| 10 Nov 2025
Organic Alkalinity modulates pH from the Sea-Surface Microlayer during a mesocosm study
Abstract. The ocean plays a central role in climate regulation by exchanging carbon dioxide (CO2) with the atmosphere. This exchange depends on the transfer efficiency across the air-sea boundary layer, the sea-surface microlayer (SML) known to be an organic-rich boundary with a thickness of less than 1 mm. The parameters dissolved inorganic carbon (DIC) and total alkalinity (TA) describe the state of the marine carbon system (MCS). However, organic alkalinity (OA), which arises from weak acid-base functional groups in dissolved organic matter, remains poorly constrained. It is known to modulate pH in organic-rich environments. Yet, to our knowledge, it has not been quantified directly in SML before. Here, we show that the enrichment of OA in the SML modulates pH and that its effect propagates further down into the underlying water (ULW). We track the evolution of the MCS during a 35-day mesocosm study where we induced a phytoplankton bloom. Three distinct bloom phases were identifyed by different biological processes dominating within the system. Dissolution dominated during the pre-bloom phase; photosynthesis and calcification prevailed during the bloom; and CO2 invasion, together with respiration, was most pronounced in the SML during the transition to the post-bloom phase. These processes provided the context for the observed variability in OA. We measured OA directly by differential potentiometric back-titration as a second titration on the same titrated TA samples. OA in the SML was persistently enriched (Enrichment Factor (EF) > 1) and reached concentrations up to 264 µmol kg⁻¹. On average, it contributed 8.4 % of TA, compared to 3.1 % in the ULW. Concurrently, the vertical pH differences between SML and ULW decreased towards zero as the bloom began and occasionally became negative. Over the study period, OA EF and ΔpH were negatively correlated (Spearman ρ = -0.75, p = 0.024), indicating that stronger OA EF dampens the pH rise associated with the bloom onset and its effect propagates further down to the ULW. Recognising that OA enrichment modulates pH in both the SML and the ULW, routine inclusion of OA in near-surface measurements and a three layer air-SML-ULW framework should guide future evaluations of air-sea CO2 exchange.
Review of “Organic Alkalinity modulates pH from the Sea-Surface Microlayer during a mesocosm study” -egusphere-2025-5265
General comments: This manuscript presents a well-designed mesocosm study investigating organic alkalinity (OA) dynamics in the sea-surface microlayer (SML) and its impact on pH modulation across phytoplankton bloom phases. The work addresses an important gap in marine carbon system research by providing direct OA measurements in the SML. The methodology is generally sound and well-described, and the conclusions are largely supported by the data. However, several points require clarification and revision before it can be accepted for publication.
Specific Comments:
Page 1, Line 29: “Spearman ρ = -0.75, p = 0.024” – The correlation is mentioned but the sample size (n=10 ?) should be stated for transparency.
Page 2, Lines 56-57: The sentence “Emiliania huxleyi consume HCO3- to form coccolith and produces acidic polysaccharides associated with dissolved oranic matter, whereas diatoms. From diatoms, Cylindrotheca closterium, release organic material relaterd to carboxylate-rich extracellular polymeric substances” contains multiple errors. “oranic” → “organic” ,“relaterd” → “related”. The sentence structure is fragmented and grammatically incorrect.
Page 3, Line 77: “under ssurfactant-enriched” → under surfactant-enriched
Page 4, Lines 100-110:The mesocosm setup description is adequate, but several details are missing: What was the light:dark cycle? How was temperature controlled/monitored? What was the actual nutrient concentration added (μM)?
Page 5, Lines 125-130: The glass plate technique sampling speed (~5 cm s-1) is reported, but the film thickness achieved is not stated. This is important for comparing with other studies.
Page 7, Lines 215-225: The bloom phase definition based on chlorophyll-a is mentioned, but actual Chl-a values are not provided in the main text, including representative values for each phase.
Page 8, Line 238: “CaCO3 dissolution appears to have been the dominant process in the SML” – This interpretation is not strongly supported by the vectors in Figure 1c, which show mixed signals. Please revise or provide additional evidence.
Page 10, Figure 2: The pie charts showing OA/TA fractions are informative, but the exact values (8.1% vs 3.1%) differ slightly from the abstract (8.4% vs 3.1%). Please clarify.
Page 12, Figure 4: The confidence interval for the slope should be reported in the figure caption.
Page 13-15, Lines 349-415: The discussion section is largely descriptive, restating the results rather than exploring mechanistic explanations for OA enrichment and pH modulation. Key mechanistic questions are unaddressed: (1) Why is OA enriched in the SML during bloom phase transitions (e.g., phytoplankton exudation vs. microbial processing vs. physical accumulation)?; (2) What specific functional groups of dissolved organic matter (DOM) drive OA in the SML?
Page 14, Lines 385-395: The comparison with literature values is useful, but the authors should address why their ULW OA values (40-100 μmol kg-1) exceed typical coastal values – could this be an artifact of the mesocosm system or the measurement method?