gels Review Dissolved Organic Matter in the Global Ocean: A Primer Dennis A. Hansell 1,* and Mónica V. Orellana 2,3 1 Department of Ocean Sciences, RSMAS, University of Miami, Miami, FL 33149, USA 2 Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA; [email protected] 3 Institute for Systems Biology, Seattle, WA 98109, USA * Correspondence: [email protected] Abstract: Marine dissolved organic matter (DOM) holds ~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. Keywords: marine dissolved organic carbon; ocean carbon cycle; marine microgels 1. Introduction Dissolved organic carbon (DOC) makes up the second largest bioavailable pools of carbon in the ocean (~660 Pg C (1 Pg = 1 × 109 metric tons); [1]) and is second only to the Citation: Hansell, D.A.; Orellana, ~50× larger pool of dissolved inorganic carbon. The size of the reservoir, and its comple- M.V. Dissolved Organic Matter in the mentary functions as a sink for autotrophically fixed carbon and as a source of substrate to Global Ocean: A Primer. Gels 2021, 7, 128. https://doi.org/10.3390/ microbial heterotrophs, indicate that DOC plays a central role in the ocean carbon cycle [2]. gels7030128 Identifying the details and mechanisms of that role in the global ocean remains a great challenge; with relevance to this Special Issue, the contribution of marine gels to those Academic Editor: Maria mechanisms is essentially unknown. While a critical percentage of DOC (varying from Valentina Dinu 10% in the coastal ocean [3] to 30% in the Arctic [4]) assemble as microgels (Orellana and Hansell, this issue), their importance in DOC basin-scale dynamics is just beginning to be Received: 28 May 2021 understood [5]. Marine gels are three-dimensional (3D), colloidal- to micrometer-sized 2+ Accepted: 26 August 2021 hydrogel networks held together by Ca ionic bonds/hydrophobic bonds that assemble Published: 28 August 2021 spontaneously from marine dissolved biopolymers [3–5]; they have been proposed to play a pivotal role in regulating ocean basin-scale biogeochemical dynamics [6]. Publisher’s Note: MDPI stays neutral In this paper, the role of DOC in the ocean carbon cycle is considered in its broadest with regard to jurisdictional claims in temporal and spatial scales, largely as a primer for those wishing to understand the global published maps and institutional affil- scale dynamics of the pool. The paper begins with an evaluation of the spatial distribution iations. of DOC at the regional and basin scales, in both the surface and deep ocean. In this context, the net production of DOC relative to the distribution and timing of marine primary production is evaluated. It briefly concludes with priorities for present and future research relevant to the role of gels in DOC dynamics. More complete and detailed reviews of DOC Copyright: © 2021 by the authors. dynamics in the ocean are available [7–13]. Licensee MDPI, Basel, Switzerland. This article is an open access article 2. DOC Concentrations and Reactivity distributed under the terms and DOC concentrations in the ocean (Figure1) range from a deep ocean low of ~35 µmolC/kg conditions of the Creative Commons (in deep waters of the Pacific that were last exposed to the atmosphere and sunlight Attribution (CC BY) license (https:// more than a millennium previously) to surface ocean highs of >80 µmolC/kg, with the creativecommons.org/licenses/by/ highest concentrations commonly found in river-influenced coastal waters [1]. Biological 4.0/). Gels 2021, 7, 128. https://doi.org/10.3390/gels7030128 https://www.mdpi.com/journal/gels Gels 2021, 7, x FOR PEER REVIEW 2 of 8 Gels 2021, 7, 128 2 of 8 Biological processes establish the vertical gradient seen in Figure 1, with net autotrophic processes establish the vertical gradient seen in Figure1, with net autotrophic production production in the sunlit surface layer and net heterotrophic consumption at depth, while in the sunlit surface layer and net heterotrophic consumption at depth, while physical physicalconditions conditions maintain maintain the gradient the gradient (i.e., high (i.e., vertical high vertical stability stability in the ocean in the water ocean column water columnlargely precludeslargely precludes ready mixing ready ofmixing DOC-enriched of DOC-enriched surface waterssurface towaters great to depths). great depths). BulkBulk DOC DOC in in the the ocean ocean is is operationally operationally resolved resolved into into at at least least three three fractions, each each qualitativelyqualitatively characterized byby its its biological biological lability lability [8 ].[8]. All All ocean ocean depths depths contain contain (1) the(1) verythe veryold, biologicallyold, biologically refractory refractory DOC (RDOCDOC (RDOC concentrations concentrations <45 µM <45 and μM bulk and radiocarbon bulk radiocar- ages bonof >6000 ages years)of >6000 [14 ].years) The RDOC[14]. The distribution RDOC dist isribution controlled is bycontrolled the global by deep the global overturning deep overturningcirculation andcirculation is thus and relatively is thus homogeneous relatively homogeneous in the deep in ocean the deep (Figure ocean1, note (Figure blues 1, noteand pinks).blues and Built pinks). upon Built the refractory upon the DOC,refractory at intermediate DOC, at intermediate (to 1000 m) (to and 1000 upper m) layerand upperdepths, layer is (2) depths, material is (2) of material intermediate of interm (or semi-)ediate lability(or semi-) (SLDOC; lability lifetime (SLDOC; of lifetime months of to monthsyears; note to years; greens, note yellows, greens, and yellows, reds in Figureand reds1). Itin is Figure this material 1). It is that this has material recently that (years) has recentlyaccumulated (years) in accumulated the surface ocean in the and surface that is ocean then mixedand that downward is then mixed into the downward ocean interior, into thethereby ocean reducing interior, thethereby vertical reducing concentration the vertic gradiental concentration and contributing gradient and to carbon contributing export to(i.e., carbon the biologicalexport (i.e., carbon the biological pump; notecarbon great pump; depth note of great green depth colors ofin green the farcolors left in (near the farIceland left (near in the Iceland North in Atlantic) the North of Atlantic) Figure1). of Concentrations Figure 1). Concentrations of this fraction of this are fraction commonly are commonly10–30 µmolC/kg 10–30 μinmolC/kg the stratified in the upperstratified ocean, upper and ocean, near zero and innear the zero deep in ocean, the deep indicating ocean, indicatingthat it is susceptible that it is susceptible to removal to over removal decades. over The decades. most biologically The most biologically labile fraction labile of DOCfrac- tion(3), withof DOC lifetimes (3), with of days lifetimes to months of days and to concentrations months and concentrations of just a few to 10of0 sjust of µamolC/kg, few to 10′ iss oflargely μmolC/kg, limited is to largely the sunlit limited layer to of the the sunlit ocean, la whereyer of itthe is producedocean, where by autotrophs it is produced daily by [2]. autotrophsReferred to daily as labile [2]. DOCReferred (LDOC), to as this labile material DOC supports(LDOC), microbialthis materi heterotrophical supports microbial processes heterotrophicin the surface processes ocean. Of in the the three surface fractions, ocean. it showsOf the thethree greatest fractions, seasonality, it shows with the greatest high net seasonality,production with rates high during net phytoplanktonproduction rate bloomss during and phytoplankton lower rates duringblooms theand autumn lower rates and duringwinter the convective autumn overturns and winter of convec the uppertive ocean.overturns of the upper ocean. FigureFigure 1. 1. VerticalVertical distribution of DOC ( μµmolC/kg) along along ocean ocean sections sections A16 A16 and and P1 P166 in in the the Atlantic Atlantic and and Pacific Pacific Oceans, Oceans, respectivelyrespectively (locations (locations given given in in Figure Figure 22).). Data density is lowlow inin thethe SouthernSouthern OceanOcean alongalong thethe sectionsection connectingconnecting thethe southernsouthern termini termini of of A16 A16 and and P16, P16, hence hence the the gap in coverage coverage.. Solid Solid arrows arrows schematically in indicatedicate the the major overturning circulationcirculation pathways inin thethe ocean,ocean, specificallyspecifically NADW, NADW, AABW, AABW AAIW, , AAIW, and and PDW. PDW. The The dashed dashed arrows arrows represent represent the apparent the ap- parentpathways pathways of sinking of sinking biogenic biogenic particles particles (exported (exported from the from surface the surface ocean) ocean) that disaggregate that disaggr whileegate sinking, while sinking, in turn in adding turn adding DOC to the deep-water column. This enrichment serves as a substrate to heterotrophs (including prokaryotes) DOC to the deep-water column.