Derivation of seawater <i>p</i>CO₂ from net community production identifies the South Atlantic Ocean as a CO₂ source

A key step in assessing the global carbon budget is the determination of the partial pressure of CO2 in seawater (pCO2 (sw)). Spatially complete observational fields of pCO2 (sw) are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of pCO2 (sw) using satellite observations. As part of this process, satellite chlorophyll a (Chl a) is often used as a proxy for the biological drawdown or release of CO2. Chl a does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP). In this study, pCO2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and either satellite-derived NCP, net primary production (NPP) or Chl a to compare which biological proxy produces the most accurate fields of pCO2 (sw). Estimates of pCO2 (sw) using NCP, NPP or Chl a were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl a or NPP. A perturbation analysis assessed the potential maximum reduction in pCO2 (sw) uncertainties that could be achieved by reducing the uncertainties in the satellite biological parameters. This illustrated further improvement using NCP compared to NPP or Chl a. Using NCP to estimate pCO2 (sw) showed that the South Atlantic Ocean is a CO2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region appears to be a sink for CO2. These results highlight that using NCP improved the accuracy of estimating pCO2 (sw) and changes the South Atlantic Ocean from a CO2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will ultimately improve our understanding and confidence in quantification of the global ocean as a CO2 sink.

Nanoscale Infrared Spectroscopy Reveals Nanoplastics at 5000 m Depth in the South Atlantic Ocean

Millimeter- and micrometer-sized plastics are well-documented in aquatic ecosystems, but little is known about sub-micrometer particles because conventional analytical techniques lack sufficient spatial resolution or the spectroscopic means to unambiguously identify individual nanometer-sized plastic particles. We combined the spatial resolution of atomic force microscopy with chemical information from infrared spectroscopy to detect, identify, and count nanoplastics down to 20 nm in diameter in samples from different depths in the South Atlantic Ocean. We present evidence for the presence of polyethylene terephthalate (PET) nanoplastics in different states of degradation at 5000 m. Using lab-based ageing of PET, we demonstrate that nanoplastics can form even without light or interaction with the plastisphere, and that macroscopic PET items are a plausible source of PET nanoplastics in the ocean.

Virus-Host Interactions Reveal Potential Roles Towards Phosphorus Cycling in South Atlantic Ocean Euphotic Zones

BackgroundDue to their role as obligate parasites of marine microorganisms, viruses are primary mediators of marine biogeochemical cycles. Recent studies have provided irrevocable evidence showing that viruses augment the metabolisms of bacteria and archaea through expression of auxiliary metabolic genes (AMGs). Several studies have shown that AMGs affect the biogeochemical recycling of sulphur and nitrogen but comparatively less is known regarding their influence on phosphorus recycling.ResultsHere, we provide the first insights regarding the potential effects of phosphorus limitation and AMGs on putative prokaryotic hosts in the euphotic zone of the South Atlantic Ocean (SAO). We identified 7,176 viral contigs that were clustered into 5,999 viral operational taxonomic units (vOTUs, >5kb). These SAO viral communities appear to be unique, as over 89% had no taxonomic assignment, possibly due to the genetic endemism in this ocean. Three phosphatases, phoN, gmhB and rnhA-cobC, were identified as P-cycle AMGs in both prokaryotic double-stranded DNA viruses and eukaryotic Nucleocytoplasmic Large DNA viruses. These genes are associated with the acquisition of inorganic phosphate from phosphate esters, the largest reservoir of P-containing compounds in the marine environment. AMGs were identified in both uncultured and unclassified prokaryotic double-stranded DNA viruses predicted to infect Bacteriodetes, Proteobacteria, Chloroflexota and Poseidonales lineages. ConclusionTogether, these results suggest that viruses modulate P-cycling in euphotic zones of the ocean and that the acquisition of these phosphatase genes may be cues of P-ester stress.