Active Carbon Transport Research Articles

The global contribution of seasonally migrating copepods to the biological carbon pump

AbstractEvery year, large numbers of zooplankton migrate from the surface ocean to depths of 500–2000 m to hibernate. Through this migration, they actively transport organic carbon to the deep ocean, where it is used to fuel metabolic needs. This active transport of carbon is thought to be highly efficient, as carbon metabolized by copepods is directly injected deep into the ocean's interior. The significance of this process in view of global carbon cycling remains an open question. Here, we focus on five representative, diapausing copepod species (Calanus finmarchicus, Calanus hyperboreus, Calanoides acutus, Calanoides natalis, and Neocalanus tonsus) distributed in the Arctic, Atlantic, Indian, and Southern Oceans. For each species, we compute both carbon injection (how much carbon is transported below the euphotic zone during zooplankton migration and left there as dissolved inorganic carbon) and carbon sequestration (the amount of carbon stored in the ocean's interior following diapausing zooplankton‐mediated injection). In total, the five species considered here contribute 0.4–0.8% of total biological carbon export, and 0.8–3.3% of total carbon sequestration mediated by the biological pump (assuming a total carbon export of ~ 10 PgC yr−1 and sequestration of ~ 1300 PgC). Including other species in this inventory would increase the contribution of diapausing copepods to the biological carbon pump, but requires more precise estimates of copepods' distribution, abundance, and metabolic requirements.

Temperature sensitivity of carbon concentrating mechanisms in the diatom Phaeodactylum tricornutum

Marine diatoms are key primary producers across diverse habitats in the global ocean. Diatoms rely on a biophysical carbon concentrating mechanism (CCM) to supply high concentrations of CO2 around their carboxylating enzyme, RuBisCO. The necessity and energetic cost of the CCM are likely to be highly sensitive to temperature, as temperature impacts CO2 concentration, diffusivity, and the kinetics of CCM components. Here, we used membrane inlet mass spectrometry (MIMS) and modeling to capture temperature regulation of the CCM in the diatom Phaeodactylum tricornutum (Pt). We found that enhanced carbon fixation rates by Pt at elevated temperatures were accompanied by increased CCM activity capable of maintaining RuBisCO close to CO2 saturation but that the mechanism varied. At 10 and 18 °C, diffusion of CO2 into the cell, driven by Pt’s ‘chloroplast pump’ was the major inorganic carbon source. However, at 18 °C, upregulation of the chloroplast pump enhanced (while retaining the proportion of) both diffusive CO2 and active HCO3− uptake into the cytosol, and significantly increased chloroplast HCO3− concentrations. In contrast, at 25 °C, compared to 18 °C, the chloroplast pump had only a slight increase in activity. While diffusive uptake of CO2 into the cell remained constant, active HCO3− uptake across the cell membrane increased resulting in Pt depending equally on both CO2 and HCO3− as inorganic carbon sources. Despite changes in the CCM, the overall rate of active carbon transport remained double that of carbon fixation across all temperatures tested. The implication of the energetic cost of the Pt CCM in response to increasing temperatures was discussed.

The Outsized Role of Salps in Carbon Export in the Subarctic Northeast Pacific Ocean.

Periodic blooms of salps (pelagic tunicates) can result in high export of organic matter, leading to an "outsized" role in the ocean's biological carbon pump (BCP). However, due to their episodic and patchy nature, salp blooms often go undetected and are rarely included in measurements or models of the BCP. We quantified salp-mediated export processes in the northeast subarctic Pacific Ocean in summer of 2018 during a bloom of Salpa aspera. Salps migrated from 300 to 750m during the day into the upper 100m at night. Salp fecal pellet production comprised up to 82% of the particulate organic carbon (POC) produced as fecal pellets by the entire epipelagic zooplankton community. Rapid sinking velocities of salp pellets (400-1,200md-1) and low microbial respiration rates on pellets (<1% of pellet C respired day-1) led to high salp pellet POC export from the euphotic zone-up to 48% of total sinking POC across the 100m depth horizon. Salp active transport of carbon by diel vertical migration and carbon export from sinking salp carcasses was usually <10% of the total sinking POC flux. Salp-mediated export markedly increased BCP efficiency, increasing by 1.5-fold the proportion of net primary production exported as POC across the base of the euphotic zone and by 2.6-fold the proportion of this POC flux persisting 100m below the euphotic zone. Salps have unique and important effects on ocean biogeochemistry and, especially in low flux settings, can dramatically increase BCP efficiency and thus carbon sequestration.

Variability in micronekton active carbon transport estimates on the Southwest Coast of Oahu using three different sampling gears

Micronekton were sampled off the southwest coast of Oahu in October of 2004 as part of the North Pacific Marine Sciences Organization (PICES) Micronekton Sampling Gear Intercalibration experiment (MIE-1). The initiative employed three different micronekton sampling gears: the Cobb Trawl, Isaacs–Kidd midwater trawl (IKMT), and Hokkaido University frame trawl (HUFT). Micronekton catches from the three sampling gears were used to quantify the rates of active carbon transport. We evaluate the differences in total active carbon transport assessed using the data collected by the three gears, with a particular emphasis on taxa- and length-specific differences across gears. The estimates of total active carbon transport derived from the HUFT were significantly lower than those derived from the IKMT. We detected no significant difference between estimates derived using the IKMT and Cobb and the Cobb and HUFT. Dominant groups contributing to active carbon transport varied across sampling gears (Cobb: myctophids and cephalopods; HUFT: decapods and stomatopods; IKMT: cephalopods, myctophids, and decapods); however, no significant differences in the contribution of individual taxa to active carbon transport across sampling gears were detected (e.g., the myctophid active carbon transport contribution for the Cobb was not significantly different from that of the IKMT). Pairwise ratios revealed size-specific differences in the contribution to total flux across gears. As expected, the HUFT and IKMT estimates of active carbon transport were higher for smaller size classes, while estimates from the Cobb were more evenly distributed across size classes. These findings suggest that the differences in total active carbon transport from the three sampling gears are driven primarily by size-based sampling biases, which produce significant differences in biomass estimates. This study provides a foundation upon which future active carbon transport studies may be adjusted to address sampling gear biases.

Fear and loathing in the pelagic: How the seascape of fear impacts the biological carbon pump

AbstractThe biological carbon pump transports photosynthetically fixed carbon from surface waters to depths. It removes carbon from the atmosphere and sequesters it in the deep ocean, playing an important role in global climate regulation. As the biological carbon pump is directly related to biological processes, it is heavily influenced by the biomass and trophic interactions between populations in the ecosystem. However, behavioral responses and adaptations to predation risk change trophic interactions, potentially having larger impacts than direct effects on trophic interactions and population abundances. Thus, predation risk may play an important role in shaping the biological carbon pump's strength (how much carbon leaves the euphotic zone) and efficiency (what fraction of detritus reaches a certain depth without being degraded). Except in the case of active carbon transport by vertically migrating organisms, this role of risk is not generally recognized. Here, we synthesize the existing knowledge on the consequences of anti‐predation responses on the biological carbon pump. First, we consider a generic anti‐predation response and investigate the different direct, indirect, and cascading effects that the response can induce. Then, we focus on pelagic anti‐predation responses and detail how they can specifically alter the different components of the pump. Finally, we discuss points to consider in biological carbon pump studies and highlight directions for future research. In particular, there is a need for more quantitative research to evaluate the importance of anti‐predation responses in shaping the biological carbon pump.

Editorial: Zooplankton and Nekton: Gatekeepers of the Biological Pump

Editorial on the Research Topic Zooplankton and Nekton: Gatekeepers of the Biological Pump Zooplankton and nekton organisms create and destroy particles in manifold ways. They feed on the diverse components of the plankton community and on detrital matter. They disaggregate these components, but also repackage them into fecal pellets. Zooplankton and nekton thereby contributes to the attenuation, but also to the export of vertically settling particles. Many zooplankton and nekton organisms also ascend to the surface layer of the ocean at dusk to feed during the dark hours, and return to midwater at the break of dawn. This diurnal vertical migration (DVM) shuttles organic matter from the surface ocean to deeper layers, where it is metabolized and excreted. This active flux (as opposed to the passive flux of sinking particles) can contribute substantially to the biological pump, the downward export of carbon and nutrients into the oceans interior. Due to their multiple roles in oceanic particle dynamics, zooplankton and nekton organisms can actually be considered the gatekeepers of the biological pump. Several articles in this Research Topic deal with the contribution of zooplankton and nekton-mediated active flux to the total export of organic matter. Using biomass and enzyme transport system (ETS) assessments of respiratory flux for both mesozooplankton and micronekton communities, Hernandez-Leon et al. estimated the total active transport of carbon (respiration, excretion, mortality, and egestion) along a transect in the Atlantic from the Canary Islands to Brazil. They found that active flux by these communities ranged from 25 to 80% of the total particulate organic carbon export at 150 m depth and that the importance of active flux increased with increasing surface productivity. Kwong et al. compared biomass, diel vertical migration, and active flux of mesozooplankton and micronekton across a range of mesoscale eddy structures along the east-coast of Australia during winter and spring. They found that although all eddy regimes had similar integrated biomass of mesozooplakton and micronekton, the organisms in the individual eddies had different migratory behavior, which resulted in contrasting importance of active flux. Kiko et al. assessed the impact of mesozooplankton DVM on elemental cycling at three stations in the Eastern Tropical North Atlantic. They found that approximately 31 to 41% of the total nitrogen loss from the upper 200 m of the water column was attributable to DVM mediated fluxes. They also suggest that gut flux-the flux created by migrators when they evacuate their gut at DVM-depth-and migrator mortality at DVM-depth contribute to an Intermediate Particle Maximum. In their study conducted in the Peruvian upwelling system (which features a severe midwater oxygen minimum zone), Kiko and Hauss concluded that the metabolic suppression

Mesozooplankton and Micronekton Active Carbon Transport in Contrasting Eddies

Mesozooplankton (June 2015 and September 2017) and micronekton (September 2017) were sampled along the eastern coast of Australia. Depth stratified mesozooplankton and micronekton were collected using a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) and an International Young Gadoid Pelagic Trawl (IYGPT) equipped with an opening/closing codend. Sampling was undertaken at the center and edge of a frontal cold-core eddy (F-CCE center and edge) in 2015, and at the center of a cold-core eddy (B-CCE) and two warm-core eddies (R-WCE and WCE) in 2017. We assess the diel vertical structure, biomass and downward active carbon transport by mesozooplankton and micronekton in eddies. Total water column mesozooplankton and micronekton biomass did not vary substantially across water masses, while the extent and depth of diel vertical migration did. Using in-situ measurements of temperature and measurements of mesozooplankton and micronekton abundance and biomass, we estimated the contribution of respiration, dissolved organic carbon (DOC) excretion, gut flux and mortality to total downward active carbon transport in each water mass. Overall, active carbon transport by mesozooplankton and micronekton below the mixed layer varied substantially across water masses. We corrected estimates of micronekton migratory biomass and active carbon transport assuming 50% net efficiency. Specifically, inIn the R-WCE mesozooplankton remained within the mixed layer during the day and night; only 50% of the total micronekton population migrated below the mixed layer contributing to carbon transport, equating to 1.52.89 mg C m-2 d-1. Mesozooplankton actively transported 16.1 and 8.0 mg C m-2 d-1 at the F-CCE Center and Edge, respectively. Mesozooplankton and micronekton active carbon transport in the B-CCE were 5.4 and 0.40.74 mg C m-2 d-1, and in the WCE 88 and 6.713.4 mg C m-2 d-1. Differences in carbon export were dependent on food availability, temperature, time spent migrating and mixed layer depth. These findings suggest that under certain conditions mesoscale eddies can act as important carbon sinks.

Large Vertical Migrations ofPyrosoma atlanticumPlay an Important Role in Active Carbon Transport

AbstractPyrosomes are efficient grazers that can form dense aggregations. Their clearance rates are among the highest of any zooplankton grazer, and they can rapidly repackage what they consume into thousands of fecal pellets per hour. In recent years, pyrosome swarms have been found outside of their natural geographical range; however, environmental drivers that promote these swarms are still unknown. During the austral spring of 2017 aPyrosoma atlanticumswarm was sampled in the Tasman Sea. Depth‐stratified sampling during the day and night was used to examine the spatial and vertical distribution ofP. atlanticumacross three eddies. Respiration rate experiments were performed onboard to determine minimum feeding requirements for the pyrosome population.P. atlanticumwas 2 orders of magnitude more abundant in the cold core eddy (CCE) compared to both warm core eddies, with maximum biomass of 360 mg WW·m−3, most likely driven by high chlorophyllaconcentrations.P. atlanticumexhibited diel vertical migration and migrated to a maximum depth strata of 800–1,000 m. Active carbon transport in the CCE was 4 orders of magnitude higher than the warm core eddies. Fecal pellet production contributed to the majority (91%) of transport, and total downward carbon flux below the mixed layer was estimated at 11 mg C·m−2·d−1. When abundant,P. atlanticumswarms have the potential to play a major role in active carbon transport, comparable to fluxes for zooplankton and micronekton communities.

Modeling the Impact of Zooplankton Diel Vertical Migration on the Carbon Export Flux of the Biological Pump

One pathway of the biological pump that remains largely unquantified in many export models is the active transport of carbon from the surface ocean to the mesopelagic by zooplankton diel vertical migration (DVM). Here, we develop a simple representation of zooplankton DVM and implement it in a global export model as a thought experiment to illustrate the effects of DVM on carbon export and mesopelagic biogeochemistry. The model is driven by diagnostic satellite measurements of net primary production, algal biomass, and phytoplankton size structure. Due to constraints on available satellite data, the results are restricted to the latitude range from 60°N to 60°S. The modeled global export flux from the base of the euphotic zone was 6.5 PgC/year, which represents a 14% increase over the export flux in model runs without DVM. The mean (± standard deviation, SD) proportional contribution of the DVM‐mediated export flux to total carbon export, averaged over the global domain and the climatological seasonal cycle, was 0.16 ± 0.04 and the proportional contribution of DVM activity to total respiration within the twilight zone was 0.16 ± 0.06. Adding DVM activity to the model also resulted in a deep local maximum in the oxygen utilization profile. The model results were most sensitive to the assumptions for the fraction of individuals participating in DVM, the fraction of fecal pellets produced in the euphotic zone, and the fraction of grazed carbon that is metabolized.

Vertical distribution and active carbon transport by pelagic decapods in the North Pacific Subtropical Gyre

Abstract Pelagic decapods were sampled during August 2011 in the central North Pacific Subtropical Gyre (NPSG). Depth-stratified samples using a MOCNESS-10 (10 m2 Multiple Opening/Closing Net and Environmental Sensing System) were collected at two stations to the west and north of the Hawaiian island of Oahu: Station Kahe: 21°20.6′N–158°16.4′W and Station ALOHA: 22°45′N–158°00′W. Total decapod abundance and biomass were 4.3 ind. m−2 and 0.71 gDW m−2. While 40 decapod taxa were identified, only 22 species were sampled sufficiently to study quantitatively their vertical migrations. All species were classified into three migration groups: full migrators (6 species); partial migrators (13 species); and non-migrators (3 species). Using measured local temperature profiles along with decapod densities and published models of respiration, excretion and mortality as well as gut fullness data, the individual and total active downward carbon flux was calculated. Active carbon flux of migrating decapods ranged from 383 to 625 µgC m−2 day−1. This active flux was equal to 4.8–7.8% of passive flux at the mean nighttime residence depth of ∼711 m), 2.1–3.4% of passive flux at the mean daytime residence depth (∼262 m), and 1.5–2.4% of passive flux at the base of the euphotic zone (∼173 m). Mortality flux accounted for ∼70% of total active flux, followed by gut flux—∼18%.

Carbon export by vertically migrating zooplankton: an optimal behavior model

Through diel vertical migration (DVM), zooplankton add an active transport to the otherwise passive sinking of detrital material that constitutes the biological pump. This active transport has proven difficult to quantify. We present a model that estimates both the temporal and depth characteristic of optimal DVM behavior based on a trade-off between feeding opportunity and predation risk; factors that vary with latitude, time of year, and the size of the migrating animal. This behavioral component, coupled to a nutrient-phytoplankton-zooplankton (NPZ) productivity model provides estimates of the active transport of carbon by different size fractions of the migrating zooplankton population as function of time and space. The approach is motivated by the difficulty in incorporating behavioral aspects of carbon transport into large scale carbon budgets of the world's oceans. The results show that despite their lower abundance, large zooplankton (length circa 1–2 mm) migrate deeper and transport approximately twice as much carbon as do the smaller zooplankton (length circa 0.2–0.3 mm). In mid- latitudes (∼30°N to ∼45°N), where pronounced spring blooms are observed, up to 20% more carbon is transported than at either equatorial or boreal latitudes. We estimate that the amount of carbon transported below the mixed layer by migrating zooplankton in the North Atlantic Ocean constitutes 27% (16–30%) of the total export flux associated with the biological pump in that region.

Feeding ecology of pelagic decapods in the North Pacific Subtropical gyre: implications for active carbon transport

Feeding ecology of pelagic decapods in the North Pacific Subtropical gyre: implications for active carbon transport

Myctophid feeding ecology and carbon transport along the northern Mid-Atlantic Ridge

Myctophids are among the most abundant fishes in the world׳s ocean and occupy a key position in marine pelagic food webs. Through their significant diel vertical migrations and metabolism they also have the potential to be a significant contributor to carbon export. We investigated the feeding ecology and contribution to organic carbon export by three myctophid species, Benthosema glaciale, Protomyctophum arcticum, and Hygophum hygomii, from a structurally and ecologically unique ecosystem- the Mid-Atlantic Ridge (MAR). Similar to the results of previous studies, the diet of these fishes was primarily copepods and euphausiids, however, gelatinous zooplankton was identified in the diet of B. glaciale for the first time. Ridge section and time of day were significant explanatory variables in the diet of B. glaciale as determined by canonical correspondence analysis, while depth was the only significant explanatory variable in the diet of P. arcticum. Daily consumption by MAR myctophids was less than 1% of dry body weight per day and resulted in the removal of less than 1% of zooplankton biomass daily. Although lower than previous estimates of carbon transport by myctophids and zooplankton in other areas, MAR myctophid active transport by diel vertical migration was equivalent to up to 8% of sinking particulate organic carbon in the North Atlantic. While highly abundant, myctophids do not impart significant predation pressure on MAR zooplankton, and play a modest role in the active transport of carbon from surface waters.

Contributions of mesozooplankton to vertical carbon export in a coastal upwelling system

Mesozooplankton can directly impact global biogeochemical cycles by repackaging particulate organic carbon (POC) into dense, rapidly sinking fecal pellets and by undertaking ver- tical migrations that transport carbon and nutrients to depth. We assessed these contributions of mesozooplankton to vertical flux in the California Current Ecosystem, a productive but spatiotem- porally variable coastal upwelling system, during cruises in April 2007 and October 2008. Sedi- ment traps and Thorium-234 ( 234 Th) disequilibrium measurements were used to assess the total passive flux of sinking POC, while pigment analyses and microscopic enumeration of sediment trap samples provided estimates of total fecal carbon transport. Identification of mesozooplankton in paired day-night, vertically stratified plankton tows allowed calculation of the active transport of carbon by the dominant taxa of vertically migrating mesozooplankton (particularly copepods and euphausiids). Across the range of 9 ecosystem conditions encountered on the cruises, recog- nizable fecal pellet mass flux varied from 3.5 to 135 mg C m �2 d �1 (3 to 94% of total passive flux) at the 100 m depth horizon. The active transport of carbon by migratory mesozooplankton taxa contributed an additional 2.4 to 47.1 mg C m �2 d �1 (1.9 to 40.5% of total passive flux). Inter-cruise comparisons suggest that fecal pellets contributed a higher portion of passive export during the productive spring cruise, when fecal material may have been responsible for close to 100% of sinking material. During the fall cruise, a gradient was observed with carbon export in productive water parcels driven by a large contribution of fecal pellets. In the less productive regions, fall ver- tical fluxes contained a higher proportion of marine snow and unidentifiable particles.

Carbon export mediated by mesopelagic fishes in the northeast Pacific Ocean

The role of fishes in the global carbon cycle is poorly known and often neglected. We show that the biomass of mesopelagic fishes off the continental USA west to longitude 141°W is positively related to annual net primary productivity, and averages 17gm−2. We estimate the export of carbon out of the epipelagic ocean mediated by mesopelagic fishes (“fish-mediated export”; FME) with individual-based metabolic modeling using the catch from 77 mesopelagic trawls distributed over the study area. FME was 15–17% (22–24mgCm−2d−1) of the total carbon exported in the study area (144mgCm−2d−1), as estimated from satellite data. FME varies spatially in both magnitude and relative importance. Although the magnitude of FME increases with increasing total export, the ratio of FME to total export decreases. FME exceeds 40% of the total carbon export in the oligotrophic North Pacific Subtropical Gyre, but forms <10% of the total export in the most productive waters of the California Current. Because the daytime residence depth of these fishes is below the depths where most remineralization of sinking particles occurs, FME is approximately equal to the passive transport at a depth of 400m. The active transport of carbon by mesopelagic fishes and zooplankton is similar in magnitude to the gap between estimates of carbon export obtained with sediment traps and by other methods. FME should be considered in models of the global carbon cycle.

Zooplankton respiration and the export of carbon at depth in the Amundsen Gulf (Arctic Ocean)

In arctic seas, lipids accumulated by zooplankton migrants in the surface layer in spring‐summer are respired at depth during the winter. The resulting active downward transport of carbon by the 200–1000 and &gt;1000 μm mesozooplankton fractions was quantified based on 41 biomass and respiration profiles from October 2007 to July 2008 in the Amundsen Gulf (Canadian Arctic Ocean). The small fraction, dominated by CII‐CIII Calanus glacialis, represented on average 12% of the overall zooplankton biomass and contributed little to the active transport of carbon by respiration. From April to July, total zooplankton ingested 17–28% of the estimated gross primary production (GPP) in the surface 100 m, and 36–59% of GPP over the entire water column. The large fraction, comprised mainly of CIV, CV and adults Calanus hyperboreus and C. glacialis that accumulate large lipid reserves, was responsible for 89% of grazing. The downward migration of large zooplankton in late summer coincided with a sharp decline in specific respiration rates signaling the start of diapause and the endogenous fuelling of metabolism. From October to April, Calanus migration‐respiration actively transported 3.1 g C m−2 beyond 100 m, a flux that represented 85 to 132% of the gravitational POC fluxes at 100 m from October to July. Our results stress the importance of including active transport by large zooplankton migrants in carbon budgets of the Arctic Ocean.

Efficiency of the CO 2 -concentrating mechanism of diatoms

Diatoms are responsible for a large fraction of CO 2 export to deep seawater, a process responsible for low modern-day CO 2 concentrations in surface seawater and the atmosphere. Like other photosynthetic organisms, diatoms have adapted to these low ambient concentrations by operating a CO 2 concentrating mechanism (CCM) to elevate the concentration of CO 2 at the site of fixation. We used mass spectrometric measurements of passive and active cellular carbon fluxes and model simulations of these fluxes to better understand the stoichiometric and energetic efficiency and the physiological architecture of the diatom CCM. The membranes of diatoms are highly permeable to CO 2 , resulting in a large diffusive exchange of CO 2 between the cell and external milieu. An active transport of carbon from the cytoplasm into the chloroplast is the main driver of the diatom CCM. Only one-third of this carbon flux is fixed photosynthetically, and the rest is lost by CO 2 diffusion back to the cytoplasm. Both the passive influx of CO 2 from the external medium and the recycling of the CO 2 leaking out of the chloroplast are achieved by the activity of a carbonic anhydrase enzyme combined with the maintenance of a low concentration of HCO 3 − in the cytoplasm. To achieve the CO 2 concentration necessary to saturate carbon fixation, the CO 2 is most likely concentrated within the pyrenoid, an organelle within the chloroplast where the CO 2 -fixating enzyme is located.

Effect of net avoidance on estimates of diel vertical migration

Diel vertical migration (DVM) has been hypothesized to actively transport organic material out of the euphotic layer, thus forming a novel part of the “biological pump.” However, quantifying DVM is made difficult by observational limitations. Conventionally, the difference between night and day biomass from net trawls in the surface has been assumed to be a consequence of species that have migrated up from their deep daytime depths. However, some of this difference might be an artifact of visual net avoidance. Here, we present a method that facilitates quantification of zooplankton that are migrating, those that are not migrating, and those able to avoid net capture. The algorithm is applied to an extensive data set gathered in the Azores Front region. Results indicate that DVM, and thus active carbon transport, calculated in the traditional manner would overestimate the true value by ~50%.

Ccm1, a regulatory gene controlling the induction of a carbon-concentrating mechanism in Chlamydomonas reinhardtii by sensing CO2 availability.

Aquatic photosynthetic organisms, including the green alga Chlamydomonas reinhardtii, induce a set of genes for a carbon-concentrating mechanism (CCM) to acclimate to CO2-limiting conditions. This acclimation is modulated by some mechanisms in the cell to sense CO2 availability. Previously, a high-CO2-requiring mutant C16 defective in an induction of the CCM was isolated from C. reinhardtii by gene tagging. By using this pleiotropic mutant, we isolated a nuclear regulatory gene, Ccm1, encoding a 699-aa hydrophilic protein with a putative zinc-finger motif in its N-terminal region and a Gln repeat characteristic of transcriptional activators. Introduction of Ccm1 into this mutant restored an active carbon transport through the CCM, development of a pyrenoid structure in the chloroplast, and induction of a set of CCM-related genes. That a 5,128-base Ccm1 transcript and also the translation product of 76 kDa were detected in both high- and low-CO2 conditions suggests that CCM1 might be modified posttranslationally. These data indicate that Ccm1 is essential to control the induction of CCM by sensing CO2 availability in Chlamydomonas cells. In addition, complementation assay and identification of the mutation site of another pleiotropic mutant, cia5, revealed that His-54 within the putative zinc-finger motif of the CCM1 is crucial to its regulatory function.

Zooplankton vertical migration and the active transport of dissolved organic and inorganic carbon in the Sargasso Sea

The least known component of the “biological pump” is the active transport of carbon and nutrients by diel vertical migration of zooplankton. We measured CO 2 respiration and dissolved organic carbon (DOC) excretion by individual species of common vertically migrating zooplankton at the US JGOFS Bermuda Atlantic Time-series Study (BATS) station. The inclusion of DOC excretion in this study builds on published research on active transport by respiration of inorganic carbon and allows a direct assessment of the role of zooplankton in the production of dissolved organic matter used in midwater microbial processes. On average, excretion of DOC makes up 24% (range=5–42%) of the total C metabolized (excreted+respired) and could represent a significant augmentation to the vertical flux that has already been documented for respiratory CO 2 flux by migrant zooplankton. Migratory fluxes were compared to other transport processes at BATS. Estimates of combined active transport of CO 2 and DOC by migrators at BATS averaged 7.8% and reached 38.6% of mean sinking POC flux at 150 m, and reached 71.4% of mean sinking POC flux at 300 m. DOC export by migrator excretion averaged 1.9% and reached 13.3% of annual DOC export by physical mixing at this site. During most of the year when deep mixing does not occur, diel migration by zooplankton could provide a supply of DOC to the deeper layers that is available for use by the microbial community. A carbon budget comparing migrant zooplankton transport to the balance of fluxes in the 300–600 m depth strata at BATS shows on average that the total migrant flux supplies 37% of the organic carbon remineralized in this layer, and that migrant DOC flux is more than 3 times the DOC flux gradient by diapycnal mixing. New estimates of active transport of both organic and inorganic carbon by migrants may help resolve observed imbalances in the C budget at BATS, but the magnitude is highly dependent on the biomass of the migrating community.