The Mesoproterozoic atmosphere had more CO2 and less O2 than at present. While the upper ocean was oxygenated, the deeper ocean was euxinic or ferruginous. Primary production was performed by Chlorobia, Cyanobacteria, Proteobacteria, and Archaeplastida. Cyanobacteria and Archaeplastida presumably performed oxygenic photosynthesis above the chemocline, while photosynthesis below the chemocline involved anoxygenic photolithotrophic oxidation of S2- or Fe2+ by Proteobacteria and S2- by Chlorobia and Cyanobacteria. Photolithotrophic growth of extant Proteobacteria with Rubisco, and Chlorobia with the reverse tricarboxylic acid cycle, involves diffusive CO2 entry; this likely also occurred under high Mesoproterozoic CO2. By contrast, Cyanobacteria may already have possessed CO2-concentrating mechanisms comparable to extant lineages. Here, we evaluate how differences in primary carboxylation pathways and inorganic carbon acquisition among these groups influence stable carbon isotope fractionation and assess how isotopic signatures in modern representatives inform interpretations of Mesoproterozoic carbon isotope records. Although culture-based fractionation patterns broadly overlap with Mesoproterozoic sedimentary δ13C values, these records are strongly biased towards continental margin settings, and direct evidence for open-ocean planktonic primary producers is lacking. The evidence reviewed highlights that physiological constraints on carbon acquisition and metabolism provide a framework for interpreting Mesoproterozoic carbon isotope signals with implications for primary production and the biological pump.

Marine heatwaves (MHWs) have increased in frequency by 34% since 1990 and are projected to rise further with global ocean change, posing significant risks to marine ecosystems. Kelps (order Laminariales) provide essential habitats and play key ecological roles, but they are increasingly threatened by MHWs. Tasmania, SE Australia, is a global warming hotspot, but the impacts of recurrent MHWs on the physiological performance of the ecologically dominant kelp Ecklonia radiata remain poorly understood. To address this, we investigated how the frequency of MHWs influenced the physiological and biochemical performance of E. radiata, both during and after MHWs, to evaluate immediate responses and recovery potential. In laboratory experiments, juvenile sporophytes were exposed to three experimental treatments: no-MHW, a single 6-day MHW, and double 6-day MHWs followed by 7-day recovery periods. Ecklonia radiata sporophytes were resilient to the single 6-day MHW, but double MHWs negatively impacted recovery, with reduced growth rates and increased tissue erosion. Although photosynthetic rates remained unaffected, changes in pigment ratios and increased antioxidant activity indicated a mitigation of physiological stress. We propose that energy may be diverted from growth toward repair processes and the maintenance of essential functions. These findings suggest there was cumulative stress caused by repeated MHWs, leading to progressive physiological decline. More frequent MHW events may hinder E. radiata's recovery capacity, with potential ecosystem implications, considering its key ecological role.

Trade-offs play vital roles in evolutionary theory, linking organism performance to changing environments in the context of global change. Marine microalgae, as one of the most important groups of primary producers in the biosphere, exhibit significant trade-offs across multiple traits in response to environmental changes, such as elevated CO2 (and consequent ocean acidification) and warming. In this review, we synthesize recent findings on the trade-offs associated with both short-term phenotypic acclimation and long-term genotypic adaptation of marine microalgae. Specifically, we discuss distinct classes of trade-offs (i.e., allocation trade-offs, acquisition trade-offs and specialist-generalist trade-offs) between multiple traits, such as growth rate, photosynthesis, nutrient acquisition, and stress tolerance. We also explored the underlying mechanisms driving these trade-offs. Finally, we discuss the broader ecological consequences of these trade-offs, such as potential shifts in species composition and ecosystem functions, and outline key research directions to better predict marine ecosystem responses to future global change scenarios.

Analysis of trait trade-offs, through which physiological traits requiring common resources are 'traded' to optimize competitive advantage, provides a route to simplify and more readily understand the complexities of ecology. The concept of trait trade-offs has found favour in plankton research, especially directed at phytoplankton, defined here as phototrophs incapable of phagotrophy. Mixoplankton, defined as protists that combine phototrophy and phagotrophy, are now recognized as being widespread and significant members of the protist plankton community; many photoflagellate 'phytoplankton' are actually mixoplankton, as are many '(microbial) zooplankton'. Mixoplankton might be expected to be dominant, being able to exploit different trophic strategies while simultaneously eliminating competitors. That mixoplankton are not dominant suggests that physiological trait trade-offs erode their apparent competitive edge. We present a systematic analysis of potential trait trade-offs in phototrophic protists focused on mixoplankton. We find no clear evidence to support trait trade-off arguments in plankton research, except perhaps for acquired phototrophy in mixoplanktonic ciliates versus zooplanktonic ciliates. Our findings suggest that the presence of various mixoplankton throughout the surface ocean waters is most likely explained by factors other than trait trade-offs. Diversities in mixoplankton form and function thus reflect that evolution of these organisms from very different lineages, provide them with advantages to function competitively in mature ecosystems with complex trophic interplay. Indeed, the complexity of those lineages is inconsistent with core trait trade-off definitions; there is no single ancestral mixoplankton nor a common environment supporting trait-trade-off-directed evolution.

Although the diverse impacts of elevated dissolved CO2 and warming on organisms within various trophic levels in marine food webs are well documented, we have yet to explore the biological links across different levels of biological organization from primary producers to secondary producers on an evolutionary time scale in a high-CO2 ocean. Here, we cultured a model marine diatom Phaeodactylum tricornutum (primary producer) in predicted future high-CO2 and/or warming conditions for similar to 1250 d with an experimental evolution approach and then fed them to the clam Coelomactra antiquata (secondary producer). We present an in-depth multi-omics analysis along the methylome (primary producer)-transcriptome (primary producer)-metabolome (primary producer)-metabolome (secondary producer) continuum. Our results showed that the downregulated terpenoid backbone biosynthesis in the methylome and transcriptome lead to decreased pyruvate levels and upregulation of some pathways (such as phenylalanine metabolism) in the metabolome of the primary producer in the long-term warming conditions. These changes in metabolomic profile in the primary producer were then transferred to the secondary producer, resulting in changes in abundance of some metabolites, such as decreases in pyruvate, and in pyruvaldhyde (also known as methylglyoxal), and increases in 2-hydroxylamino-4,6-dinitrotoluene. Our study provides a new insight into the molecular mechanisms underlying the trophic transfer from primary to secondary producers in a future high-CO2 ocean and may provide more accurate projections of marine ecosystem services and functions over the next century.

Marine microalgae demonstrate a notable capacity to adapt to high CO2 and warming in the context of global change. However, the dynamics of their evolutionary processes under simultaneous high CO₂ and warming conditions remain poorly understood. Here, we analyze the dynamics of evolution in experimental populations of a model marine diatom Phaeodactylum tricornutum. We conducted whole-genome resequencing of populations under ambient, high-CO2, warming and high CO2 + warming at 2-yr intervals over a 4-yr adaptation period. The common genes selected between 2- and 4-yr adaptation were found to be involved in protein ubiquitination and degradation and the tricarboxylic acid (TCA) cycle, and were consistently selected regardless of the experimental conditions or adaptation duration. The unique genes selected only by 4-yr adaptation function in respiration, fatty acid, and amino acid metabolism, facilitating adaptation to prolonged high CO2 with warming conditions. Corresponding changes at the metabolomic level, with significant alterations in metabolites abundances involved in these pathways, support the genomic findings. Our study, integrating genomic and metabolomic data, demonstrates that long-term adaptation of microalgae to high CO2 and/or warming can be characterized by a complex and dynamic genetic process and may advance our understanding of microalgae adaptation to global change.

Kelp forests are threatened by ocean warming, yet effects of co-occurring drivers such as CO 2 are rarely considered when predicting their performance in the future. In Australia, the kelp Ecklonia radiata forms extensive forests across seawater temperatures of approximately 7–26°C. Cool-edge populations are typically considered more thermally tolerant than their warm-edge counterparts but this ignores the possibility of local adaptation. Moreover, it is unknown whether elevated CO 2 can mitigate negative effects of warming. To identify whether elevated CO 2 could improve thermal performance of a cool-edge population of E. radiata , we constructed thermal performance curves for growth and photosynthesis, under both current and elevated CO 2 (approx. 400 and 1000 µatm). We then modelled annual performance under warming scenarios to highlight thermal susceptibility. Elevated CO 2 had minimal effect on growth but increased photosynthesis around the thermal optimum. Thermal optima were approximately 16°C for growth and approximately 18°C for photosynthesis, and modelled performance indicated cool-edge populations may be vulnerable in the future. Our findings demonstrate that elevated CO 2 is unlikely to offset negative effects of ocean warming on the kelp E. radiata and highlight the potential susceptibility of cool-edge populations to ocean warming.

Zostera marina is a seagrass, a group of angiosperms that evolved from land to live submerged in seawater, an environment of high salinity, alkaline pH and usually very low NO3 - . In 2000, we reported the first physiological evidence for the Na+ -dependent high-affinity NO3 - uptake in this plant. Now, to determine the molecular identity of this process, we searched for NO3 - transporters common to other vascular plants encoded in Z. marina's genome. We cloned two candidates, ZosmaNPF6.3 and ZosmaNRT2 with its partner protein ZosmaNAR2. ZosmaNAR2 expression levels increase up to 4.5-fold in Z. marina leaves under NO3 - -deficiency, while ZosmaNRT2 and ZosmaNPF6.3 expressions were low and unaffected by NO3 - . NO3 - transport capacity, kinetic properties and H+ or Na+ -dependence were examined by heterologous expression in the Hansenula polymorpha high-affinity NO3 - transporter gene disrupted strain (∆ynt1). ZosmaNPF6.3 functions as a H+ -dependent NO3 - transporter, without functionality at alkaline pH and apparent dual kinetics (KM  = 11.1 µM at NO3 - concentrations below 50 µM). ZosmaNRT2 transports NO3 - in a H+ -independent but Na+ -dependent manner (KM  = 1 mM Na+ ), with low NO3 - affinity (KM  = 30 µM). When ZosmaNRT2 and ZosmaNAR2 are co-expressed, a Na+ -dependent high-affinity NO3 - transport occurs (KM  = 5.7 µM NO3 - ), mimicking the in vivo value. These results are discussed in the physiological context, providing evidence that ZosmaNRT2 is a Na+ -dependent high-affinity NO3 - transporter, the first of its kind to be functionally characterised in a vascular plant, that requires ZosmaNAR2 to achieve the necessary high-affinity for nitrate uptake from seawater.

The essential elements Ca and P, taken up and used metabolically as Ca2+ and H2PO4-/HPO42- respectively, could precipitate as one or more of the insoluble forms calcium phosphate (mainly apatite) if the free ion concentrations and pH are high enough. In the cytosol, chloroplast stroma, and mitochondrial matrix, the very low free Ca2+ concentration avoids calcium phosphate precipitation, apart from occasionally in the mitochondrial matrix. The low free Ca2+ concentration in these compartments is commonly thought of in terms of the role of Ca2+ in signalling. However, it also helps avoids calcium phosphate precipitation, and this could be its earliest function in evolution. In vacuoles, cell walls, and xylem conduits, there can be relatively high concentrations of Ca2+ and inorganic orthophosphate, but pH and/or other ligands for Ca2+, suggests that calcium phosphate precipitates are rare. However, apatite is precipitated under metabolic control in shoot trichomes, and by evaporative water loss in hydathodes, in some terrestrial flowering plants. In aquatic macrophytes that deposit CaCO3 on their cell walls or in their environment as a result of pH increase or removal of inhibitors of nucleation or crystal growth, phosphate is sometimes incorporated in the CaCO3. Calcium phosphate precipitation also occurs in some stromatolites.