Nathan G. Walworth

Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC's reports indicate that rapid abatement of CO₂ emissions and development of carbon dioxide removal (CDR) strategies are needed to address legacy and difficult-to-abate emissions sources. These CDR methods must efficiently and safely sequester gigatons of atmospheric CO₂. Coastal enhanced weathering (CEW) via the addition of the common mineral olivine to coastal waters is one promising approach to enhance ocean alkalinity for large-scale CDR. As olivine weathers, it releases several biologically active dissolution products, including alkalinity, trace metals, and the nutrient silicate. Released trace metals can serve as micronutrients but may also be toxic at high concentrations to marine biota, including phytoplankton, which lie at the base of marine food webs. We grew six species representing several globally important phytoplankton species under elevated concentrations of olivine dissolution products via a synthetic olivine leachate (OL) based on olivine's elemental composition. We monitored their physiological and biogeochemical responses, which allowed us to determine physiological impacts and thresholds at elevated olivine leachate concentrations, in addition to individual effects of specific constituents. We found both positive and neutral responses but no evident toxic effects for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria. In both single and competitive co-cultures, silicifiers and calcifiers benefited from olivine dissolution products like iron and silicate or enhanced alkalinity, respectively. The non-N₂-fixing picocyanobacterium could use synthetic olivine-derived iron for growth, while N₂-fixing cyanobacteria could not. However, other trace metals like nickel and cobalt supported cyanobacterial growth across both groups. Growth benefits to phytoplankton groups in situ will depend on species-specific responses and ambient concentrations of other required nutrients. Results suggest olivine dissolution products appear unlikely to cause negative physiological effects for any of the phytoplankton examined, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term eco-evolutionary responses to olivine exposure and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.

Marine microbes like diatoms make up the base of marine food webs and drive global nutrient cycles. Despite their key roles in ecology, biogeochemistry, and biotechnology, we have limited empirical data on how forces other than adaptation may drive diatom diversification, especially in the absence of environmental change. One key feature of diatom populations is frequent extreme reductions in population size, which can occur both in situ and ex situ as part of bloom-and-bust growth dynamics. This can drive divergence between closely related lineages, even in the absence of environmental differences. Here, we combine experimental evolution and transcriptome landscapes (t-scapes) to reveal repeated evolutionary divergence within several species of diatoms in a constant environment. We show that most of the transcriptional divergence can be captured on a reduced set of axes, and that repeatable evolution can occur along a single major axis of variation defined by core ortholog expression comprising common metabolic pathways. Previous work has associated specific transcriptional changes in gene networks with environmental factors. Here, we find that these same gene networks diverge in the absence of environmental change, suggesting these pathways may be central in generating phenotypic diversity as a result of both selective and random evolutionary forces. If this is the case, these genes and the functions they encode may represent universal axes of variation. Such axes that capture suites of interacting transcriptional changes during diversification improve our understanding of both global patterns in local adaptation and microdiversity, as well as evolutionary forces shaping algal cultivation.

One of the greatest threats facing the planet is the continued increase in excess greenhouse gasses, with CO₂ being the primary driver due to its rapid increase in only a century. Excess CO₂ is exacerbating known climate tipping points that will have cascading local and global effects including loss of biodiversity, global warming, and climate migration. However, global reduction of CO₂ emissions is not enough. Carbon dioxide removal (CDR) will also be needed to avoid the catastrophic effects of global warming. Although the drawdown and storage of CO₂ occur naturally via the coupling of the silicate and carbonate cycles, they operate over geological timescales (thousands of years). Here, we suggest that microbes can be used to accelerate this process, perhaps by orders of magnitude, while simultaneously producing potentially valuable by-products. This could provide both a sustainable pathway for global drawdown of CO₂ and an environmentally benign biosynthesis of materials. We discuss several different approaches, all of which involve enhancing the rate of silicate weathering. We use the silicate mineral olivine as a case study because of its favorable weathering properties, global abundance, and growing interest in CDR applications. Extensive research is needed to determine both the upper limit of the rate of silicate dissolution and its potential to economically scale to draw down significant amounts (Mt/Gt) of CO₂. Other industrial processes have successfully cultivated microbial consortia to provide valuable services at scale (e.g., wastewater treatment, anaerobic digestion, fermentation), and we argue that similar economies of scale could be achieved from this research.

Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC calls for both the rapid abatement of CO₂ emissions and development of carbon dioxide removal (CDR) strategies that can address legacy emissions and difficult to abate emissions sources. These CDR methods must efficiently and safely sequester gigatons of atmospheric CO₂. Coastal Enhanced Weathering (CEW) via the addition of the common mineral olivine to coastal waters is one promising approach to enhance ocean alkalinity for large-scale CDR. As olivine weathers, it releases several biologically active dissolution products, including alkalinity, trace metals, and the nutrient silicate. Released trace metals can serve as micronutrients but may also be toxic at high concentrations to marine biota including phytoplankton that lie at the base of marine food webs. We grew several globally important phytoplankton functional groups under elevated concentrations of olivine dissolution products using a synthetic olivine leachate (OL) based on olivine elemental composition, and monitored their physiological and biogeochemical responses. This allowed us to determine physiological impacts and thresholds at elevated olivine leachate concentrations, in addition to individual effects of specific constituents. We found both positive and neutral responses but no evident toxic effects for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria. In both single and competitive co-cultures, silicifiers and calcifiers benefited from olivine dissolution products like iron and silicate or enhanced alkalinity, respectively. The non-N₂-fixing picocyanobacterium could use synthetic olivine-derived iron for growth, while N₂-fixing cyanobacteria could not. However, other trace metals like nickel and cobalt supported cyanobacterial growth across both groups. Growth benefits to particular phytoplankton groups in situ will depend on species-specific responses and ambient concentrations of other required nutrients. Results suggest olivine dissolution products appear unlikely to cause negative effects for marine phytoplankton, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term evolutionary responses to olivine exposure, and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.

Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC calls for both the rapid abatement of CO₂ emissions and development of carbon dioxide removal (CDR) strategies that can address legacy emissions and difficult to abate emissions sources. These CDR methods must efficiently and safely sequester gigatons of atmospheric CO₂. Coastal Enhanced Weathering (CEW) via the addition of the common mineral olivine to coastal waters is one promising approach to enhance ocean alkalinity for large-scale CDR. As olivine weathers, it releases several biologically active dissolution products, including alkalinity, trace metals, and the nutrient silicate. Released trace metals can serve as micronutrients but may also be toxic at high concentrations to marine biota including phytoplankton that lie at the base of marine food webs. We grew several globally important phytoplankton functional groups under elevated concentrations of olivine dissolution products using a synthetic olivine leachate (OL) based on olivine elemental composition, and monitored their physiological and biogeochemical responses. This allowed us to determine physiological impacts and thresholds at elevated olivine leachate concentrations, in addition to individual effects of specific constituents. We found both positive and neutral responses but no evident toxic effects for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria. In both single and competitive co-cultures, silicifiers and calcifiers benefited from olivine dissolution products like iron and silicate or enhanced alkalinity, respectively. The non-N₂-fixing picocyanobacterium could use synthetic olivine-derived iron for growth, while N₂-fixing cyanobacteria could not. However, other trace metals like nickel and cobalt supported cyanobacterial growth across both groups. Growth benefits to particular phytoplankton groups in situ will depend on species-specific responses and ambient concentrations of other required nutrients. Results suggest olivine dissolution products appear unlikely to cause negative effects for marine phytoplankton, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term evolutionary responses to olivine exposure, and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.

Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC calls for both the rapid abatement of CO₂ emissions and development of carbon dioxide removal (CDR) strategies that can address legacy emissions and difficult to abate emissions sources. These CDR methods must efficiently and safely sequester gigatons of atmospheric CO₂. Coastal Enhanced Weathering (CEW) via the addition of the common mineral olivine to coastal waters is one promising approach to enhance ocean alkalinity for large-scale CDR. As olivine weathers, it releases several biologically active dissolution products, including alkalinity, trace metals, and the nutrient silicate. Released trace metals can serve as micronutrients but may also be toxic at high concentrations to marine biota including phytoplankton that lie at the base of marine food webs. We grew several globally important phytoplankton functional groups under elevated concentrations of olivine dissolution products using a synthetic olivine leachate (OL) based on olivine elemental composition, and monitored their physiological and biogeochemical responses. This allowed us to determine physiological impacts and thresholds at elevated olivine leachate concentrations, in addition to individual effects of specific constituents. We found both positive and neutral responses but no evident toxic effects for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria. In both single and competitive co-cultures, silicifiers and calcifiers benefited from olivine dissolution products like iron and silicate or enhanced alkalinity, respectively. The non-N₂-fixing picocyanobacterium could use synthetic olivine-derived iron for growth, while N₂-fixing cyanobacteria could not. However, other trace metals like nickel and cobalt supported cyanobacterial growth across both groups. Growth benefits to particular phytoplankton groups in situ will depend on species-specific responses and ambient concentrations of other required nutrients. Results suggest olivine dissolution products appear unlikely to cause negative effects for marine phytoplankton, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term evolutionary responses to olivine exposure, and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.

Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC reports indicate that rapid abatement of CO ₂ emissions and development of carbon dioxide removal (CDR) strategies are needed to address legacy and difficult to abate emissions sources. These CDR methods must efficiently and safely sequester gigatons of atmospheric CO ₂ . Coastal Enhanced Weathering (CEW) via the addition of the common mineral olivine to coastal waters is one promising approach to enhance ocean alkalinity for large-scale CDR. As olivine weathers, it releases several biologically active dissolution products, including alkalinity, trace metals, and the nutrient silicate. Released trace metals can serve as micronutrients but may also be toxic at high concentrations to marine biota including phytoplankton that lie at the base of marine food webs. We grew six species representing several globally important phytoplankton species under elevated concentrations of olivine dissolution products via a synthetic olivine leachate (OL) based on olivine elemental composition. We monitored their physiological and biogeochemical responses, which allowed us to determine physiological impacts and thresholds at elevated olivine leachate concentrations, in addition to individual effects of specific constituents. We found both positive and neutral responses but no evident toxic effects for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria. In both single and competitive co-cultures, silicifiers and calcifiers benefited from olivine dissolution products like iron and silicate or enhanced alkalinity, respectively. The non-N ₂ -fixing picocyanobacterium could use synthetic olivine-derived iron for growth, while N ₂ -fixing cyanobacteria could not. However, other trace metals like nickel and cobalt supported cyanobacterial growth across both groups. Growth benefits to phytoplankton groups in situ will depend on species-specific responses and ambient concentrations of other required nutrients. Results suggest olivine dissolution products appear unlikely to cause negative physiological effects for any of the phytoplankton examined, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term eco-evolutionary responses to olivine exposure and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.

Unicellular photosynthetic marine microbes, or phytoplankton, make up the base of marine food webs and drive global nutrient cycles. Despite their key roles in ecology and biogeochemistry, we have a limited understanding of how the basic features of their demographics along with dynamic environments affect trait evolution. A key feature of diatom ecology is frequent extreme reductions in population size, both as part of their bloom-and-bust growth dynamics, and as a result of living within ocean currents. Here, we use experimental evolution to understand which metabolic pathways and functions readily diversify in diatom populations following population bottleneck events. We subjected replicate populations of six genetically distinct diatom strains to population bottlenecks and then subsequently allowed them to evolve as large populations in the absence of environmental change. Phylogenies and global expression of orthologs were generally strain-specific, indicating that vertical (inherited) evolutionary constraints largely determine the occupation of specific locations in the transcriptional landscape (i.e. tran-scape). Following bottlenecks and subsequent evolution as large populations, transcriptional networks of most populations returned to those of the ancestral population. However, at least one replicate population per lineage migrated in the tran-scape, demonstrating that evolutionary changes in gene expression patterns and transcriptional relationships can be driven by population bottlenecks even in the absence of environmental change. Importantly, the orthologs dominating transcriptional diversification resided in common, central metabolic pathways. These data advance our understanding of constraints and patterns of transcriptional relationships underlying trait evolution in microbes that drive global food webs and elemental cycles.

Marine diatoms are important primary producers that can undergo multitrait changes in response to environmental or demographic perturbations. Here, we investigated how flexible multitrait correlations are over hundreds of generations, and how lineage diversity and phenotype diversity are related. We used a perturbation-evolution experiment with Thalassiosira diatoms in which they were “knocked-off” their ancestral multitrait phenotype peak using repeated population bottlenecks, and were then allowed to grow unhindered as large populations. We found that most multitrait phenotypes were isolated but that in 10% of cases, phenotypes were connected by viable trajectories. Following the bottleneck perturbation, evolving in a +4°C environment favoured the discovery of novel multitrait phenotypes, while evolving in the ancestral environment most often resulted in the ancestral phenotype re-evolving. Finally, both trait values and trait correlations evolved in this experiment, with the most drastic changes in trait correlations being associated with lower growth rates. Our findings demonstrate that demographic events that result in decreases in fitness, such as population bottlenecks, have the potential to play an important role in the realised connectivity of phenotypes, as well as in shaping the relationship between lineage and phenotypic diversity in diatoms.