Marine dissolved organic matter (DOM) holds similar to 660 billion metric tons of carbon, making it one of Earth's major carbon reservoirs that is exchangeable with the atmosphere on annual to millennial time scales. The global ocean scale dynamics of the pool have become better illuminated over the past few decades, and those are very briefly described here. What is still far from understood is the dynamical control on this pool at the molecular level; in the case of this Special Issue, the role of microgels is poorly known. This manuscript provides the global context of a large pool of marine DOM upon which those missing insights can be built.

We describe a framework for incorporating cross-disciplinary oceanographic research into high school curriculum modules and discuss how this framework could be adopted broadly by ocean scientists to build cohesive broader impacts programs nested within individual oceanographic research programs. The framework has brought ocean science to over one million students in the form of "curriculum modules," one of which has been adopted as an official high school curriculum by the California State Board of Education. The framework for developing these curricular modules is easy to replicate and could help to scale up education and outreach efforts to advance ocean science in classrooms.

The US National Science Foundation (NSF) requires grant-funded research to include broader impacts activities. According to NSF's Proposal and Award Policies and Procedures Guide, broader impacts activities should "benefit society and contribute to the achievement of specific, desired societal outcomes" while connecting to the funded research. NSF expects broader impacts activities to be inclusive of underrepresented groups in science, technology, engineering, and mathematics (STEM). While scientists acknowledge the significance of integrating outreach into their research programs, there are few models for achieving sustained, long-term, scalable impacts for underrepresented groups. The goal of this commentary is to empower our colleagues to broadly disseminate their research to K–12 school systems.

The interseasonal and interannual variability of total organic carbon (TOC) was assessed in the deep northeastern Pacific Ocean, an area characterized by high primary production and organic matter export. Samples were collected from throughout the deeper water column (>250 m) seasonally in 2017 and 2018 along the Line P transect, as well as one distribution of dissolved organic carbon (DOC) in spring 2018. High heterogeneity was observed in TOC concentrations at depths greater than 1,000 m, both within seasons (concentration differences of ~2–6 μmol kg^-1) and across seasons (~2–12 μmol kg^-1), coinciding with changes in fluorescence in the overlying waters. Such observations suggest that biogenic particles sinking from the upper ocean are seasonally delivering observable TOC to depth. The presence of these particles also appeared to contribute to the DOC pool, as suggested by differences in the TOC and DOC distributions in spring 2018. Seasonal TOC net accumulation and removal rates differed between the years: 0.5 and 2.1 μmol kg^-1 day^-1 (accumulation) and 0.6 and 2.1 μmol kg^-1 day^-1 (removal) for 2017 and 2018, respectively. The rate estimates indicated that introduction of organic carbon to the bathypelagic occurred at approximately the same rate as removal post-bloom, demonstrating the efficient removal of seasonally produced organic carbon. High abundances of gelatinous zooplankton in spring 2018, supported by higher abundances of phytoplankton, enhanced the export of organic carbon to the bathypelagic zone during the seasonal bloom, resulting in localized TOC concentrations up to 148 μmol kg^-1 in the bathypelagic. These results indicate high variability in bathypelagic TOC concentrations at high latitude, unlike oligotrophic systems.

Lipid accumulation due to nitrogen depletion has been studied extensively in Chlamydomonas reinhardtii and the metabolic changes that lead to triacylglycerol biosynthesis have been of particular interest to researchers in the biodiesel industry. The induction of programmed cell death (PCD) in response to nitrogen starvation has also been documented in related chlorophytes. Here, we examined the temporal and metabolic overlap of lipid accumulation and PCD in response to nitrogen starvation in the important model organism C. reinhardtii. Nitrogen starvation induced physiological stress, measured by the progressive decline in chlorophyll a fluorescence, reduced photosynthetic efficiency and decreased growth. In keeping with previous reports, cells accumulated lipids reaching a peak after 2–3 days. At the same time, DNA nicking and caspase‐like protease activity was observed in a proportion of cells, and ultrastructural observations confirmed that death was via PCD. Our results demonstrate that DNA nicking and caspase‐like activity are observed during PCD in C. reinhardtii in response to nitrogen starvation, and that death occurs at the same time as lipid biosynthesis. Microalgal lipid production due to nitrogen depletion in C. reinhardtii is limited by the decrease in culture growth and knowing that the loss of culture density is, at least in part, due to PCD is important for the biotechnology industry.

Field investigations of the properties of heavily melted "rotten" Arctic sea ice were carried out on shorefast and drifting ice off the coast of Utqiagvik (formerly Barrow), Alaska, during the melt season. While no formal criteria exist to qualify when ice becomes rotten, the objective of this study was to sample melting ice at the point at which its structural and optical properties are sufficiently advanced beyond the peak of the summer season. Baseline data on the physical (temperature, salinity, density, microstructure) and optical (light scattering) properties of shorefast ice were recorded in May and June 2015. In July of both 2015 and 2017, small boats were used to access drifting rotten ice within ~32 km of Utqiagvik. Measurements showed that pore space increased as ice temperature increased (–8 to 0°C), ice salinity decreased (10 to 0 ppt), and bulk density decreased (0.9 to 0.6 g cm^-3). Changes in pore space were characterized with thin-section microphotography and X-ray micro-computed tomography in the laboratory. These analyses yielded changes in average brine inclusion number density (which decreased from 32 to 0.01 mm^-3), mean pore size (which increased from 80 μm to 3 mm), and total porosity (increased from 0% to > 45%) and structural anisotropy (variable, with values of generally less than 0.7). Additionally, light-scattering coefficients of the ice increased from approximately 0.06 to > 0.35 cm^-1 as the ice melt progressed. Together, these findings indicate that the properties of Arctic sea ice at the end of melt season are significantly distinct from those of often-studied summertime ice. If such rotten ice were to become more prevalent in a warmer Arctic with longer melt seasons, this could have implications for the exchange of fluid and heat at the ocean surface.

The fate of diatoms in future acidified oceans could have dramatic implications on marine ecosystems, because they account for ~40% of marine primary production. Here, we quantify resilience of Thalassiosira pseudonana in mid-20th century (300 ppm CO₂) and future (1000 ppm CO₂) conditions that cause ocean acidification, using a stress test that probes its ability to recover from incrementally higher amount of low-dose ultraviolet A (UVA) and B (UVB) radiation and re-initiate growth in day–night cycles, limited by nitrogen. While all cultures eventually collapse, those growing at 300 ppm CO₂ succumb sooner. The underlying mechanism for collapse appears to be a system failure resulting from "loss of relational resilience," that is, inability to adopt physiological states matched to N-availability and phase of the diurnal cycle. Importantly, under elevated CO₂ conditions diatoms sustain relational resilience over a longer timeframe, demonstrating increased resilience to future acidified ocean conditions. This stress test framework can be extended to evaluate and predict how various climate change associated stressors may impact microbial community resilience.

Diatoms are responsible for ~40% of marine primary productivity, fuelling the oceanic carbon cycle and contributing to natural carbon sequestration in the deep ocean. Diatoms rely on energetically expensive carbon concentrating mechanisms (CCMs) to fix carbon efficiently at modern levels of CO₂. How diatoms may respond over the short and long term to rising atmospheric CO₂ remains an open question. Here we use nitrate-limited chemostats to show that the model diatom Thalassiosira pseudonana rapidly responds to increasing CO₂ by differentially expressing gene clusters that regulate transcription and chromosome folding, and subsequently reduces transcription of photosynthesis and respiration gene clusters under steady-state elevated CO₂. These results suggest that exposure to elevated CO₂ first causes a shift in regulation, and then a metabolic rearrangement. Genes in one CO₂-responsive cluster included CCM and photorespiration genes that share a putative cAMP-responsive cis-regulatory sequence, implying these genes are co-regulated in response to CO₂, with cAMP as an intermediate messenger. We verified cAMP-induced downregulation of CCM gene δ-CA3 in nutrient-replete diatom cultures by inhibiting the hydrolysis of cAMP. These results indicate an important role for cAMP in downregulating CCM and photorespiration genes under elevated CO₂ and provide insights into mechanisms of diatom acclimation in response to climate change.

Marine diatoms are important primary producers that thrive in diverse and dynamic environments. They do so, in theory, by sensing changing conditions and adapting their physiology accordingly. Using the model species Thalassiosira pseudonana, we conducted a detailed physiological and transcriptomic survey to measure the recurrent transcriptional changes that characterize typical diatom growth in batch culture. Roughly 40% of the transcriptome varied significantly and recurrently, reflecting large, reproducible cell-state transitions between four principal states: (i) "dawn," following 12 h of darkness; (ii) "dusk," following 12 h of light; (iii) exponential growth and nutrient repletion; and (iv) stationary phase and nutrient depletion. Increases in expression of thousands of genes at the end of the reoccurring dark periods (dawn), including those involved in photosynthesis (e.g., ribulose-1,5-bisphosphate carboxylase oxygenase genes rbcS and rbcL), imply large-scale anticipatory circadian mechanisms at the level of gene regulation. Repeated shifts in the transcript levels of hundreds of genes encoding sensory, signaling, and regulatory functions accompanied the four cell-state transitions, providing a preliminary map of the highly coordinated gene regulatory program under varying conditions. Several putative light sensing and signaling proteins were associated with recurrent diel transitions, suggesting that these genes may be involved in light-sensitive and circadian regulation of cell state. These results begin to explain, in comprehensive detail, how the diatom gene regulatory program operates under varying environmental conditions. Detailed knowledge of this dynamic molecular process will be invaluable for new hypothesis generation and the interpretation of genetic, environmental, and metatranscriptomic data from field studies.