Sampling was conducted during the TransPEGASO cruise aboard the Spanish R/V Hespérides, from 20 October to 21 November 2014. A total of 41 stations were sampled within a transit across the Atlantic Ocean from Cartagena (SE Spain) to Punta Arenas (S Chile, Fig. 1). During the cruise, the ship crossed six biogeographical provinces (Longhurst, 1998): the Northeastern Subtropical Gyre, the Canary Current Coastal, the North Atlantic Tropical Gyre, the Western Tropical Atlantic, the South Tropical Gyre and the SWAS. Seawater was collected from 4 m depth using the ship's underway pump (BKMKC–10.11, Tecnium, Manresa, Spain) and screened through a 150 µm Nylon mesh to remove large particles. Temperature and salinity were measured continuously using a SBE21 Sea Cat Thermosalinograph. Solar irradiance was also measured continuously using a LI-COR Biospherical PAR Sensor. The rest of the variables were collected twice a day (09:00:00 and 16:00:00 local time) with the ship moving at approximately 10 knots.

We used R software packages lmodel2 and ggplot2 (RStudio Team, 2016) to test for covariations and to explore the potential controlling variables of TEP distribution across the Atlantic Ocean. We performed pairwise Spearman correlation analyses between TEP and POC concentrations. We performed bivariate and multiple regression analyses (ordinary least squares, OLS) between TEP concentrations and several physical, chemical and biological variables. Data were log transformed to fulfil the requirements of parametric tests. Ranged major axis (RMA) regression would have been more suitable since there were errors in both our dependent and independent variables. However, we decided to perform OLS regressions for a better comparison of slopes between our study and those available in the literature. The nonparametric Wilcoxon–Mann–Whitney test was carried out to compare variables, like TEPs and POC, among regions. Two main regions were analyzed separately due to remarkable differences in nutrient, Chl a and TEP concentration: the open Atlantic Ocean (OAO, n=30), with exclusion of the single sample from the edge of the Canary Coastal Upwelling (CU), which had a much higher TEP concentration; and the SWAS (n=10).

TEP concentrations ranged from 18.3 to 446.8 µg XG eq L-1 along the entire Atlantic Ocean transect. Across OAO, CU included, nitrate and phosphate concentrations were low and relatively homogeneous (nitrate: 0.47±0.51 µmol L-1; phosphate: 0.11±0.06 µmol L-1). Silicate ranged between 0.20 and 1.42 µmol L-1, and presented the minimum concentrations in the CU station and surroundings, and the maximum concentration at station 14. The temperatures ranged from 20.7 to 29.6 ∘C (25.6±23.8 ∘C), with maximum values in the Equatorial Counter Current (∼0–20∘ N, 29.1–29.6 ∘C), and minimum values around the CU and in the southernmost stations of the OAO (22.6–23.6 ∘C). The salinity ranged between 34.8 and 37.4, with the minimum values in the Equatorial Counter Current and the maximum values around 10–30∘ S. The Chl a concentration was low and quite homogeneous (0.36±0.22 mg m-3), even at the CU (0.25 mg m-3).

In the Northeastern Subtropical Gyre and the Canary Current Coastal (stations 1 to 7, Fig. 1) Chl a concentration ranged from 0.24 to 0.37 mg m-3. The phytoplankton biomass was generally dominated by Prochlorococcus, with an average of 1.68×105±0.81×105 cells mL-1, which corresponded to a biomass of 8.58±4.16 µg C L-1. TEP concentration in this region ranged from 54.2 to 131.7 µg XG eq L-1 (average 73.9±27.3 µg XG eq L-1). In the station 8 we sampled the edge of the CU. The decrease in silicate (0.26 µmol L-1) was accompanied by a relative increase in diatoms (9.4-fold increase) and dinoflagellates (1.3-fold increase) with respect to surrounding stations (Fig. 2b, e). Prochlorococcus abundance decreased to 9×103 cell mL-1 and a biomass of 0.46 µg C L-1. In this station, TEP concentrations were the highest found along the whole transect (446.7 µg XG eq L-1) but the Chl a concentration (0.25 mg m-3) was lower than in the neighboring region. Consequently the TEP:Chla ratio was the highest of the whole transect (1760.4). Moving south, the North Tropical Gyre (stations 9 to 13) showed an increase in silicate concentration, from 0.20 to 0.79 µmol L-1. The Chl a concentration ranged from 0.41 to 0.57 mg m-3 (Fig. 2c). In the northernmost part of this region (stations 9 to 11), phytoplankton biomass was dominated by Synechococcus, with an average of 7.7×104±0.8×104 cells mL-1, which corresponded to a biomass of 13.5±1.4 µg C L-1. By contrast, the southernmost stations (12 and 13) were dominated by Prochlorococcus, with an average of 2.6×105±0.5×105 cells mL-1, that corresponded to a biomass of 13.2±2.7 µg C L-1 (Fig. 2e). TEP concentrations were similar to those in the Northeastern Subtropical Gyre and the Canary Current Coastal, ranging between 78.1 and 123.9 µg XG eq L-1. Station 14, with a relatively high temperature (29.0 ∘C) and low salinity (35.2) was probably the most influenced by the Equatorial Counter Current. In this station, the silicate concentration (1.41 µmol L-1) was the maximum observed in the whole transect, and there was an increase in dinoflagellates and “other microalgae”, and a decrease in Prochlorococcus. The Chl a concentration (0.48 mg m-3) was similar to the surrounding stations and TEPs were 49.4 µg XG eq L-1. Moving further south, in the Western Tropical and the South Tropical Gyre (stations 15 to 31) Chl a ranged from 0.20 to 0.41 mg m-3 and the silicate concentration decreased (0.42–1.39 µmol L-1). TEP presented the lowest average values of the whole transect, ranging from 25.5 to 80.4 µg XG eq L-1. Overall in the OAO (excluding CU), TEPs ranged from 18.3 to 131.7 µg XG eq L-1 (average 59.9±27.4 µg XG eq L-1) and the TEP:Chla ratio ranged between 81 and 360 (average 183±56; Table 1).

The southernmost part of the cruise transect corresponded to the SWAS (stations 32 to 41). In this region, temperature (7.6–13.9 ∘C) and salinity (32.6–33.6) were lower on average than those found in the OAO (Table 1). The SWAS could be further divided into two regions according to different inorganic nutrient (nitrate and phosphate) concentrations (p<0.05) and phytoplankton composition. The northern SWAS (stations 32 to 36) presented lower nitrate (0.16 to 4.15 µmol L-1) and phosphate (0.31 to 0.62 µmol L-1) concentrations than the southern SWAS (stations 37 to 41; nitrate: 2.16 to 8.92 µmol L-1, phosphate: 0.51 to 0.89 µmol L-1). Silicate was more homogeneous throughout (0.31 to 1.27 µmol L-1). Chl a concentration across the entire SWAS (1.07–3.75 mg m-3) was significantly higher than in the OAO, with no major differences between the northern and the southern parts. In most of the northern SWAS, phytoplankton biomass was dominated by “other microalgae”, with an average of 10.2×105±6.1×105 cells L-1, which corresponded to a biomass of 43.7±25.8 µg C L-1. In station 35, an increase in diatoms (58 121 cells L-1 and a biomass of 145.2 µg C L-1) and dinoflagellates (44 896 cells L-1 and a biomass of 3.3 µg C L-1) was observed, coinciding with a decrease in silicate (0.32 µmol L-1). Here in northern SWAS, TEPs ranged from 98.6 to 427.2 µg XG eq L-1, with the maxima in stations 34 and 35 (Fig. 2f). In the southern SWAS (stations 37 to 41), phytoplankton biomass was dominated by picoeukaryotes, with an average of 6.34×104±1.93×104 cells mL-1, which corresponded to a biomass of 83.6±25.5 µg C L-1. TEP concentration ranged 168.6–395.7 µg XG eq L-1. Overall in the SWAS, TEPs ranged from 98.6 to 427.2 µg XG eq L-1 (average 255.7±130.4 µg XG eq L-1) and the TEP:Chla ratio ranged from 31 to 165 (average 97±42) (Table 1).

TEPs and POC covaried significantly and positively across the entire TransPEGASO transect (Spearman rs analysis, r=0.91, p<0.01, n=17). The contribution of TEP–C to the POC pool (TEP–C%POC) ranged between 34 % and 103 % in the OAO (average 66±19 %), and between 28 % and 110 % in the SWAS (average 73±36 %). POC was not analyzed in the CU (Fig. 3). To better explore the importance of TEP–C with respect to other major quantifiable POC pools, we estimated phytoplankton biomass (phyto–C) and HP biomass (HP–C) throughout the whole cruise (Fig. 2). It is worth mentioning that POC also includes other fractions of nonliving non-TEP organic carbon (e.g., cell fragments and Coomassie stainable particles), but phytoplankton and heterotrophic prokaryotes are generally considered the most abundant in open sea water (Ortega-Retuerta et al., 2009b; Yamada et al., 2015). TEP–C contributed the most to the POC pool in the OAO, where it represented twice the share of phyto–C and HP–C. In the SWAS, conversely, TEP–C was not significantly different than phyto–C, and was 3 times higher than HP–C (Fig. 3).

TEPs were significantly and positively related to Chl a along the entire transect (R2=0.61, p<0.001, n=39, Table 3). The regression equation for log converted TEP vs. Chl a was log TEP=2.09(±0.04)+0.66(±0.08)×log⁡ Chl a. Considering the two study regions separately, only in the OAO was the relationship significant, with a higher slope than in the entire transect (log TEP = 2.31(±0.10)+1.13(±0.20)×log⁡ Chl a; R2=0.56, p<0.001, n=29).

Across the whole transect, TEPs presented a significant (p<0.05) positive relationship with total phytoplankton biomass (Table 3) and with some phytoplankton biomass groups: Synechococcus (R2=0.30), picoeukaryotes (R2=0.49), diatoms (R2=0.19) and “other microalgae” (R2=0.27), and with HPA (R2=0.60). TEPs were negatively related to silicate (R2=0.19) and coccolithophores (R2=0.15). Some differences arose from examining the two regions separately. Within the OAO, TEPs presented a significant (p<0.001) positive relationship with Chl a (R2=0.56), total phytoplankton biomass (R2=0.47) and some phytoplankton groups (Synechococcus, picoeukaryotes, diatoms, dinoflagellates and “other microalgae”, Table 3), but not with HPA. TEPs showed a significant (p<0.001) negative relationship with the previous 24 h averaged solar irradiance (R2=0.43, Fig. 4). Multiple regression analyses showed the combined positive effect of Chl a and HPA on TEP distribution in the OAO (Table 4). By contrast, within the SWAS, TEPs only presented a significant (p<0.05) positive relationship with total phytoplankton biomass (R2=0.62) and HNA (R2=0.46, Table 3).

We present the first distribution of surface (4 m) TEP concentration along a latitudinal gradient in the Atlantic Ocean, covering both open sea and shelf waters. It is worth mentioning that vertical variability within the top surface meters (<4 m) has sometimes been observed (Wurl et al., 2009), but 4 m is usually considered “surface ocean” in studies where samples are collected with either an oceanographic rosette or an underway pumping system. The existing information about TEP distribution in surface waters of the open oceans is compiled in Table 2. The TEP concentrations that we measured across the OAO (CU included) generally fall within the range reported in other studies from the open ocean (Table 2). However, our levels are higher than those observed in the Mediterranean Sea (Ortega-Retuerta et al., 2010, 2017), Pacific Ocean (Ramaiah et al., 2005; Kodama et al., 2014; Iuculano et al., 2017b) and one study in the northwestern Atlantic Ocean (Cisternas-Novoa et al., 2015), and lower than that reported in the eastern Mediterranean Sea (Bar-Zeev et al., 2011). We believe that one of the reasons for the higher values found in our study compared with these previous studies is the depth. Mean TEP values in some of them (Ortega-Retuerta et al., 2010; Kodama et al., 2014; Cisternas-Novoa et al., 2015; Ortega-Retuerta et al., 2017) correspond to the above mixed layer depth or from 0 to 100 or 200 m. As TEPs tend to accumulate in the surface and our values correspond only to the surface (4 m), this could explain the higher values obtained in our dataset. Another reason seems to be the different Chl a concentrations, as the main TEP producer is phytoplankton. Chl a concentration in the OAO (0.4±0.2 mg m-3 (0.2–0.6 mg m-3)) was generally higher than in the other studies referred in the Table 2. For example, in Iuculano et al. (2017b) Chl a ranged 0.05–0.31 mg m-3, and in Kodama et al. (2014) it averaged 0.05±0.01 mg m-3. We also cannot discard the possibility that differences in TEP chemical composition could cause differences in staining capacity.

We found maximum TEP concentrations in the regions with high nutrient supply, namely in the station located in the CU and within the SWAS. Ours are the first TEP concentrations ever measured in the SWAS (Table 1), and only three more studies have reported TEP concentrations in coastal or shelf waters of the Atlantic Ocean (Harlay et al., 2009, 2010; Jennings et al., 2017). The SWAS is a high-nutrient region due to the arrival of cold nutrient-rich subantarctic water with the Malvinas Current. This current collides near 40∘ S with the southward-flowing Brazil Current (Gordon, 1989; Piola and Gordon, 1989; Peterson and Stramma, 1991; Palma et al., 2008). The nutrient-rich water in the region is responsible for the proliferation of phytoplankton and HP, which could partly explain the high TEP concentrations in this region. It is also known that large freshwater discharges occur in the shelf (Piola, 2005). These discharges could bring allochtonous HP directly to the shelf or bring DOM loads, which would stimulate autochtonous microbes. Besides, DOM inputs associated with freshwater discharges could also contain TEPs and their precursors. Although no previous information on TEP distribution exists for this area, previous studies in similarly productive areas or during phytoplankton blooms already observed high TEP concentrations (Long and Azam, 1996; Harlay et al., 2009; Klein et al., 2011). The TEP levels we measured at the SWAS are generally within the range of those reported for coastal areas (Passow and Alldredge, 1995; Passow et al., 1995; Riebesell et al., 1995; Kiorboe et al., 1996; Hong et al., 1997; Jähmlich et al., 1998; Wild, 2000; Ramaiah et al., 2001; Engel et al., 2002b; García et al., 2002; Radic et al., 2005; Scoullos et al., 2006; Sugimoto et al., 2007; Harlay et al., 2009, 2010; Wurl et al., 2009; Fukao et al., 2011; Klein et al., 2011; Sun et al., 2012; Van Oostende et al., 2012; Dreshchinskii and Engel, 2017; Jennings et al., 2017). Only two studies, in the western Baltic Sea and the Dona Paula Bay (Arabian Sea), reported TEP levels higher than ours (Engel, 2000; Bhaskar and Bhosle, 2006).

The significant positive correlation between TEPs and POC observed in our study highlighted the importance of TEP-determining POC horizontal variations in the surface Atlantic Ocean, suggesting a high contribution of TEPs to this pool. A few values of TEP–C%POC were unrealistically higher than 100 %, a feature that has also been observed in other studies (Engel and Passow, 2001; Bar-Zeev et al., 2011; Yamada et al., 2015). This suggests the inaccuracy of the use of standard TEP-to-carbon conversion factors (CFs, 0.51 µg TEP–C L-1 (µg X eq L-1)-1 in our case). Therefore there is a need to define specific CFs for diverse regions or environmental conditions. Nonetheless, an alternative explanation for the apparent oversizing of the relative TEP–C pool may be strictly methodological: TEPs are determined on filters of 0.4 µm pore size, whereas POC is measured on glass fibre filters with nominal pore size of 0.7 µm. It is plausible, thus, that some of the smaller TEPs are not taken into account in the POC measurement.

All in all, our results clearly show that TEP–C constituted an important portion of the POC pool in the Atlantic Ocean (from 28 % to 110 %). This contribution is comparable to that reported in the eastern Mediterranean Sea (Bar-Zeev et al., 2011; Parinos et al., 2017), lower than in the western Arctic (Yamada et al., 2015), but higher than in the northeastern Atlantic Ocean (Harlay et al., 2009, 2010). Both in the OAO and SWAS, TEPs comprised the largest share of the POC pool, with phyto–C being equal or the second most important contributor to POC (Fig. 3). Phyto–C surpassed TEP–C in only one station in the SWAS. The contribution of phyto–C and HP–C to the POC pool should be considered with caution, as the glass fibre filters (nominal pore size 0.7 µm) used to analyze POC could have not retained all the small phytoplankton organisms and prokaryotes (Gasol and Morán, 1999), causing underestimation of the actual POC pool. Furthermore, conversion factors carry quite an uncertainty, as pointed out in the Methods section.

A previous study in a eutrophic system reported TEP–C as the dominant POC contributor (Yamada et al., 2015), whereas others found that phyto–C represented the largest share to POC compared to TEP–C and HP–C (Bhaskar and Bhosle, 2006; Ortega-Retuerta et al., 2009b; de Vicente et al., 2010). With our results taken all together, we hypothesize that in oligotrophic conditions TEP–C is the predominant POC fraction, because nutrient limitation favors TEP production by phytoplankton and limits TEP consumption by bacteria. Conversely, in eutrophic conditions, the predominant POC fraction depends on many variables like the community composition, the bloom stage and sources of TEPs other than phytoplankton.

In order to better understand and even predict the occurrence of TEPs in the surface ocean, it is important to describe their distribution together with those of their main putative sources (phytoplankton and heterotrophic prokaryotes), sinks and environmental modulators, across large-scale gradients. However, most of the previous studies of TEPs in the Atlantic Ocean were restricted to local areas, and, to our knowledge, only one included a complete description of these variables together in a long transect (Mazuecos, 2015).

Our dataset suggests that phytoplankton is the main driver of TEP distribution in the surface Atlantic Ocean at the horizontal scale, since significant positive relationships were observed between TEPs and both Chl a and phytoplankton biomass (Table 3). It is worth noting that Chl a was a good estimator of phytoplankton biomass when the entire cruise was considered, as these variables were tightly related (R2=0.79, p value < 0.001, n=36). The slope of the log converted TEP–Chl a relationship for the whole study (β=0.66±0.08, Table 3) was within the upper range amongst published data (Fig. 5), and the slope in the OAO (β=1.13±0.20) was the highest reported so far (Table 3, Fig. 5). In the SWAS, the TEP–Chl a relationship was not significant (p value > 0.05), yet it was for TEP–phytoplankton biomass (see below).

TEP:Chla ratios were significantly (p<0.001) higher in the OAO (both including or excluding the CU) than in the SWAS (Table 1), with the maximum value in the station located in the CU. TEP:Chla values in the OAO (CU included) were comparable to those observed in other oligotrophic areas (Riebesell et al., 1995; García et al., 2002; Prieto et al., 2006; Harlay et al., 2009; Ortega-Retuerta et al., 2010; Kodama et al., 2014; Iuculano et al., 2017b; Parinos et al., 2017) (Table 2), while the values in the SWAS were comparable to those reported in eutrophic waters (Hong et al., 1997; Ramaiah et al., 2001; Engel et al., 2002b; Corzo et al., 2005; Ortega-Retuerta et al., 2009b). The higher TEP:Chla ratios in oligotrophic waters (Prieto et al., 2006) are related to nutrient scarcity, which is suggested to enhance TEP production by phytoplankton and prokaryotes (Myklestad, 1977; Guerrini et al., 1998; Mari et al., 2005; Beauvais et al., 2006). The highest TEP:Chla ratio of the entire transect observed in the station located in the CU was probably associated with the high relative abundance of diatoms and dinoflagellates. These groups are known to be strong TEP producers (Passow and Alldredge, 1994), and besides, previous studies have shown that TEP production rates reach maxima at late stages of the growth cycle, once nutrients have been exhausted (Corzo et al., 2000; Pedrotti et al., 2010; Borchard and Engel, 2015). In the CU, the relatively low Chl a level along with low silicate concentrations suggests that the upwelling–triggered bloom maximum had already passed, which resulted in a high TEP:Chla ratio. Although POC was not measured in the CU, high TEP:Chla suggests a high proportion of TEPs with respect to other organic particles. In the SWAS, the lower TEP:Chla ratios could be related with a lower rate of TEP production under relatively replete nutrient conditions. Extending our comparison to the literature, the TEP:Chla ratio is generally higher in oligotrophic regions (Prieto et al., 2006; Ortega-Retuerta et al., 2010; Kodama et al., 2014; Iuculano et al., 2017b) than in eutrophic regions (Hong et al., 1997; Engel et al., 2002b, 2017; Corzo et al., 2005; Ortega-Retuerta et al., 2009b; Klein et al., 2011).

In the OAO, the phytoplankton groups that showed a significant (p<0.05) positive relationship to TEPs and hence were candidates to be considered as the main producers of TEPs or their precursors were Synechococcus, picoeukaryotes, diatoms, dinoflagellates and “other microalgae” (Table 3). All the abovementioned groups have been reported to produce TEPs (see references in the introduction). Conversely, coccolithophores and Prochlorococcus did not present a significant relationship with TEP. It has been shown in cultures that coccolithophores do not produce high amounts of TEPs (Passow, 2002b), and a previous study showed temporal disconnections between coccolithophores and TEP maxima (Ortega-Retuerta et al., 2018). However, in a previous study in the Atlantic Ocean, Leblanc et al. (2009) found an association of TEPs with coccolithophores.

The oligotrophic ocean covers a big portion of the global ocean and it is mostly dominated by picophytoplankton (Agawin et al., 2000), chiefly Prochlorococcus and Synechococcus (Partensky et al., 1999). Iuculano et al. (2017b) reported relatively high rates of TEP production by Prochlorococcus in culture, and Mazuecos (2015) found a significant and positive relationship of TEPs with Prochlorococcus abundance in the low-latitude oceans. The absence of significant covariation between TEPs and the abundant Prochlorococcus in our study suggests that these picophytoplankters are not the main TEP producers, or their production is strongly modulated by environmental conditions. It is remarkable that, amongst the phytoplankton groups of the present study, Synechococcus biomass presented the highest correlation (R2=0.72) with TEP concentration in the OAO. Deng et al. (2016) demonstrated TEP production by marine Synechococcus in a laboratory study, but only Mazuecos (2015) had previously found a significant and positive relationship (R2=0.26–0.36) between these two variables in the ocean, particularly in the Atlantic, North Pacific and Indian oceans. This author actually found that Synechococcus was the phytoplankton group with the highest relationship with TEP concentration. Our study supports the importance of Synechococcus as a TEP source in the oligotrophic ocean.

In the SWAS, unlike in the OAO, the significant relationship between TEPs and the total phytoplankton biomass (R2=0.62) was not accompanied by any relationship to any phytoplankton group (Table 3). This could be due to the high variability of the phytoplankton composition in the SWAS stations. Since many phytoplankton taxa are capable of TEP production, it is difficult to discern one group playing the main role. Moreover, as mentioned before, in these shelf waters TEP formation could have been further modulated by aggregation of colloids carried by freshwater discharges.

Regarding the influence of abiotic factors in TEP distribution, we found a negative relationship (R2=0.43) between TEP concentration and the 24 h averaged solar irradiance in the OAO (Fig. 4). The OAO stations were exposed to high solar radiation due to water transparency and their location in tropical and subtropical regions. Ultraviolet (UV) radiation causes TEP loss by photolysis (Ortega-Retuerta et al., 2009a) and inhibits TEP formation from precursors (Orellana and Verdugo, 2003). However, it has also been proved that solar radiation harms picophytoplanktonic cells through photobiological stress, inducing TEP production (Agustí and Llabrés, 2007; Iuculano et al., 2017b). Our results suggest that the roles of UV radiation in breaking up TEPs and/or limiting their formation from precursors overcome UV stress-induced TEP production.

The role of HPs as potential drivers of TEP distribution is not straightforward, since their net effect on TEP accumulation depends on local conditions. Across the entire transect, TEP concentration was significantly (p<0.001) and positively related to HPA (Table 3). However, the relationship was not significant considering the regions separately, and only in the SWAS were TEPs significantly (p<0.05) and positively related to HNA, considered to be a proxy of the more active cells (Servais et al., 1999; Lebaron et al., 2001). This relationship in the SWAS could indicate that HPs used TEPs as a significant carbon source or that both HPs and TEPs were controlled by the same drivers, such as the presence of dissolved polysaccharides, which are substrates for HP as well as TEP precursors (Mari and Kiorboe, 1996). In the OAO, despite the lack of a paired relationship between TEPs and HPA, multiple regression analyses showed that both phytoplankton and HPs contributed significantly to explain TEP concentration variance (Table 4).

In summary, our study describes for the first time the horizontal distribution of TEPs across a north–south transect in the Atlantic Ocean. TEPs constituted a large portion of the POC pool, larger than phytoplankton at most stations and always larger than heterotrophic prokaryotic biomass. This supports the important role of TEPs in the carbon cycle. The drivers of TEP distribution were primarily phytoplankton and, to a lesser extent, heterotrophic prokaryotes among sources, with Synechococcus playing an outstanding role in the oligotrophic ocean. In the oligotrophic ocean, solar irradiance was also a major identifiable sink. We call for the need to carry out more extensive studies in the ocean, across both space and time, in order to better predict the occurrence of TEPs and incorporate diagnostic relationships in model projections. These diagnostic studies must be combined with further process studies if we are to relate TEP concentrations to important biogeochemical processes such as microbial colonization of particles, organic matter export to the deep ocean, gas exchange at the air–water interface and organic aerosol formation.