Misconceptions of the marine biological carbon pump in a changing climate: Thinking outside the "export" box.

The marine biological carbon pump (BCP) stores carbon in the ocean interior, isolating it from exchange with the atmosphere and thereby coregulating atmospheric carbon dioxide (CO2 ). As the BCP commonly is equated with the flux of organic material to the ocean interior, termed "export flux," a change in export flux is perceived to directly impact atmospheric CO2 , and thus climate. Here, we recap how this perception contrasts with current understanding of the BCP, emphasizing the lack of a direct relationship between global export flux and atmospheric CO2 . We argue for the use of the storage of carbon of biological origin in the ocean interior as a diagnostic that directly relates to atmospheric CO2 , as a way forward to quantify the changes in the BCP in a changing climate. The diagnostic is conveniently applicable to both climate model data and increasingly available observational data. It can explain a seemingly paradoxical response under anthropogenic climate change: Despite a decrease in export flux, the BCP intensifies due to a longer reemergence time of biogenically stored carbon back to the ocean surface and thereby provides a negative feedback to increasing atmospheric CO2 . This feedback is notably small compared with anthropogenic CO2 emissions and other carbon-climate feedbacks. In this Opinion paper, we advocate for a comprehensive view of the BCP's impact on atmospheric CO2 , providing a prerequisite for assessing the effectiveness of marine CO2 removal approaches that target marine biology.

Implications of plastic pollution on global marine carbon cycling and climate.

Plastic pollution can both chemically and physically impede marine biota. But it can also provide novel substrates for colonization, and its leachate might stimulate phytoplankton growth. Plastic contains carbon, which is released into the environment upon breakdown. All of these mechanisms have been proposed to contribute global impacts on open ocean carbon cycling and climate from ubiquitous plastic pollution. Laboratory studies produce compelling data showing both stimulation and inhibition of primary producers and disruption of predatory lifecycles at individual scale, but global carbon cycle impacts remain mostly unquantified. Preliminary modelling estimates ecosystem alterations and direct carbon release due to plastic pollution will remain vastly less disruptive to global carbon cycling than the direct damage wrought by fossil fuel carbon emissions. But when considered by mass, carbon in the form of bulky, persistent plastic particles may be disproportionally more influential on biogeochemical cycling than carbon as a gas in the atmosphere or as a dissolved component of seawater. Thus, future research should pay particular attention to the optical and other physical effects of marine plastic pollution on Earth system and ecological function, and resulting impacts on oxygen and nutrient cycling. Improved understanding of the breakdown of plastics in the marine environment should also be considered high-priority, as any potential perturbation of biological carbon cycling by plastic pollution is climate-relevant on centennial timescales and longer.

Enhanced silica export in a future ocean triggers global diatom decline.

Diatoms account for up to 40% of marine primary production1,2 and require silicic acid to grow and build their opal shell3. On the physiological and ecological level, diatoms are thought to be resistant to, or even benefit from, ocean acidification4-6. Yet, global-scale responses and implications for biogeochemical cycles in the future ocean remain largely unknown. Here we conducted five in situ mesocosm experiments with natural plankton communities in different biomes and find that ocean acidification increases the elemental ratio of silicon (Si) to nitrogen (N) of sinking biogenic matter by 17 ± 6 per cent under [Formula: see text] conditions projected for the year 2100. This shift in Si:N seems to be caused by slower chemical dissolution of silica at decreasing seawater pH. We test this finding with global sediment trap data, which confirm a widespread influence of pH on Si:N in the oceanic water column. Earth system model simulations show that a future pH-driven decrease in silica dissolution of sinking material reduces the availability of silicic acid in the surface ocean, triggering a global decline of diatoms by 13-26 per cent due to ocean acidification by the year 2200. This outcome contrasts sharply with the conclusions of previous experimental studies, thereby illustrating how our current understanding of biological impacts of ocean change can be considerably altered at the global scale through unexpected feedback mechanisms in the Earth system.

What are small-size microplastic distributions telling us?

Writing in this issue of Global Change Biology, Shiye Zhao and co-authors report a microplastic soup extending thousands of meters below the South Atlantic Subtropical Gyre. This discovery contributes two pieces to the missing plastic puzzle- (1) nets have been under-sampling the smallest microplastic size fraction, which is actually not missing when using high-volume filtration, and (2) small microplastics in particular find their way below the surface. But their results also contribute several new questions, because some of what they found is quite surprising. This article is a Research Article on Zhao et al., https://doi.org/10.1111/gcb.16089.