Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth's biogeochemical cycle. The two are in a constant state of dynamic change as a result of physical and biochemical processes; however, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the seasonal transition of DOM and POM pools and their drivers in a sub-Arctic fjord by means of monthly environmental sampling and by performing experiments at selected time points. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm) or left unfiltered, followed by 36 h incubations. Before and after incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), extracellular polymeric substances (EPSs), microbial community (flow cytometry), and molecular composition of DOM (high-performance liquid chromatography coupled to high-resolution mass spectrometry – HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed an increase in POC concentrations. While POC concentrations increased in September, DOM lability decreased based on changes in the average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December and February), when environmental POC concentrations were low, lower concentrations of POC were measured at the end of the experiments compared to at the start. The change in POC concentrations was significantly different between the biologically productive period and the winter period (t test; p<0.05). Simultaneously, the DOM pool became more labile during the incubation period, as indicated by changes in the average hydrogen saturation, aromaticity, and oxygen saturation, with implications for carbon cycling. The change in POC was not directly associated with an antagonistic change in DOC concentrations, highlighting the complexity of organic matter transformations, making the dynamics between POC and DOC difficult to quantify. However, in both periods, bacterial activity and EPS concentrations increased throughout the incubations, showing that bacterial degradation and physical DOM aggregation drive the transformations of POM and DOM in concert but at varying degrees under different environmental conditions.

Cyanobacteria have high iron requirements due to iron-rich photosynthetic machineries. Despite the high concentrations of iron in the Earth’s crust, iron is limiting in many marine environments due to iron’s low solubility. Oxic conditions leave a large portion of the ocean’s iron pool unavailable for biotic uptake, and so the physiochemical properties of iron are hugely important for iron’s bioavailability. Our study is the first to investigate the effect of iron source on iron internalization and extracellular reduction by Synechococcus sp. PCC 7002. The results indicated that the amorphous iron hydrolysis species produced by FeCl₃ better support growth in Synechococcus through more efficient iron internalization and a larger degree of extracellular reduction of iron than the crystalline FeO(OH). An analysis of dissolved iron (II) indicated that biogenic reduction took place in cultures of Synechococcus grown on both FeCl₃ and FeO(OH).

Iron is an essential, yet scarce, nutrient in marine environments. Phytoplankton, and especially cyanobacteria, have developed a wide range of mechanisms to acquire iron and maintain their iron-rich photosynthetic machinery. Iron limitation studies often utilize either oceanographic methods to understand large scale processes, or laboratory-based, molecular experiments to identify underlying molecular mechanisms on a cellular level. Here, we aim to highlight the benefits of both approaches to encourage interdisciplinary understanding of the effects of iron limitation on cyanobacteria with a focus on avoiding pitfalls in the initial phases of collaboration. In particular, we discuss the use of trace metal clean methods in combination with sterile techniques, and the challenges faced when a new collaboration is set up to combine interdisciplinary techniques. Methods necessary for producing reliable data, such as High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS), Flow Injection Analysis Chemiluminescence (FIA-CL), and 77K fluorescence emission spectroscopy are discussed and evaluated and a technical manual, including the preparation of the artificial seawater medium Aquil, cleaning procedures, and a sampling scheme for an iron limitation experiment is included. This paper provides a reference point for researchers to implement different techniques into interdisciplinary iron studies that span cyanobacteria physiology, molecular biology, and biogeochemistry.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.