Bacterioplankton are heterotrophic bacteria and archaea who are responsible for decomposing and demineralizing dissolved organic matter. Specifically, these microorganisms are regulated by abiotic factors, including light, climate, and the amount of nutrients available in the water, given seasonal conditions. The effects of large-scale environmental factors over a period of days to weeks are impacted by the grazing, photosynthesis, molecule exchanges, viral lysis, and grazing. These factors determine the success and abundance of specific bacterioplankton populations^[1].

Bacterioplankton contribute significantly and play a key role in Carbon cycling, but these contributions can be difficult to measure. However, as a part of the Export Process in the Oceans from RemoTe Sensing (EXPORTS) Program, previous research has been able to measure the influence of bacterioplankton communities and dissolved organic matter (DOM) composition on the Carbon Cycle^[2]. Notably, recorded measurements of Bacterioplankton production rates related positively with the rate of DOM degradation. Significant contributions were also found to be from the Flavobacteriaceae family^[2]. DOM became less degraded over time, leading bacterioplankton productivity to increase, and create a more diverse community^[2].

The biological carbon pump is both a process that processes and separates organic matter. This is accomplished by re-mineralizing particulate and dissolved sources of carbon, as occurs in the passive sinking flux of particulate organic matter (POM), the physical deep mixing of Dissolved organic matter (DOM), and the movement of carbon that occurs by the vertical migration of zooplankton. These pathways are able to transport carbon sources to depths where carbon is sequesters and sedimented into the ocean floor after the death of zooplankton. These pathways also help to transport organic carbon from the atmosphere through photosynthesis, where carbon can be stored for decades or centuries^[3]. The carbon biomass contributed by Bacterioplankton to the carbon cycle changes depending on whether the species is in oligotrophic or eutrophic waters.

The release of carbon biomass is contributed by bacterioplankton through the extracellular release of phytoplankton cells, grazing activity such as sloppy feeding, excretion, or egestion by microzooplankton grazers, and the dissolving of organic particles into marine environments. Bacterioplankton, as heterotrophic bacteria, act as the primary conduit for taking up bioavailable DOM and moving it to higher trophic levels or remineralization, vital to the microbial loop as a critical flow of carbon that influences and modifies the ocean carbon cycle^[4].

The annual net primary production (NPP), by comparison, is produced by phytoplankton found globally across the world's oceans, and despite making up less than 0.2% of photosynthetic biomass, maintain rapid turn over times that contribute for nearly half of the NPP by the amount of oxygen and new organic matter they produce^[5].

By comparison, the Bacterioplankton Carbon Demand (BCD) is estimated to account for 20 to 100% of the decrease in the sinking Particulate Organic Carbon flux with depth^[6]. The estimated net BCD can be estimated as the sum of net amount of heterotrophic bacterioplankton to the carbon that is respired in the form of carbon dioxide.

Comparing the BCD to NPP allows researchers to evaluate how much NPP can support BCD. The ratio between BCD:NPP have been reported as low as 0, while exceeding 1 in other regions. The BCD:NPP ratio is important to identify regions of net primary productivity. Understanding the location of these regions provides scientists with information on regions where there is a net release of CO2 from ocean systems, and serve as a source of DOC accumulation on seasonal time scales. In regions where BCD:NPP is low, the microbial loop has been determined to be "malfunctioning" by some researchers. This can occur for several reasons including phytoplankton growth exceeding the metabolic capacity of heterotrophic consumption, the production of the product or precursors to recalcitrant compounds, or the inability of phytoplankton to grow on the available organic matter within the region. Research has also found genetic or community expression, including competition between bacterioplankton to phytoplankton in the region may influence the BCD:NPP ratio. Previous studies have found that BCD may more closely track grazer or viral mediated release of DOM, rather than DOM being provided by an sudden influx caused by phytoplankton production. The disconnect between BCD and NPP can arise from either one, a combination of, or all of the described conditions above^[5].

Dissolved organic matter (DOM) is a key nutrient and contributor to the total amount of carbon within ecosystems, including forest ecosystems where DOC provides 40 to 50% of the carbon used by soil microbes. In high concentration DOM environments, bacterial abundance increases. Notably, substrate-specific bacterial communities is able to shape DOM composition and diversity^[7]. The bioavailability of Dissolved Organic Matter (DOM) is critical to the composition of microbial communities. DOM is made bioavailable by the chemical composition of organic matter (OM) and the microbial community composition. When OM is more bioavailable, previous studies have found that there was a three-fold increase in availability from the classes Bacteroidetes, Alphaproteobacteria, and Gammaproteobacteria, of which are heterotrophic bacteriophages. The results showed that it is possible that multiple classes are responsible for removing organic carbon and the degrading of OM by heterotrophic bacteria^[8]. Labile DOM is dissolved organic matter (DOM) which is bioavailable in the water column^[8].

The degradation state of DOM is critical in the structuring of microbial communities. Specifically, the amount of DOM plays a role in the establishment of which communities are abundant in the water column, selecting for specific taxa based on their metabolic qualities to use different carbon quantities^[9]. Labile DOM is dissolved organic matter (DOM) which is bioavailable in the water column. Bacterioplankton are limited to utilizing low molecular weight (<600 Da) DOM. Thereby, in order to facilitate the movement of carbon, bacterioplankton must hydrolyze and consume high molecular weight or Particular Organic Matter (POM) to low molecular weight compounds^[8]. Specifically, labile DOM shifts microbial populations towards r-strategists, which are microbes who maintain fast copy numbers and high growth rates by exploiting nutrient resources within the water column, such as in river plumes. Due to this, competition among species can increase within the water column. Some species of R-strategist, or specialist, bacterioplankton include Bacteroidetes, which dominate plume waters. In contrast, nutrient poor conditions with low labile DOM concentrations are flow growing and have low ribosomal RNA, and are K-strategists. K-strategist, or generalist, Bacterioplankton are characterized for their slow growth and rely on cooperating among species to mitigate the scarcity of labile DOM. Previous studies suggest that labile DOM that favors r-strategist bacterioplankton taxa nearshore may become ecologically relevant offshore due to reduced bioavailable DOM concentrations^[9]. Dominant taxa included Alphaproteobacteria, Cyanobacteria, Gammaproteobacteria and bacteroidetes. Salinity also plays a key role in community composition; Cyanobacteria as well as bacteroidetes decreases at higher salinity, whereas Gammaproteobacteria increased at higher salinities. Researchers studying bacterial communities in the South China Sea have found that the alignment between salinity and overall DOM concentrations to bacterioplankton communities is significant. Compounds including proteins, amino sugars, and lipids expressed a negative correlation with bacterial diversity regardless of salinity concentrations. Notably, carbohydrates only exhibited a similar, negative relationship when salinity was above 31.0. Other DOM compounds were found to display positive correlations to bacterial diversity throughout the salinity gradient^[10]. As well, the high consumption of DOM near coastal communities has been found to act as a region of high carbon sequestration compared to open waters^[10]. Overall, microbial responses to DOM alter community compositions, and have been found to determine regions of high carbon sequestration.

Ocean Station PAPA (OSP) is located at the subarctic North East Pacific, and experiences seasonal changes in both the biomass and productivity if bacterioplankton. Recent changes observed in the 2018 Export Process in Ocean from Remote Sensing (EXPORTS) field study found changes in the OM Bioavailability at Ocean Station PAPA. Previous recording of bacterial growth efficiency (BGE) collected at OSP in 2018 has indicated tat heterotrophic bacterioplanktons play a significant role in the recycling of carbon within the carbon cycle; BGE values averaged 31 ± 7% during studies conducted in the late summer. These results support the hypothesis that marine microbes can efficiently convert organic carbon into biomass while also contributing to carbon remineralization in high-nutrient, iron-limited regions^[2].Bacterioplankton production, as well as net primary production can nearly double during the spring and summer season at OSP despite iron limitations that cause high-nutrient, low chlorophyll conditions^[8]. At OSP, BGE is directly influenced by the composition of DOM and the activity of Methylophilaceae, and Flavobacteriaceae, two important taxa of heterotrophic bacterioplankton. Surface water measurements at 5m and above showed bacteria increased by 0.7 ± 0.2 micromoles of C per liter by the stationary phase. Further, carbon remineralization rates averaged 0.19 ± 0.09 micromoles of Carbon per litre per day over a 6 to 10 day period. Researchers also found the relationship between the removal rate of Carbon (C) to the initial composition of total hydrolysable amino acids were significantly correlated. Carbon removal rates were found to be high when amino acids were able to be easily changed^[9].

BGE measurements are important to how carbon is sequestered and metabolized in ocean ecosystems, and helps researchers to determine whether bacterioplankton will re-mineralize organic carbon through increased respiration, or convert organic carbon into biomass that can be consumed and take carbon into higher trophic levels^[11]. It is calculated by the net amount of bacterial production (BP) over the amount of bacterial carbon demand (BCD) (BGE = BP/BCD). However, there are still knowledge gaps and uncertainties about the measurement of BGE values across ocean ecosystems; with few seasonal studies available, the true impact of bacterial production (BP) resulting from the consumption of labile DOM or bacterial carbon demand (BCD) is not universal and is uncertain^[11]. ***!!!Can add to this in the future, not yet.

The ocean carbon pump is the process by which organic carbon from the oceans surface in the form of CO2 is moved from the ocean surface to deeper ocean waters. This carbon pumps involved three pathways, being the carbonate pump, soft tissue pump, and solubility pump. The carbonate and soft tissue pump are the product of the flux from organic and calcium carbonate detritus respectively, which float down by gravity from the oceans surface. In contrast, the solubility pump results from an increase in the solubility of carbon dioxide into cold water, and its movement to the deep ocean through downwelling^[12][13]. As cold water retains a greater density and greater ability to take in gases, cold water is able to transport large volumes of atmospheric and organic carbon dioxide into the deep ocean^[13]. By comparison, upwelling allows deeper, cold water to return to the surface, bringing nutrient rich waters^[14]. These aspects demonstrate an abiotic and one-dimensional transport of carbon by either upwelling or downwelling which facilitates abiotic carbon transport from the euphotic zone into deeper, mesopelagic waters^[12].

The Export Process in the Ocean from RemoTe Sensing (EXPORTS) program is a campaign responsible for developing a predictive understanding and fate of the ocean's primary productivity, and how this affects the global carbon cycle in the ocean, as well as future climates. It is a multi-institutional effort which uses ocean optics, remote sensing, and molecular biology to understand the transport of carbon from the ocean's surface to deeper, mesopelagic waters^[15]. Led by NASA, the data collected by satellite provides global observations of the amount of phytoplankton stocks and rates of primary production. Based on this data, EXPORTS allows scientists to create a understanding of the export and production of organic carbon from the euphotic zone, and its transport below 500 m, also known as the mesopelagic zone. While most of the carbon is stored, a fraction of the carbon transported is respired back into the atmosphere in the form of carbon dioxide^[16]. Overall, EXPORTS provides a detailed and up-to-date information on the transport of carbon by phytoplankton primary production within the euphotic zone. The ultimate goal of EXPORTS is to provide new insights on ocean carbon cycling^[16].

The export of organic carbon to mesopelagic waters and its sequestration to the bottom of the ocean is supported by marine microbes, which regulate atmospheric CO2 levels and climate. EXPORTS maps how phytoplankton create, transform and transport organic carbon and organic matter into the deep ocean. The research team at EXPORTS focuses on the sinking particles throughout ocean depths and the diverse communities of marine microbes that produce, transform, and feed on organic carbon. This organic carbon can include marine snow, which is a mixture of organic carbon such as dead plankton, mucus, and waste. However, it is important to note the cycling of marine snow and organic carbon sequestration is difficult to measure, because we do not fully understand the export of carbon from the surface through the midwater, and thereby it can be difficult to quantify^[15].

The EXPORTS Field Campaigns goals are to address three questions: How do the three carbon transport pathways of organic matter move carbon from the surface to deeper ocean waters, what controls the efficiency of that transfer, and how can the uncertainty of these estimates be reduced? One such campaign at OSP was conducted, characterized as a high nutrient zone that experiences very little recycling as a result of its ocean mixed layer. Researchers in this campaign found that the mixed layer located at the surface of the ocean and zooplankton populations was found to mediate most of the transport of organic matter to deeper oceanic depths below 500m^[17].

Another contribution to our understanding of microbial life is by the research completed by the EXPORTS program on the food web rate and export flux determination within ocean currents and biogeochemical fields in different seasons. The campaign was conducted in the North Pacific Ocean, where the ocean current conditions and biogeochemical fields, as well as low temporal variability, characterized the fall season at OSP^[17]. These findings allowed researchers to better understand the biological carbon pump during the fall season at OSP.

Heterotrophic bacterioplankton and archaea play a dual role in the ocean's biological carbon pump; they both form sinking POC and are the primary drivers of degrading POC^[18]. Remineralization is carried out by heterotrophic bacteria and archaea, which respire organic carbon into CO₂. As a result, almost half of the carbon photosynthetically fixed is also respired, and only a small fraction of organic carbon is exported to the deep sea^[19]. Previous studies have found that microbes with sinking particles may influence the efficiency of carbon transport by facilitating aggregation, POM degradation and the tropic movement of POM^[18]. Sinking particles are a critical conduit for moving organic material at the surface of the ocean to deeper ocean waters^[20]. Sinking particles, also known as Marine snow, is formed by phytoplankton and cyanobacteria which collide and stick together to form larger aggregates. This stick material is called a transparent exopolymer particle (TEP)^[21]. For example, phytoplankton and bacteria produce TEP that act as sticky binding agents that enhance the aggregation of smaller organic carbon particles into heavier, more dense and faster sinking marine snow. Sinking organic particles also include zooplankton fecal pellets, which is produced when zooplankton graze on phytoplankton. As these pellets are more dense, they often sink faster than aggregates^[22]. In addition, sloppy feeding by Zooplankton, which occurs as zooplankton consume phytoplankton, cause other organic and fecal matter in addition to dead phytoplankton to form marine snow^[22]. Mineral ballast, including calcium carbonate from coccolithophores or silica from diatoms may increase the density of sinking particles and cause them to sequester faster than lighter particles^[23]. In addition, viral lysis of phytoplankton release intracellular organic matter which promotes marine snow formation. This mechanism is known as the viral shuttle^[24].

Sinking particles, including marine snow, are dissolved by heterotrophic bacteria which colonize and consume these particles as they sink. The depth of POC dissolving regulates how long carbon can be stored in the ocean, as well as the atmospheric CO₂ concentration; CO₂ is released to the mesopelagic zone, but can return to the atmosphere within decades up. POC dissolving at depths greater than 1000m leads to carbon sequestration greater than a century. To measure the efficiency of sinking POC before dissolving in the deep ocean, researchers use particle transfer efficiency, or T_eff. Currently, factors that regulate POC dissolution are poorly understood compared to the production of POC by autotrophs near the surface of the ocean. If systematic spatial variations in T_eff can be understood, it may become possible to understand the underlying environmental or ecological causes of POC re-mineralization^[25].

To re-mineralize POC, microbes rapidly colonize sinking particles to form biogeochemical hotspots^[26]. These dense communities produce extracellular enzymes which break down complex organic compounds rich in organic carbon, such as POC, into smaller, more liable compounds like DOC^[27]. Enzymatic hydrolysis, which uses lipases, carbohydrases, and proteases to breaks down POC. This POC will become DOC, of which is respired as CO₂^[28]. For instance, particle-associated bacteria will use these enzymes listed to hydrolyze POC into DOC, which will then be respired as CO₂^[29].This mechanism acts as the primary mechanism responsible for the attenuation of POC flux at greater ocean depths. As well, as organic particles release DOC into the surrounding water, heterotrophic microbial communities obtain an influx of organic nutrients that extends re-mineralization^[30]. In regions where the OSP is located, including the North Pacific Subtropical Gyre, Deep-sea Gammaproteobacteria, as well as Epsilonproteobacteria made up 80% of the bacterial community^[20].

The Microbial Carbon Pump (MCP) describes the transformation of bioavailable and labile DOM into recalcitrant dissolved organic matter (RDOC). This occurs through various microbial processes and contributes to the long-term storage of organic carbon by sequestering^[31]. The MCP operates through several microbial metabolic and ecological processes, but operates through microbial metabolism in the dissolved phase to produce carbon compounds that are resistant to degradation. This process is significant because small changes in the persistence of DOC, even compared to the atmospheric amount of CO₂, can influence the global carbon cycle^[32].

Microbial communities convert labile DOM into RDOC through selective consumption, transformation, or the re-synthesis of organic compounds. During these processes, microbes will use easily-degradable organic carbon substrates and release less-bioavailable carbon compounds. As well, altered polysaccharides, amino acid derivatives, or humic-like substances that are released as waste products can accumulate as RDOC^[31][32]. For example, previous research has found that bacterioplankton will transform simple organic compounds into RDOC by using repeated cycle of uptaking and releasing labile DOC. Transforming DOC into RDOC by this pathway reduces the likelihood that carbon will be respired back into CO₂^[33].

RDOC production is important for long-term carbon sequestration. As RDOC is resistant to microbial degradation, it is able to remain sequestered from any time scale from decades to a millennia; this allows for organic carbon sources to move from short-term biogeochemical cycling to long-term storage^[34]. As a result, the MCP provides a mechanism to store carbon sources independently of particle sinking, and allow carbon storage to occur beyond the biological pump. Moreover, abiotic conditions including temperature, nutrient availability, and microbial community composition can influence the efficiency of RDOC production^[35].

Iron is a key limiting micronutrient in marine environments, especially in high-nutrient, low chlorophyll (HNLC) regions including the Pacific and Southern Ocean. The Martin Iron Hypothesis states that the abundance of phytoplankton depends on the bioavailability of iron, which regulates their growth specifically within HNLC regions. Increasing iron supply, as such, has been suggested to enhance primary productivity and draw down more atmospheric CO₂. This hypothesis was originally proposed by John Martin, who famously sumarized the hypothesis in the idea as such: "Give me a half tanker of iron, and I will give you an ice age"^[36].

The most notable of HNLC enrichment experiments was IronEx II, which demonstrated that adding bioavailable iron to HNLC waters led to rapid phytoplankton growth and the development of phytoplankton blooms dominated by large diatoms^[37]. Primary productivity in HNCL regions are significantly increased and temporarily enhances carbon uptake and sequestration through the biological carbon pump via sinking particles. However, it is important to note other experiments such as SOIREE or EIFEX showed that much of the fixed carbon is rapidly re-mineralized in the surface of the ocean, with very limited long-term sequestration^[38].

As well, microbial productivity and community structure is strongly influenced by the addition of iron. When more iron is available, phytoplankton communities can shift from consisting of small picoplankton to larger diatoms; With more larger diatoms present, food webs dynamics are shifted, and export efficiency is increased. Heterotrophic bacteria will respond to increase organic matter production caused by greater volumes of diatoms, and result in greater microbial respiration and recycling of carbon and nutrients^[39]. In HNLC regions, iron limitation can also constrain nitrogen fixation. As Diazotrophs require iron-rich enzymes such as nitrogenase, which requires iron, iron availability constrains broader nutrient cycling^[40].

Re-mineralization is the conversion of organic nutrients back into inorganic forms that can be reused by primary producers, such as the cycling of nitrogen (N) and phosphorous (P). During the degradation of organic matter, heterotrophic bacteria release ammonium (NH₄⁺) and phosphate (PO₄³⁻) and replenish nutrient concentrations including N and P in the water column^[41][42]. This microbial recycling is important, especially in oligotrophic regions where nutrient supply from deeper water is limited and nutrients are in low concentrations^[43].

In the nitrogen cycle, nitrification, denitrification, and anammox are mediated by marine microbes. Nitrification is the oxidation of ammonium (NH₄⁺) to ammonia (NO₃⁻), whereas denitrification and anammox remove nitrogen from the system by converting NH₄⁺ into N₂ gas^[44]. By comparison, nitrogen fixation is carried out by diazotrophic microorganisms such as Trichodesmium. This species of marine microbe introduces nitrogen from a new source into the ocean, tightly regulated by environmental conditions such as oxygen and nutrient availability^[45].

Similar to nitrogen cycling, phosphorous cycling is regulated by Trichodesmium. These microbes can access dissolved organic phosphorous (DOP) which are not bioavailable in the water column through the production of enzymes such as alkaline phosphatase. Producing these enzymes allows this genus of marine microbes to utilize otherwise unavailable nutrient pools, and provides them a competitive advantage in low-phosphate environments^[46]. Due to its uptake of phosphorous and access to a limited, unavailable resource, Trichodesmium can also influence community composition.

Nutrient cycling stoichiometry is often described using the Redfield Ratio (C:N:P ≈ 106:16:1). The Redfield ratio describes the ratio of organic nutrients on average across oceans globally^[47]. Microbial remineralization tends to occur in proportions similar to this ratio, and thereby connects the influence of the nitrogen cycle, phosphorous cycle, and carbon cycle to the activity of microbes such as Trichodesmium. However, deviations from the Redfield ratio are common and reflect variations in microbial community structure, nutrient available and environmental conditions^[48]. The deviations in these environments can influence carbon export efficiency; as primary production influences carbon fixation and re-mineralization, the communities in these regions are also impacted and can vary between environments. As a result, communities can become more or less productive and change the balance between carbon fixation and re-mineralization.