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The study by Franz et al. provides exciting new insights into deep biosphere processes during the 'boring billion', indicating that fungi-like eukaryotic organisms developed before 1 Ga and that a deep continental biosphere was already present in the Early Mesoproterozoic/Late Paleoproterozoic.

Reaching the Paris Agreement targets to limit global warming to 1.5 or 2°C implies not only reducing emissions, but also active carbon dioxide removal from the atmosphere. While land-based carbon dioxide removal or negative emission technologies have received most attention, ocean solutions are increasingly being considered. Ocean alkalinity enhancement (OAE), or alkalinization, is one promising ocean-based carbon dioxide technology. However, any new carbon dioxide removal technique needs thorough investigations for its effectiveness, longevity, benefits and lack of disbenefits, financial viability, social acceptance and governability. The paper by Bach is a nice illustration of the type of research that must be done if we are to consider large scale application of OAE. It focuses on the additionality problem of ocean alkalinity enhancement, specifically it investigates how the addition of alkalinity modifies the natural alkalinity cycle and in that way the efficiency of carbon dioxide sequestration.

The study covers a relevant contemporary topic (wildfire) and focusses on the recent major European summer of wildfires. The authors demonstrate that the assessment of burned area alone is not enough to fully characterize the impact of fires. Novel methods to assess burned area and above ground biomass losses are described to provide a more informative impact-based characterization of fires.

Marine warming and deoxygenation are projected to intensify and drive a relative decrease in global habitat viability penetrating to all depths with warming dominating at the surface and deoxygenation becomes increasingly important with depth. In a 2°C scenario of global warming, epipelagic species' habitat losses are generally in the order of 0.1-0.5 million km3, while mesopelagic habitat losses are 0.01-0.15 million km3 and demersal losses are in the order of about 0.00025 million km3.

Arctic greenhouse gas fluxes are key for climate feedback but the Arctic greenhouse gas balance is poorly constrained due to a limited understanding of the spatial variation in these fluxes. This study combines extensive chamber-based flux measurements and remote sensing data to develop a machine-learning model to predict greenhouse gas fluxes across a tundra landscape in Finland. The analysis revealed that the system was a net greenhouse gas sink and showed widespread CH4 uptake in upland vegetation types, almost surpassing the high wetland CH4 emissions at the landscape scale.

This study is the first to quantify land carbon cycle feedbacks under decreasing atmospheric CO2 concentration following negative CO2 emissions and compare them to feedbacks under positive emissions. The novel approach presented here reduces the carbon cycle inertia in the phase where atmospheric CO2 concentration decreases in order to improve the quantification of carbon cycle feedbacks under negative emissions. This approach reduced the effectivity of negative emissions in reducing atmospheric CO2, due to larger concentration-carbon and climate-carbon feedbacks.

The paper by Turner and co-authors tackles the very timely question on how well we can reconstruct carbon inventories given the sparse observations. Using an Ensemble Optimal Interpolation approach and synthetic observations, the authors show that a large fraction of ocean carbon and its variability can be reconstructed using temperature and salinity measurements in the top 100 meter, however, reconstruction skill decreases when the top 2000 meters are considered. The authors propose a new way to use sparse observations to better understand historical carbon cycle changes, i.e., an important quantity in light of future changes driven by man-made emissions.

This is an excellent review of gross primary production (GPP) and net community production (NCP) measurements in the world's oceans using a new fleet of autonomous ocean-going instruments called biogeochemical-Argo (BGC-Argo) floats. The number of BGC-Argo floats has recently been surging with their users, so this review is timely and helpful for many oceanographers and biogeochemists to understand the advantages and challenges in the measurements of GPP and NCP in the oceans with the recently emerged technology.

The development of hyperspectral core scanning has facilitated high spatial/temporal pigment reconstructions indicative of changes in bioproductivity and was so far limited to those bacteriochlorophylls derived from Chromatiaceae (purple sulfur bacteria) to reconstruct changes in water column redox conditions and meromixis. This is the first study that reconstructs and quantifies Chlorobi-derived bacteriochlorophylls and its derivatives at high resolution in lake sediments. Therefore, this paper yields more insight into temporal variations of oxygenic and anoxygenic phototrophic communities, and provides a non-destructive tool for more detailed reconstructions of photic zone euxinia in meromictic lakes.

Intensifying wildfire regimes in many parts of the world are increasing the production of pyrogenic organic matter (PyOM), with potential implications for water supplies that are critical for domestic, industrial, agricultural, and ecological needs. This study provides a novel assessment of the influence of PyOM on aquatic ecosystems and showed that PyOM can be actively transformed in aquatic ecosystems and may be an increasing source of C emissions to the atmosphere as the prevalence of wildfires increases.

Nitrite is a nexus in nitrogen turnover, especially in oxygen minimum zones. This study works out new details of its specific role and adds new evidence for anaerobic nitrite oxidation. Furthermore, it shows that nitrogen recycling (nitrate reduction and nitrite oxidation) is quantitatively more important than loss processes.

This study documents the formation of Fe, Mn and Si amorphous mineral phases by very small cyanobacteria living in oxic, alkaline lakes. This finding has implications for using sedimentary Fe/Mn enrichments as a proxy for redox cycling in the past and extends the documented evidence for biomineralization of Si-rich phases.