laterite or limestone, by comparing the allocation of P among foliar P fractions. METHODS: We carried out pot experiments with slow-growing B. attenuata, which resprouts after fire, and faster growing opportunistic B. sessilis, which is killed by fire, on substrates with different P availability using a randomized complete block design. We measured leaf P and N concentrations, photosynthesis, leaf mass per area, relative growth rate and P allocated to major biochemical fractions in B. attenuata and B. sessilis. KEY RESULTS: The two species had similarly low foliar total P concentrations, but distinct patterns of P allocation to P-containing fractions. The foliar total N concentration of B. sessilis was greater than that of B. attenuata on all substrates. The foliar total P and N concentrations in both species decreased with decreasing P availability. The relative growth rate of both species was positively correlated with concentrations of both foliar nucleic acid P and total N, but there was no correlation with other P fractions. Faster growing B. sessilis allocated more P to nucleic acids than B. attenuata did, but other fractions were similar. CONCLUSIONS: The nutrient allocation patterns in faster growing opportunistic B. sessilis and slower growing B. attenuata revealed different strategies in response to soil P availability which matched their contrasting growth strategy.

Macroalgae are important components on coral reefs that underpin food webs but are also indicators of declines in coral reef condition. The sensitivity of macroalgal communities to environmental factors, including climate and pollutants from the land, is important for the future state of coral reefs. We assessed regional variation in the photosynthetic physiology of macroalgae on the Great Barrier Reef and Moreton Bay, Queensland, Australia, using delta C-13, tissue chemistry, and metabolic indicators over broad environmental gradients from six sites spanning 13 degrees of latitude and varying distance from shore. Our data set included 568 samples from 54 genera and 19 macroalgal orders. The delta C-13 of tissues had a strong taxonomic basis (explaining 66% of the variation in delta C-13), varying significantly among algal orders. Particularly low values of delta C-13, indicating direct use of CO2(aq) in photosynthesis, were mainly associated with lineages within the Rhodophyta. We observed declines in delta C-13 with increasing depth (inshore only) and latitude, from offshore to inshore sites and with increasing tissue N and N : P ratio. Variation in delta C-13 of tissues among macroalgal lineages suggests evolution of a range of CO2 acquisition pathways. Patterns in delta C-13 availability are most consistent with higher availability of CO2(aq) in cooler water and inshore sites. delta C-13 in macroalgae could provide an important tool for monitoring changes in CO2(aq) with increasing atmospheric CO2, with changes in ocean circulation and with changes to runoff from the land to the coastal oceans.

Bloom et al. (2019) proposed that rising atmospheric CO2 concentrations “inhibit malate production in chloroplasts and thus impede assimilation of nitrate into protein of C3 plants, a phenomenon that will strongly influence primary productivity and food security under the environmental conditions anticipated during the next few decades”. Previously we argued that the weight of evidence in the literature indicated that elevated atmospheric [CO2] does not inhibit NO3‐ assimilation in C3 plants (Andrews et al. 2019). New data for common bean (Phaseolus vulgaris) and wheat (Triticum aestivum) were presented that supported this view and indicated that the effects of elevated atmospheric [CO2] on nitrogen (N) assimilation and growth of C3 vascular plants were similar regardless of the form of N assimilated. Bloom et al. (2019) strongly criticised the arguments presented in Andrews et al. (2019). Here we respond to these criticisms and again conclude that the available data indicate that elevated atmospheric [CO2] does not inhibit NO3‐ assimilation of C3 plants. Measurement of the partitioning of NO3‐ assimilation between root and shoot of C3 species under different NO3‐ supply, at ambient and elevated CO2 would determine if their NO3‐ assimilation is inhibited in shoots but enhanced in roots at elevated atmospheric CO2.

Generation of ion electrochemical potential differences by primary active transport can involve energy inputs from light, from exergonic redox reactions and from exergonic ATP hydrolysis. These electrochemical potential differences are important for homoeostasis, for signalling, and for energizing nutrient influx. The three main ions involved are H+, Na+(efflux) and Cl−(influx). In prokaryotes, fluxes of all three of these ions are energized by ion-pumping rhodopsins, with one archaeal rhodopsin pumping H+intothe cells; among eukaryotes there is also an H+influx rhodopsin inAcetabulariaand (probably) H+efflux in diatoms. Bacteriochlorophyll-based photoreactions export H+from the cytosol in some anoxygenic photosynthetic bacteria, but chlorophyll-based photoreactions in marine cyanobacteria do not lead to export of H+. Exergonic redox reactions export H+and Na+in photosynthetic bacteria, and possibly H+in eukaryotic algae. P-type H+- and/or Na+-ATPases occur in almost all of the photosynthetic marine organisms examined. P-type H+-efflux ATPases occur in charophycean marine algae and flowering plants whereas P-type Na+-ATPases predominate in other marine green algae and non-green algae, possibly with H+-ATPases in some cases. An F-type Cl−-ATPase is known to occur inAcetabularia. Some assignments, on the basis of genomic evidence, of P-type ATPases to H+or Na+as the pumped ion are inconclusive.

The efficiency of carbon sequestration by the biological pump could decline in the coming decades because respiration tends to increase more with temperature than photosynthesis. Despite these differences in the short‐term temperature sensitivities of photosynthesis and respiration, it remains unknown whether the long‐term impacts of global warming on metabolic rates of phytoplankton can be modulated by evolutionary adaptation. We found that respiration was consistently more temperature dependent than photosynthesis across 18 diverse marine phytoplankton, resulting in universal declines in the rate of carbon fixation with short‐term increases in temperature. Long‐term experimental evolution under high temperature reversed the short‐term stimulation of metabolic rates, resulting in increased rates of carbon fixation. Our findings suggest that thermal adaptation may therefore have an ameliorating impact on the efficiency of phytoplankton as primary mediators of the biological carbon pump., The primary production of marine phytoplankton could decline with warming if rates of respiration increase more with rising temperature than photosynthesis. We found that respiration was consistently more temperature dependent than photosynthesis across 18 diverse marine phytoplankton taxa, resulting in universal declines in the rate of carbon fixation with short‐term increases in temperature. Whereas, by contrast, long‐term experimental evolution under high temperature acted to reverse the short‐term stimulation of metabolic rates, resulting in increased rates of carbon fixation.

BackgroundMost terrestrial vascular plants can assimilate soil obtained NO3- in their root and shoot.ScopeData from the literature are collated and analysed with respect to genotype and environmental effects on the partitioning of NO3- assimilation between root and shoot of terrestrial vascular plants.ConclusionsTemperate evergreen woody species in the Ericaceae and Pinaceae carry out most of their NO3- assimilation in the root when growing in low (0.5 mM) up to at least 5 mM soil NO3-. The root is the main site of NO3- assimilation for temperate deciduous woody species and perennial and annual herbaceous legume species at 0.5–1 mM NO3- but for many, shoot assimilation increases in importance with increased NO3- supply. Temperate perennial grasses and annual non-legume species and tropical/ sub-tropical species regardless of life-form, carry out a substantial, usually major proportion of their NO3- assimilation in shoots at NO3- concentrations above 0.5 mM. Furthermore, high NH4+ supply, mycorrhizal infection and infection by parasitic plants can increase the proportion of total plant NO3- assimilation carried out in the shoot while abiotic stress and elevated atmospheric [CO2] can cause this to decrease. Shoot NO3- assimilation is an advantage under non-stress conditions due to its positive effect on leaf expansion but can be a disadvantage under freezing and chilling stress conditions. Increased reliance on root NO3- assimilation at elevated CO2 was associated with increased and conversely decreased plant growth and NO3- assimilation depending on study. Resolution of these different findings across studies is an important area for further research.

Across the marine landscape, from estuaries to the open ocean, biota take up silicon (Si) as monosilicic acid and deposit it into their tissues as biogenic silica (BSi). Along the coast, vegetated ecosystems, such as salt marshes and mangroves, sequester a significant amount of Si in their tissues and likely help regulate the availability of Si in surrounding waters (Carey & Fulweiler, 2014; Elizondo et al., 2021). Si is also accumulated by sponges, euglyphid amoebae, radiolarians, silicoflagellates, and choanoflagellates, as well as a few coccolithophores, Prasinophyceae, and picocyanobacteria (Raven & Giordano, 2009; Gadd & Raven, 2010; Baines et al., 2012). The dominant driver of coastal (and open ocean) Si cycling, however, is generally thought to be diatoms. These siliceous phytoplankton require Si on a 1 : 1 molar ratio with nitrogen (N). Diatoms are responsible for 40–50% of global marine primary production (Field et al., 1998; Rousseaux & Gregg, 2013) and form the base of the marine food web in many parts of the ocean, especially coastal temperate regions (Irigoien et al., 2002).

Carbon sequestration is defined as the secure storage of carbon-containing molecules for >100 years, and in the context of carbon dioxide removal for climate mitigation, the origin of this CO2 is from the atmosphere. On land, trees globally sequester substantial amounts of carbon in woody biomass, and an analogous role for seaweeds in ocean carbon sequestration has been suggested. The purposeful expansion of natural seaweed beds and aquaculture systems, including into the open ocean (ocean afforestation), has been proposed as a method of increasing carbon sequestration and use in carbon trading and offset schemes. However, to verify whether CO2 fixed by seaweeds through photosynthesis leads to carbon sequestration is extremely complex in the marine environment compared to terrestrial systems, because of the need to jointly consider: the comparatively rapid turnover of seaweed biomass, tracing the fate of carbon via particulate and dissolved organic carbon pathways in dynamic coastal waters, and the key role of atmosphere-ocean CO2 exchange. We propose a Forensic Carbon Accounting approach, in which a thorough analysis of carbon flows between the atmosphere and ocean, and into and out of seaweeds would be undertaken, for assessing the magnitude of CO2 removal and robust attribution of carbon sequestration to seaweeds.

Our scientific understanding of climate change makes clear the necessity for both emission reduction and carbon dioxide removal (CDR). The ocean with its large surface area, great depths and long coastlines is central to developing CDR approaches commensurate with the scale needed to limit warming to below 2 °C. Many proposed marine CDR approaches rely on spatial upscaling along with enhancement and/or acceleration of the rates of naturally occurring processes. One such approach is 'ocean afforestation', which involves offshore transport and concurrent growth of nearshore macroalgae (seaweed), followed by their export into the deep ocean. The purposeful occupation for months of open ocean waters by macroalgae, which do not naturally occur there, will probably affect offshore ecosystems through a range of biological threats, including altered ocean chemistry and changed microbial physiology and ecology. Here, we present model simulations of ocean afforestation and link these to lessons from other examples of offshore dispersal, including rafting plastic debris, and discuss the ramifications for offshore ecosystems. We explore what additional metrics are required to assess the ecological implications of this proposed CDR. In our opinion, these ecological metrics must have equal weight to CDR capacity in the development of initial trials, pilot studies and potential licensing.