Cycling of dissolved and particulate organic matter in the pelagic marine environment : Impact of phytoplankton community mortality and microbial degradation

Cell lysis, as a consequence of adverse conditions, has been recognized as an important loss process among phytoplankton, in addition to the well-known loss processes of grazing and sinking. Cell lysis has been connected to increased release of carbon fixed by phytoplankton as dissolved organic carbon (DOC), the primary carbon source for pelagic heterotrophic bacteria. This has the potential to enhance pelagic remineralization at the cost of reduced sedimentation of organic carbon. Cell lysis may, therefore, have global consequences as the ratio of pelagic remineralization to sedimentation widely determines whether oceans function as a source or a sink of atmospheric carbon. However, the subject has been studied predominantly in oceans and oligotrophic marine regions. The Baltic Sea is different from these environments and the causes and consequences of phytoplankton cell lysis may, therefore, be expected to differ. The studies included in this thesis are the first attempt to study phytoplankton cell lysis and its effect on carbon cycling in the Baltic Sea. The focus of the thesis is mainly on elucidating the abiotic and biological controls of cell lysis and its relationship with pelagic DOC concentration. These were studied on a spatial scale during a spring bloom on an area covering the Gulf of Finland, the Åland Sea and the Baltic Proper, and on a temporal scale during a two-year long monitoring campaign in an estuary in the northern Gulf of Finland. In both studies the proportion of cells undergoing lysis was measured using a membrane impermeable nucleic acid stain to indicate cells with compromised membrane integrity. The spatial monitoring study revealed considerable variation in the proportion of cells undergoing lysis with generally higher proportion of dying cells in deep water (1-10 m: average 84%, range 67-91%; 30 m: average 77%, range 62-90%; 60 m: average 71%, range 58-86%) and among nanophytoplankton (surface water average: 64%), as compared to smaller eukaryotic picophytoplankton (surface water average: 88%) and picocyanobacteria (surface water average: 82%). No clear correlations between cell lysis and nutrient concentrations were found, although there was a weak correlation between the proportion of intact eukaryotic picophytoplankton and phosphate concentration (R2 = 0.13, p = 0.029). No connection between cell lysis and DOC concentration was found. Also during the temporal monitoring campaign variation of cells undergoing lysis was high (surface water average: 62%, range 18-97%). Again, no correlation between nutrient concentrations and cell lysis was found, although this time there was a weak negative relationship between the proportion of cells undergoing lysis and DOC concentration (R2 = 0.15, p = 0.0185). In both studies some indication was found that phytoplankton lysis is less prevalent in conditions where interspecific phytoplankton competition is low. Details of the flow of carbon from phytoplankton to pelagic heterotrophic bacteria was studied experimentally using two phytoplankton species (a dinoflagellate Apocalathium malmogiense and a cryptophyte Rhodomonas marina). Contrasting species specific differences were found in their ability to transfer carbon from the inorganic pool via DOC to bacterial biomass and in the composition of the emerging bacterial community. The smaller R. marina released more bioavailable DOC and attracted a bacterial community mainly consisting of copiotrophs (bacteria thriving when DOC is abundant and highly bioavailable), which likely directs more carbon towards microbial loop. The DOC released by the larger A. malmogiense was less bioavailable. If these results can be generalized to other taxa of similar size, the fast consumption of DOC released by R. marina may partially explain why no relationship between the lysis of small phytoplankton and DOC concentration was found. The overarching conclusion from the two field studies is that the environmental conditions, such as nutrient limitation, that have been shown to promote cell lysis in oligotrophic marine regions are not the main determinants of cell lysis in the Baltic Sea. Also, the high ambient DOC concentration and terrestrial runoff in the Baltic Sea seem to mask the effect of cell lysis on DOC concentration. The group and species specific differences in both cell lysis and carbon cycling indicate that investigating cell lysis on lower taxonomic levels will help to connect cell lysis to carbon cycling.Kasviplanktonin yhteyttämisessään sitoma hiili siirtyy eteenpäin kasviplanktonsolujen upotessa, tullessa syödyksi tai hajotessa. Uppoaminen kuljettaa hiiltä vesipatsaan tuottavasta pintakerroksesta syvempiin vesikerroksiin ja meren pohjan eliöstölle, ja mahdollisesti poistaa hiiltä pinnan hiilenkierrosta pitkäksi aikaa. Eläinplanktonin laidunnus siirtää hiiltä suuremmille eläimille pinnan ravintoverkossa. Kasviplanktonsolujen hajoamisen vaikutusta hiilen kiertoon on tutkittu kahta muuta kuolleisuusmekanismia vähemmän eikä sen kaikkia seurauksia tiedetä. Väitöskirjassani selvitin hajoavien kasviplanktonsolujen osuutta kasviplanktonyhteisössä sekä kuolevien solujen vaikutusta erityisesti vesipatsaan liuenneen orgaanisen aineen määrään avoimella Itämerellä kevätkukinnan aikana sekä kahden vuoden seurannalla Suomenlahden pohjoisrannikolla. Selvitin myös ympäristön olosuhteiden vaikutusta kuolevien kasviplanktonsolujen osuuteen. Kenttätutkimusten lisäksi tutkin kokeellisesti kasviplanktonista lähtöisin olevan orgaanisen aineen käyttöä vesipatsaan bakteerien toimesta. Hajoavien solujen osuus oli alimmillaan 18% ja korkeimmillaan 97%. Toisin kuin valtamerissä, alhaiset ravinnepitoisuudet eivät selittäneet vaihtelua hajoavien solujen osuudessa Itämerellä. Hajoavia soluja oli vähiten silloin, kun kasviplanktonyhteisö koostui lähinnä yhdestä lajista, mikä voi viitata haitallisiin lajienvälisiin vuorovaikutuksiin yhtenä solujen hajoamista edistävänä tekijänä. Hajoavien kasviplanktonsolujen vuosittainen vaihtelu korreloi hieman liuenneen orgaanisen aineen pitoisuuden kanssa. Hajoavien kasviplanktonien määrällistä vaikutusta orgaanisen aineen kiertoon ei kuitenkaan pystytty määrittämään. Kokeissa havaittiin eroja eri kasviplanktonlajien tuottaman orgaanisen aineen määrässä ja laadussa sekä sen hajotuksessa ja sitä käyttävän mikrobiyhteisön koostumuksessa. Itämeren korkea liuenneen orgaanisen aineen määrä ja tuoreen orgaanisen aineen nopea hajotus bakteerien toimesta todennäköisesti selittävät, miksi valtamerissä havaittua yhteyttä liuenneen orgaanisen aineen pitoisuuksissa ja kasviplanktonin hajoamisessa ei havaittu Itämerellä yhtä selkeästi. Jotta Itämeren kasviplanktonin eri kuolleisuusmekanismien tarkka vaikutus hiilen kiertoon pystyttäisiin määrittämään, erot tärkeimpien kasviplanktonlajien tuottamassa liuenneessa orgaanisessa aineessa ja sen hajotuksessa tulisi tuntea paremmin

Contrasting patterns of carbon cycling and dissolved organic matter processing in two phytoplankton-bacteria communities

Microbial consumption of phytoplankton-derived organic carbon in the pelagic food web is an important component of the global C cycle. We studied C cycling in two phytoplankton-bacteria systems (non-axenic cultures of a dinoflagellate Apocalathium malmogiense and a cryptophyte Rhodomonas marina) in two complementary experiments. In the first experiment we grew phytoplankton and bacteria in nutrient-replete conditions and followed C processing at early exponential growth phase and twice later when the community had grown denser. Cell-specific primary production and total community respiration were up to 4 and 7 times higher, respectively, in the A. malmogiense treatments. Based on the optical signals, accumulating dissolved organic C (DOC) was degraded more in the R. marina treatments, and the rate of bacterial production to primary production was higher. Thus, the flow of C from phytoplankton to bacteria was relatively higher in R. marina treatments than in A. malmogiense treatments, which was further supported by faster C-14 transfer from phytoplankton to bacterial biomass. In the second experiment we investigated consumption of the phytoplankton-derived DOC by bacteria. DOC consumption and transformation, bacterial production, and bacterial respiration were all higher in R. marina treatments. In both experiments A. malmogiense supported a bacterial community predominated by bacteria specialized in the utilization of less labile DOC (class Bacteroidia), whereas R. marina supported a community predominated by copiotrophic Alphaand Gammaproteobacteria. Our findings suggest that large dinoflagellates cycle relatively more C between phytoplankton biomass and the inorganic C pool, whereas small cryptophytes direct relatively more C to the microbial loop.Peer reviewe

Viability of pico- and nanophytoplankton in the Baltic Sea during spring

Phytoplankton cell death is an important process in marine food webs, but the viability of natural phytoplankton communities remains unexplored in many ecosystems. In this study, we measured the viability of natural pico- and nanophytoplankton communities in the central and southern parts of the Baltic Sea (55°21′ N, 17°06′ E–60°18′ N, 19°14′ E) during spring (4th–15th April 2016) to assess differences among phytoplankton groups and the potential relationship between cell death and temperature, and inorganic nutrient availability. Cell viability was determined by SYTOX Green cell staining and flow cytometry at a total of 27 stations representing differing hydrographic regimes. Three general groups of phytoplankton (picocyanobacteria, picoeukaryotes, and nanophytoplankton) were identified by cytometry using pigment fluorescence and light scatter characteristics. The picocyanobacteria and picoeukaryotes had significantly higher cell viability than the nanophytoplankton population at all depths throughout the study area. Viability correlated positively with the photosynthetic efficiency (Fv/Fm, maximum quantum yield of photosystem II) as measured on the total phytoplankton community. However, an anticipated correlation with dissolved organic carbon was not observed. We found that the abiotic factors suggested to affect phytoplankton viability in other marine ecosystems were not as important in the Baltic Sea, and other biotic processes, e.g. processes related to species succession could have a more pronounced role.peerReviewe

Copernicus Ocean State Report, issue 6

The 6th issue of the Copernicus OSR incorporates a large range of topics for the blue, white and green ocean for all European regional seas, and the global ocean over 1993–2020 with a special focus on 2020

Phytoplankton and bacteria incubations in part 2 (dissolved organic matter consumption) of the microcosm experiment, Gulf of Finland, Baltic Sea

The data were collected from an experiment using phytoplankton cultures (Apocalathium malmogiense and Rhodomonas marina). The aim of the experiment was to study carbon cycling among phytoplankton and bacteria, and the effects on the dissolved organic matter (DOM) pool. Measured variables include phytoplankton and bacterial abundance, primary production, bacterial production and respiration, 14C-transfer from phytoplankton to DOM and bacteria, concentrations of particulate and dissolved organic carbon, nitrate, phosphate and chlorophyll a, and optical characteristics of dissolved organic matter. The experiment was conducted at Tvärminne Zoological Station, Hanko, Finland with non-axenic unialgal phytoplankton cultures and bacteria originating from the Baltic Sea. The experiment was conducted between Dec. 2017 and Apr. 2018. The experiment consisted of two parts, the DOM release experiment (part 1) and the DOM consumption experiment (part 2). Separate triplicate batch cultures of both phytoplankton species were grown for each experiment. In the DOM release experiment the cultures were grown for over 4 months and three day-long incubations (key point incubations, KPI's) were initiated on three occasions; the first KPI at early exponential growth phase and the second and third KPI's when the phytoplankton had grown more abundant. During each KPI and aliquot of the culture was inoculated with freshly collected sea water bacteria, and bacterial community composition was measured. This aliquot was then divided into two further aliquots; one was incubated with radioisotopes for productivity (primary and bacterial production) and 14C-flow analyses (production line) and one filtered through 0.8 µm for analysis of DOM optical properties. During the KPI's measurements were taken at 0, 4, 8 and 12 h. Nutrient concentrations (measured from non-filtered and 0.8 µm filtered samples) and concentration of dissolved organic carbon were measured only at 0 and 12 h. Concentrations of particulate organic carbon and nitrogen and chlorophyll a were measured only once for each KPI at the beginning of the incubation. In the DOM consumption experiments the cultures were grown to high abundance, after which the phytoplankton and most of the bacteria were filtered out. The filtrate was then inoculated with freshly collected sea water bacteria, after which it was incubated for 7 days. Bacterial abundance, production, respiration, and community composition, and concentration and optical properties of DOM were measured daily. The experimental design is explained in figure 1 of the associated publication. This data table contains measurements collected during the 7-day incubation of part 2 of the experiment (DOM consumption experiment). The measured variables are concentrations of nitrate, phosphate and dissolved organic carbon, incorporation rates of 3H-thymidine and 14C-leucine and bacterial production calculated based on these, abundance of high and low nucleic acid and total bacteria, flow cytometric side scatter of high and low nucleic acid bacteria, optical properties of DOM, and bacterial respiration rate. Daily respiration rate is calculated from continues oxygen measurement using optodes as explained in the associated publication

Production line part of the key point incubations in part 1 (dissolved organic matter release) of the microcosm experiment, Gulf of Finland, Baltic Sea

The data were collected from an experiment using phytoplankton cultures (Apocalathium malmogiense and Rhodomonas marina). The aim of the experiment was to study carbon cycling among phytoplankton and bacteria, and the effects on the dissolved organic matter (DOM) pool. The experiment was conducted at Tvärminne Zoological Station, Hanko, Finland with non-axenic unialgal phytoplankton cultures and bacteria originating from the Baltic Sea. The experiment was conducted between Dec. 2017 and Apr. 2018. The experiment consisted of two parts, the DOM release experiment (part 1) and the DOM consumption experiment (part 2). Separate triplicate batch cultures of both phytoplankton species were grown for each experiment. In the DOM release experiment the cultures were grown for over 4 months and three day-long incubations (key point incubations, KPI's) were initiated on three occasions; the first KPI at early exponential growth phase and the second and third KPI's when the phytoplankton had grown more abundant. During each KPI and aliquot of the culture was inoculated with freshly collected sea water bacteria, and bacterial community composition was measured. This aliquot was then divided into two further aliquots; one was incubated with radioisotopes for productivity (primary and bacterial production) and 14C-flow analyses (production line) and one filtered through 0.8 µm for analysis of DOM optical properties. During the KPI's measurements were taken at 0, 4, 8 and 12 h. Nutrient concentrations (measured from non-filtered and 0.8 µm filtered samples) and concentration of dissolved organic carbon were measured only at 0 and 12 h. Concentrations of particulate organic carbon and nitrogen and chlorophyll a were measured only once for each KPI at the beginning of the incubation. In the DOM consumption experiments the cultures were grown to high abundance, after which the phytoplankton and most of the bacteria were filtered out. The filtrate was then inoculated with freshly collected sea water bacteria, after which it was incubated for 7 days. Bacterial abundance, production, respiration, and community composition, and concentration and optical properties of DOM were measured daily. The experimental design is explained in figure 1 of the associated publication. This data table contains measurements taken during the production line, i.e. all the measurements involving radioisotopes. It is structured based on two light and one dark measurements of primary production. Primary production measurements themselves are given in https://doi.pangaea.de/10.1594/PANGAEA.937723. From each of these subsamples (2 light and 1 dark) the following variables were measured: 14C-DOM production from 14C-NaHCO3, bacterial incorporation of 14C originating from 14C-NaHCO3, 3H-thymidine incorporation rate, and 3H-thymidine based bacterial production (calculated from thymidine incorporation rate). Raw reads from the scintillation counting are not given, only the calculated production rates calculated as explained in the methods of the associated publication. This data table is explained in figure 2 of the associated publication

Monitoring of the growth of phytoplankton batch cultures in part 1 (dissolved organic matter release) of the microcosm experiment, Gulf of Finland, Baltic Sea

The data were collected from an experiment using phytoplankton cultures (Apocalathium malmogiense and Rhodomonas marina). The aim of the experiment was to study carbon cycling among phytoplankton and bacteria, and the effects on the dissolved organic matter (DOM) pool. The experiment was conducted at Tvärminne Zoological Station, Hanko, Finland with non-axenic unialgal phytoplankton cultures and bacteria originating from the Baltic Sea. The experiment was conducted between Dec. 2017 and Apr. 2018. The experiment consisted of two parts, the DOM release experiment (part 1) and the DOM consumption experiment (part 2). Separate triplicate batch cultures of both phytoplankton species were grown for each experiment. In the DOM release experiment the cultures were grown for over 4 months and three day-long incubations (key point incubations, KPI's) were initiated on three occasions; the first KPI at early exponential growth phase and the second and third KPI's when the phytoplankton had grown more abundant. This data table contains measurements collected during monitoring of the growth of phytoplankton batch cultures in part 1 of the experiment (DOM release experiment), i.e., before and in between of the KPIs. These phytoplankton cultures were used in the three key point incubations of the DOM release experiment. The variables measured during the monitoring, and included in this data file, are phytoplankton abundance, abundance of A. malmogiense cells with lower chlorophyll a fluorescence, percentage of phytoplankton cells with intact membranes, and optical properties of DOM. The experimental design is explained in figure 1 of the associated publication

Seasonal variation in estuarine phytoplankton viability and its relationship with carbon dynamics in the Baltic Sea

Cell death drives the magnitude and community composition of phytoplankton and can result in the conversion of particulate organic carbon to dissolved organic carbon (DOC), thereby affecting carbon cycling in the aquatic food web. We used a membrane integrity probe (Sytox Green) to study the seasonal variation in the percentage of viable cells in the phytoplankton population in an estuary in the northern Baltic Sea for 21 months. The associated dissolved and particulate organic matter concentrations were also studied. The viable fraction of phytoplankton cells varied from < 20% to almost 100%, with an average of 62%. Viability was highest when a single phytoplankton group (diatoms or dinoflagellates) dominated the community. Viability of sinking phytoplankton cells, including some motile species, was in general as high as in surface water. Changes in viability were not closely related to nutrient concentrations, virus-like particle abundance, seawater temperature or salinity. There was a weak but significant negative correlation between viability and DOC, although at this location, the DOC pool was mainly influenced by the inflow of riverine water. This study demonstrates that cell viability, and its relationship with carbon export, is highly variable in the complex microbial populations common within estuarine and coastal marine ecosystems

Key point incubations in part 1 (dissolved organic matter release) of the microcosm experiment, Gulf of Finland, Baltic Sea

The data were collected from an experiment using phytoplankton cultures (Apocalathium malmogiense and Rhodomonas marina). The aim of the experiment was to study carbon cycling among phytoplankton and bacteria, and the effects on the dissolved organic matter (DOM) pool. Measured variables include phytoplankton and bacterial abundance, primary production, bacterial production and respiration, 14C-transfer from phytoplankton to DOM and bacteria, concentrations of particulate and dissolved organic carbon, nitrate, phosphate and chlorophyll a, and optical characteristics of dissolved organic matter. The experiment was conducted at Tvärminne Zoological Station, Hanko, Finland with non-axenic unialgal phytoplankton cultures and bacteria originating from the Baltic Sea. The experiment was conducted between Dec. 2017 and Apr. 2018. The experiment consisted of two parts, the DOM release experiment (part 1) and the DOM consumption experiment (part 2). Separate triplicate batch cultures of both phytoplankton species were grown for each experiment. In the DOM release experiment the cultures were grown for over 4 months and three day-long incubations (key point incubations, KPI's) were initiated on three occasions; the first KPI at early exponential growth phase and the second and third KPI's when the phytoplankton had grown more abundant. During each KPI and aliquot of the culture was inoculated with freshly collected sea water bacteria, and bacterial community composition was measured. This aliquot was then divided into two further aliquots; one was incubated with radioisotopes for productivity (primary and bacterial production) and 14C-flow analyses (production line) and one filtered through 0.8 µm for analysis of DOM optical properties. During the KPI's measurements were taken at 0, 4, 8 and 12 h. Nutrient concentrations (measured from non-filtered and 0.8 µm filtered samples) and concentration of dissolved organic carbon were measured only at 0 and 12 h. Concentrations of particulate organic carbon and nitrogen and chlorophyll a were measured only once for each KPI at the beginning of the incubation. In the DOM consumption experiments the cultures were grown to high abundance, after which the phytoplankton and most of the bacteria were filtered out. The filtrate was then inoculated with freshly collected sea water bacteria, after which it was incubated for 7 days. Bacterial abundance, production, respiration, and community composition, and concentration and optical properties of DOM were measured daily. The experimental design is explained in figure 1 of the associated publication. This data table contains the measurements taken during the KPIs of part 1 (DOM release experiment). Measurements are shown for all replicates (Replicate) of both phytoplankton treatments (Species) at each KPI (Exp run) at each measurement time point of the incubation (Inc dur). The measured variables include concentrations of nitrate, phosphate, chlorophyll a, particulate organic carbon and nitrogen, dissolved organic carbon, and total inorganic carbon, primary production, incorporation rates of 3H-thymidine and 14C-leucine and bacterial production calculated based on these, abundance of high and low nucleic acid and total bacteria, flow cytometric side scatter of high and low nucleic acid bacteria, optical properties of DOM, phytoplankton abundance, abundance of A. malmogiense cells with lower chlorophyll a fluorescence, and percentage of phytoplankton cells with intact membranes. Primary production is calculated using two subsamples incubated in light, and one subsample incubated in dark, and the dark measurement was subtracted from the mean of the light measurements. These light and dark samples were also used for calculating bacterial production and the 14C-flow (table 2 of the associated publication). For measurements taken in these light and dark samples (i.e. production line) see https://doi.pangaea.de/10.1594/PANGAEA.937723

Monitoring of the growth phase of phytoplankton cultures in part 2 (dissolved organic matter consumption) of the microcosm experiment, Gulf of Finland, Baltic Sea

The data were collected from an experiment using phytoplankton cultures (Apocalathium malmogiense and Rhodomonas marina). The aim of the experiment was to study carbon cycling among phytoplankton and bacteria, and the effects on the dissolved organic matter (DOM) pool. Measured variables include phytoplankton and bacterial abundance, primary production, bacterial production and respiration, 14C-transfer from phytoplankton to DOM and bacteria, concentrations of particulate and dissolved organic carbon, nitrate, phosphate and chlorophyll a, and optical characteristics of dissolved organic matter. The experiment was conducted at Tvärminne Zoological Station, Hanko, Finland with non-axenic unialgal phytoplankton cultures and bacteria originating from the Baltic Sea. The experiment was conducted between Dec. 2017 and Apr. 2018. The experiment consisted of two parts, the DOM release experiment (part 1) and the DOM consumption experiment (part 2). Separate triplicate batch cultures of both phytoplankton species were grown for each experiment. In the DOM release experiment the cultures were grown for over 4 months and three day-long incubations (key point incubations, KPI's) were initiated on three occasions; the first KPI at early exponential growth phase and the second and third KPI's when the phytoplankton had grown more abundant. During each KPI and aliquot of the culture was inoculated with freshly collected sea water bacteria, and bacterial community composition was measured. This aliquot was then divided into two further aliquots; one was incubated with radioisotopes for productivity (primary and bacterial production) and 14C-flow analyses (production line) and one filtered through 0.8 µm for analysis of DOM optical properties. During the KPI's measurements were taken at 0, 4, 8 and 12 h. Nutrient concentrations (measured from non-filtered and 0.8 µm filtered samples) and concentration of dissolved organic carbon were measured only at 0 and 12 h. Concentrations of particulate organic carbon and nitrogen and chlorophyll a were measured only once for each KPI at the beginning of the incubation. In the DOM consumption experiments the cultures were grown to high abundance, after which the phytoplankton and most of the bacteria were filtered out. The filtrate was then inoculated with freshly collected sea water bacteria, after which it was incubated for 7 days. Bacterial abundance, production, respiration, and community composition, and concentration and optical properties of DOM were measured daily. The experimental design is explained in figure 1 of the associated publication. This data table contains measurements collected during monitoring of the growth of phytoplankton batch cultures in part 2 of the experiment (DOM consumption experiment), i.e., before the initiation of the incubation. These phytoplankton cultures were used in the incubation of the DOM consumption experiment. The measured variables are phytoplankton abundance and abundance of A. malmogiense cells with lower chlorophyll a fluorescence