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Abstract Kelp forests form some of the most productive areas on earth and are proposed to sequester carbon in the ocean, largely in the form of released dissolved organic carbon (DOC). Here we investigate the role of environmental, seasonal and age-related physiological gradients on the partitioning of net primary production (NPP) into DOC by the canopy forming giant kelp (Macrocystis pyrifera). Rates of DOC production were strongly influenced by an age-related decline in physiological condition (i.e. senescence). During the mature stage of giant kelp development, DOC production was a small and constant fraction of NPP regardless of tissue nitrogen content or light intensity. When giant kelp entered its senescent phase, DOC production increased substantially and was uncoupled from NPP and light intensity. Compositional analysis of giant kelp-derived DOC showed that elevated DOC production during senescence was due to the solubilization of biomass carbon, rather than by direct exudation. We coupled our incubation and physiological experiments to a novel satellite-derived 20-year time series of giant kelp canopy biomass and physiology. Annual DOC production by giant kelp varied due to differences in standing biomass between years, but on average, 74% of the annual DOC production by giant kelp was due to senescence. This study suggests DOC may be a more important fate of macroalgal NPP than previously recognized. 

Abstract Ammonia-oxidizing archaea are the most abundant chemolithoautotrophs in the ocean and are assumed to dominate carbon fixation below the sunlit surface layer. However, the supply of reduced nitrogen delivered from the surface in sinking particulate organic matter is insufficient to support the amount of nitrification required to sustain measured carbon fixation rates in the dark ocean. Here we attempt to reconcile this observed discrepancy by quantifying the contribution of ammonia oxidizers to dark carbon fixation in the eastern tropical and subtropical Pacific Ocean. We used phenylacetylene—a specific inhibitor of the ammonia monooxygenase enzyme—to selectively inhibit ammonia oxidizers in samples collected throughout the water column (60–600 m depth). We show that, despite their high abundances, ammonia oxidizers contribute only a small fraction to dark carbon fixation, accounting for 4–25% of the total depth-integrated rates in the eastern tropical Pacific. The highest contributions were observed within the upper mesopelagic zone (120–175 m depth), where ammonia oxidation could account for ~50% of dark carbon fixation at some stations. Our results challenge the current view that carbon fixation in the dark ocean is primarily sustained by nitrification and suggest that other microbial metabolisms, including heterotrophy, might play a larger role than previously assumed. 

{"Abstract":["This data package contains water chemistry measurements taken\n monthly at these reefs in the nearshore areas of the Santa Barbra\n Channel, CA, USA: Arroyo Quemado, Bullito, Naples, Arroyo Burro,\n Mohawk and Carpinteria. Measurements include standard CTD\n parameters, nutrients, pigments, particulate CN, total dissolved N\n and P, stable isotopes of C and N (not all parameters are measured\n at all stations). Sampling began in November 2000. Some stations\n are sampled only occasionally. During the first 2 years, CTD data\n were collected with a SBE19 Seacat Profiler and water samples with\n a pump. Starting in February 2003, a SBE19-Plus with a rosette\n sampler was used.\n \n \n There are 3 tables in this dataset. Water chemistry and profiles\n from "registered stations" (see geographic coverage) are\n in 2 tables with "registered" in their name. CTD\n profiles are often collected ad hoc, or as "stations of\n opportunity". These have been collected in a third table as\n "non-registered". The station codes for these may be\n reused, and are not recorded in metadata (but can be found in\n data)."]} 

Abstract The size distribution of submicron particles is essential for understanding their biogeochemical and optical roles, but it has seldom been measured. This study utilizes ViewSizer 3000, an instrument that tracks Brownian motions of particles, to measure the particle size distributions (PSD) from 250 to 1,050 nm in the North Pacific Ocean (NP) and the North Atlantic Ocean (NA) at depths from 5 to 500 m. The concentration of particles varies over one order of magnitude at any given size bin, with greater variations up to two orders of magnitude at sizes >600 nm. In both locations, concentrations decrease with depth. Bacterioplankton are a dominant component, accounting for 65%–90% of the submicron particles in the surface waters (<100 m) and approximately 30%–40% at depths >150 m at both sites. In the NP, the volume mean diameter increased approximately 5% from the morning to noon at the surface, probably resulting from the diurnal growth of bacterioplankton. In the NA, the concentration and mean size increased by >60% and ∼10% respectively after one storm that introduced a different particle population into the study area. 

Abstract Climate‐driven warming is projected to intensify wildfires, increasing their frequency and severity globally. Wildfires are an increasingly significant source of atmospheric deposition, delivering nutrients, organic matter, and trace metals to coastal and open ocean waters. These inputs have the potential to fertilize or inhibit microbial growth, yet their ecological impacts remain poorly understood. This study examines how ash leachate, derived from the 2017 Thomas Fire in California and lab‐produced ash from Oregon vegetation, affects coastal plankton communities. Shipboard experiments off the California coast examined how pre‐existing plankton biomass concentrations mediate responses to ash leachates. We found that ash leachate contained dissolved organic matter (DOM) that significantly increased bacterioplankton specific growth rates and DOM remineralization rates but had a negligible effect on bacterioplankton growth efficiency, suggesting low DOM bioavailability. Furthermore, ash‐derived DOM had a higher potential to accumulate in high biomass water, where pre‐existing DOM substrates may better support bacterial metabolism. Ash leachate had a neutral to negative effect on phytoplankton division rates and decreased microzooplankton grazing rates, particularly in low biomass water, leading to increased phytoplankton accumulation. Nanoeukaryotes accumulated in low biomass water, whereas picoeukaryotes andSynechococcusaccumulated in high biomass water. Our findings suggest that the influence of ash deposition on DOM cycling, phytoplankton accumulation, and broader marine food web dynamics depends on pre‐existing biomass levels. Understanding these interactions is critical for predicting the biogeochemical consequences of increasing wildfire activity on marine ecosystems. 

Abstract We introduce the Global rRNA Universal Metabarcoding Plankton database (GRUMP), which consists of 1194 samples that were collected from 2003–2020 and cover extensive latitudinal and longitudinal transects, as well as depth profiles in all major ocean basins. DNA from unfractionated (>0.2 µm) seawater samples was amplified using the 515Y/926 R universal three-domain rRNA gene primers, simultaneously quantifying the relative abundance of amplicon sequencing variants (ASVs) from bacteria, archaea, eukaryotic nuclear 18S, and eukaryotic plastid 16S. Thus, the ratio between taxa in one sample is directly comparable to the ratio in any other GRUMP sample, regardless of gene copy number differences. This obviates a problem in prior global studies that used size-fractionation and different rRNA gene primers for bacteria, archaea, and eukaryotes, precluding comparisons across size fractions or domains. On average, bacteria contributed 71%, eukaryotes 19%, and archaea 8% to rRNA gene abundance, though eukaryotes contributed 32% at latitudes >40°. GRUMP is publicly available on the Simons Collaborative Marine Atlas Project (CMAP), promoting the global comparison of marine microbial dynamics. 

ABSTRACT Dissolved organic matter (DOM) comprises diverse compounds with variable bioavailability across aquatic ecosystems. The sources and quantities of DOM can influence microbial growth and community structure with effects on biogeochemical processes. To investigate the chemodiversity of labile DOM in tropical reef waters, we tracked microbial utilisation of over 3000 untargeted mass spectrometry ion features exuded from two coral and three algal species. Roughly half of these features clustered into over 500 biologically labile spectral subnetworks annotated to diverse structural superclasses, including benzenoids, lipids, organic acids, heterocyclics and phenylpropanoids, comprising on average one‐third of the ion richness and abundance within each chemical class. Distinct subsets of these labile compounds were exuded by algae and corals during the day and night, driving differential microbial growth and substrate utilisation. This study expands the chemical diversity of labile marine DOM with implications for carbon cycling in coastal environments. 

Heterotrophic bacteria and archaea (“heteroprokaryotes”) drive global carbon cycling, but how to quantitatively organize their functional complexity remains unclear. We generated a global-scale understanding of marine heteroprokaryotic functional biogeography by synthesizing genetic sequencing data with a mechanistic marine ecosystem model. We incorporated heteroprokaryotic diversity into the trait-based model along two axes: substrate lability and growth strategy. Using genetic sequences along three ocean transects, we compiled 21 heteroprokaryotic guilds and estimated their degree of optimization for rapid growth (copiotrophy). Data and model consistency indicated that gradients in grazing and substrate lability predominantly set biogeographical patterns, and we identified deep-ocean “slow copiotrophs” whose ecological interactions control the surface accumulation of dissolved organic carbon.