Rate and fate of dissolved organic carbon release by seaweeds: A missing link in the coastal ocean carbon cycle

Abstract

Dissolved organic carbon (DOC) release by seaweeds (marine macroalgae) is a critical component of the coastal ocean biogeochemical carbon cycle but is an aspect of seaweed carbon physiology that we know relatively little about. Seaweed‐derived DOC is found throughout coastal ecosystems and supports multiple food web linkages. Here, we discuss the mechanisms of DOC release by seaweeds and group them into passive (leakage, requires no energy) and active release (exudation, requires energy) with particular focus on the photosynthetic “overflow” hypothesis. The release of DOC from seaweeds was first studied in the 1960s, but subsequent studies use a range of units hindering evaluation: we convert published values to a common unit (μmol C · g DW−1 · h−1) allowing comparisons between seaweed phyla, functional groups, biogeographic region, and an assessment of the environmental regulation of DOC production. The range of DOC release rates by seaweeds from each phylum under ambient environmental conditions was 0–266.44 μmol C · g DW−1 · h−1 (Chlorophyta), 0–89.92 μmol C · g DW−1 · h−1 (Ochrophyta), and 0–41.28 μmol C · g DW−1· h−1 (Rhodophyta). DOC release rates increased under environmental factors such as desiccation, high irradiance, non‐optimal temperatures, altered salinity, and elevated dissolved carbon dioxide (CO2) concentrations. Importantly, DOC release was highest by seaweeds that were desiccated (<90 times greater DOC release compared to ambient). We discuss the impact of future ocean scenarios (ocean acidification, seawater warming, altered irradiance) on DOC release rates by seaweeds, the role of seaweed‐derived DOC in carbon sequestration models, and how they inform future research directions.

Forensic carbon accounting: Assessing the role of seaweeds for carbon sequestration

“…Larger pieces of seaweed tissue (debris) are eaten by macro-invertebrates such as abalone and sea urchins (Elliott Smith and Fox 2021). Dissolved organic carbon (DOC) is defined as organic carbon molecules which pass through a filter pore size 0.22-0.7 lm (GF/F) and provide substrate for heterotrophic bacteria and microheterotrophic eukaryotes, which in turn support progressively higher trophic levels (Paine et al 2021). Seaweed beds support substantial numbers of sessile and mobile invertebrates (Taylor and Cole 1994, Hepburn and Hurd 2005, Poore et al 2012, Su arez-Jim enez et al 2017, Bu e et al 2020, which respirerelease CO 2continuously (R H ; Fig.…”

“…However, a fraction of the deep ocean refractory DOC can be upwelled and broken down photochemically into more labile forms resulting in micro-heterotrophic remineralization to DIC (Shen andBenner 2018, Paine et al 2021). The fate of seaweed-derived DOC in biogeochemical carbon cycles is thought to be extremely important, but we have only a rudimentary understanding of its bioavailability and fate in the oceanic food webs (Paine et al 2021).…”

“…Dissolved organic carbon (DOC) is defined as organic carbon molecules which pass through a filter pore size 0.22–0.7 μm (GF/F) and provide substrate for heterotrophic bacteria and micro‐heterotrophic eukaryotes, which in turn support progressively higher trophic levels (Paine et al. 2021). Seaweed beds support substantial numbers of sessile and mobile invertebrates (Taylor and Cole 1994, Hepburn and Hurd 2005, Poore et al.…”

“…The fate of seaweed‐derived DOC in biogeochemical carbon cycles is thought to be extremely important, but we have only a rudimentary understanding of its bioavailability and fate in the oceanic food webs (Paine et al. 2021).…”

Quantifying habitat provisioning at macroalgal cultivation sites

“…51 A greater understanding of the microbiome of macroalgal cultivation is therefore needed to determine both its ecological value in macroalgal cultivation, 8 and understand how the macroalgal microbiome is regulated by other associated biodiversity present at cultivation sites, which may mitigate disease outbreaks. 35,52 Macroalgal cultivation sites may also potentially affect microorganisms in the water column and benthic sediments, through production of soluble dissolved and particulate organic matter, 53 deposition of detritus and the potential attraction of waste-producing species such as fish (Figure 1). Microorganisms in the surrounding environment, however, are unlikely to be affected to the extent seen in other aquaculture species, particularly fishfarms, 54 as waste production is comparatively low (although less is known about the amounts produced at larger offshore sites).…”