Siderophores as an iron source for Prochlorococcus in deep chlorophyll maximum layers of the oligotrophic ocean

Prochlorococcus is one of the most abundant photosynthesizing organisms in the oligotrophic oceans. Gene content variation among Prochlorococcus populations in separate ocean basins often mirrors the selective pressures imposed by the region's distinct biogeochemistry. By pairing genomic datasets with trace metal concentrations from across the global ocean, we show that the genomic capacity for siderophore-mediated iron uptake is widespread in low-light adapted Prochlorococcus populations from iron-depleted regions of the oligotrophic Pacific and S. Atlantic oceans: Prochlorococcus siderophore consumers were absent in the N. Atlantic ocean (higher iron flux) but constituted up to half of all Prochlorococcus genomes from metagenomes in the N. Pacific (lower iron flux). Prochlorococcus siderophore consumers, like many other bacteria with this trait, also lack siderophore biosynthesis genes indicating that they scavenge exogenous siderophores from seawater. Statistical modeling suggests that the capacity for siderophore uptake is endemic to remote ocean regions where atmospheric iron fluxes are the smallest, particularly at deep chlorophyll maximum and primary nitrite maximum layers. We argue that abundant siderophore consumers at these two common oceanographic features could be a symptom of wider community iron stress, consistent with prior hypotheses. Our results provide a clear example of iron as a selective force driving the evolution of Prochlorococcus.

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Dynamics of the Deep Chlorophyll Maximum in the Black Sea as depicted by BGC-Argo floats

10.5194/egusphere-egu21-3191 ◽

2021 ◽

Author(s):

Arthur Capet ◽

florian ricour ◽

Fabrizio D'Ortenzio ◽

Bruno Delille ◽

Marilaure Grégoire

Keyword(s):

Black Sea ◽

Global Ocean ◽

The Black Sea ◽

Second Phase ◽

Deep Chlorophyll Maximum ◽

Phytoplankton Productivity ◽

Spatial Homogeneity ◽

Argo Floats ◽

Chlorophyll Maximum ◽

Basin Scale

The deep chlorophyll maximum (DCM) is a well known feature of the global ocean. However, its description and the study of its formation are a&#160; challenge, especially in the peculiar environment that is the Black Sea. The retrieval of chlorophyll a (Chla) from fluorescence (Fluo) profiles recorded by biogeochemical-Argo (BGC-Argo) floats is not trivial in the Black Sea, due to the very high content of colored dissolved organic matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase of the Chla concentration with depth.Here, we revised Fluo correction protocols for the Black Sea context using co-located in-situ high-performance liquid chromatography (HPLC) and BGC-Argo measurements. The processed set of Chla data (2014&#8211;2019) is then used to provide a systematic description of the seasonal DCM dynamics in the Black Sea and to explore different hypotheses concerning the mechanisms underlying its development.Our results show that the corrections applied to the Chla profiles are consistent with HPLC data. In the Black Sea, the DCM begins to form in March, throughout the basin, at a density level set by the previous winter mixed layer. During a first phase (April-May), the DCM remains attached to this particular layer. The spatial homogeneity of this feature suggests a hysteresis mechanism, i.e., that the DCM structure locally influences environmental conditions rather than adapting instantaneously to external factors.In a second phase (July-September), the DCM migrates upward, where there is higher irradiance, which suggests the interplay of biotic factors. Overall, the DCM concentrates around 45 to 65% of the total chlorophyll content within a 10 m layer centered around a depth of 30 to 40 m, which stresses the importance of considering DCM dynamics when evaluating phytoplankton productivity at basin scale.

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Dynamics of the Deep Chlorophyll Maximum in the Black Sea as depicted by BGC-Argo floats

10.5194/bg-2020-295 ◽

2020 ◽

Author(s):

Florian Ricour ◽

Arthur Capet ◽

Fabrizio D'Ortenzio ◽

Bruno Delille ◽

Marilaure Grégoire

Keyword(s):

Black Sea ◽

High Performance ◽

Global Ocean ◽

The Black Sea ◽

Fluorescence Signal ◽

Deep Chlorophyll Maximum ◽

Lateral Loads ◽

Spatial Homogeneity ◽

Argo Floats ◽

Chlorophyll Maximum

Abstract. The Deep Chlorophyll Maximum (DCM) is a well known feature of the global ocean. However, its description and the study of its formation are a challenge, especially in the peculiar Black Sea environment. The retrieval of Chlorophyll a (Chla) from fluorescence (Fluo) profiles recorded by Biogeochemical-Argo (BGC-Argo) floats is not trivial in the Black Sea, due to the very high content of Colored Dissolved Organic Matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase of the Chla concentration with depth. Here we revised Fluo correction protocols for the Black Sea context using co-located in-situ High-Performance Liquid Chromatography (HPLC) and BGC-Argo measurements. The processed set of Argo Chla data (2014–2019) is then used to provide a systematic description of the seasonal DCM dynamics in the Black Sea, and to explore different hypotheses concerning the mechanisms underlying its development. Our results show that the corrections applied to Chla profiles are consistent with HPLC data. In the Black Sea, the DCM is initiated in March, throughout the basin, at a pycnal level set by the previous winter mixed layer. The DCM then remains attached to this particular layer until the end of September. The spatial homogeneity of this feature suggests a self-sustaining DCM structure, locally influencing environmental conditions rather than adapting instantaneously to external factors. In summer, the DCM concentrates around 50 to 65 % of the total chlorophyll content around a depth of 30 m, where light conditions ranged from 0.5 to 4.5 % of surface incoming irradiance. In October, as the DCM structure is gradually eroded, a longitudinal gradient appears in the DCM pycnal depth, indicating that autumnal mixing induces a relocation of the DCM which is this time driven by regional factors, such as nutrients lateral loads and turbidity.

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Independent iron and light limitation in a low-light-adapted Prochlorococcus from the deep chlorophyll maximum

The ISME Journal ◽

10.1038/s41396-020-00776-y ◽

2020 ◽

Vol 15 (1) ◽

pp. 359-362

Author(s):

Nicholas J. Hawco ◽

Feixue Fu ◽

Nina Yang ◽

David A. Hutchins ◽

Seth G. John

Keyword(s):

Light Intensity ◽

High Efficiency ◽

Iron Concentration ◽

Euphotic Zone ◽

Theoretical Models ◽

Light Limitation ◽

Deep Chlorophyll Maximum ◽

North Pacific Subtropical Gyre ◽

Low Light ◽

Chlorophyll Maximum

AbstractThroughout the open ocean, a minimum in dissolved iron concentration (dFe) overlaps with the deep chlorophyll maximum (DCM), which marks the lower limit of the euphotic zone. Maximizing light capture in these dim waters is expected to require upregulation of Fe-bearing photosystems, further depleting dFe and possibly leading to co-limitation by both iron and light. However, this effect has not been quantified for important phytoplankton groups like Prochlorococcus, which contributes most of the productivity in the oligotrophic DCM. Here, we present culture experiments with Prochlorococcus strain MIT1214, a member of the Low Light 1 ecotype isolated from the DCM in the North Pacific subtropical gyre. Under a matrix of iron and irradiance matching those found at the DCM, the ratio of Fe to carbon in Prochlorococcus MIT1214 cells ranged from 10–40 × 10−6 mol Fe:mol C and increased with light intensity and growth rate. These results challenge theoretical models predicting highest Fe:C at lowest light intensity, and are best explained by a large photosynthetic Fe demand that is not downregulated at higher light. To sustain primary production in the DCM with the rigid Fe requirements of low-light-adapted Prochlorococcus, dFe must be recycled rapidly and at high efficiency.

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Dynamics of the deep chlorophyll maximum in the Black Sea as depicted by BGC-Argo floats

Biogeosciences ◽

10.5194/bg-18-755-2021 ◽

2021 ◽

Vol 18 (2) ◽

pp. 755-774

Author(s):

Florian Ricour ◽

Arthur Capet ◽

Fabrizio D'Ortenzio ◽

Bruno Delille ◽

Marilaure Grégoire

Keyword(s):

Black Sea ◽

Global Ocean ◽

The Black Sea ◽

Second Phase ◽

Deep Chlorophyll Maximum ◽

Phytoplankton Productivity ◽

Chl A ◽

Argo Floats ◽

Chlorophyll Maximum ◽

Basin Scale

Abstract. The deep chlorophyll maximum (DCM) is a well-known feature of the global ocean. However, its description and the study of its formation are a challenge, especially in the peculiar environment that is the Black Sea. The retrieval of chlorophyll a (chl a) from fluorescence (Fluo) profiles recorded by Biogeochemical Argo (BGC-Argo) floats is not trivial in the Black Sea, due to the very high content of coloured dissolved organic matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase in the chl a concentration with depth. Here, we revised Fluo correction protocols for the Black Sea context using co-located in situ high-performance liquid chromatography (HPLC) and BGC-Argo measurements. The processed set of chl a data (2014–2019) is then used to provide a systematic description of the seasonal DCM dynamics in the Black Sea and to explore different hypotheses concerning the mechanisms underlying its development. Our results show that the corrections applied to the chl a profiles are consistent with HPLC data. In the Black Sea, the DCM begins to form in March, throughout the basin, at a density level set by the previous winter mixed layer. During a first phase (April–May), the DCM remains attached to this particular layer. The spatial homogeneity of this feature suggests a hysteresis mechanism, i.e. that the DCM structure locally influences environmental conditions rather than adapting instantaneously to external factors. In a second phase (July–September), the DCM migrates upward, where there is higher irradiance, which suggests the interplay of biotic factors. Overall, the DCM concentrates around 45 % to 65 % of the total chlorophyll content within a 10 m layer centred around a depth of 30 to 40 m, which stresses the importance of considering DCM dynamics when evaluating phytoplankton productivity at basin scale.

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Contribution of Rhizosolenia eriensis and Cyclotella spp. to the Deep Chlorophyll Maximum of Sproat Lake, British Columbia, Canada

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/f90-013 ◽

1990 ◽

Vol 47 (1) ◽

pp. 128-135 ◽

Cited By ~ 10

Author(s):

Leland J. Jackson ◽

John G. Stockner ◽

Paul J. Harrison

Keyword(s):

Spatial Separation ◽

Deep Chlorophyll Maximum ◽

Nitrogen And Phosphorus ◽

Low Light ◽

Temporal And Spatial Distribution ◽

Neutral Buoyancy ◽

Chlorophyll Maximum ◽

Centric Diatoms ◽

Temporal And Spatial ◽

Lake Fertilization

Experimental fertilization of Sproat Lake with nitrogen and phosphorus greatly increased the abundance of two centric diatoms: Cyclotella spp. and Rhizosolenia eriensis. A decrease in sinking rates to neutral buoyancy at 17.5–22.5 m, an area of high nutrients and low light, coupled with sedimentation estimates of 106–107 celis∙m−2∙d−1, provide strong evidence that diatoms contribute to the formation of a seasonal deep chlorophyll maximum (DCM). The position of the Sproat Lake DCM, occurring at or just above the 1% light depth, appears to be largely determined by the light regime. R. eriensis bloomed and sank out of the mixed layer early in the spring before lake fertilization began. Immediately after fertilization, concentrations of nitrate and phosphate were elevated for 1 h only in the top 1 m of the water column. Most R. eriensis cells were well below 1 m and benefited little from the nutrient addition because of temporal and spatial separation. Cyclotella spp. occurred in the upper epilimnion and bloomed later in the year and consequently benefited (by large density increases) from fertilization. It is important to consider the temporal and spatial distribution of phytoplankton in determining which species will increase in abundance as a result of areal fertilization.

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Low light adaptation and export production in the deep chlorophyll maximum layer in the northern Indian Ocean

Deep Sea Research Part II Topical Studies in Oceanography ◽

10.1016/0967-0645(93)90055-r ◽

1993 ◽

Vol 40 (3) ◽

pp. 737-752 ◽

Cited By ~ 33

Author(s):

Falk Pollehne ◽

Bert Klein ◽

Bernt Zeitzschel

Keyword(s):

Indian Ocean ◽

Light Adaptation ◽

Deep Chlorophyll Maximum ◽

Northern Indian Ocean ◽

Low Light ◽

Export Production ◽

Chlorophyll Maximum

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Atmospheric Control of Deep Chlorophyll Maximum Development

Geosciences ◽

10.3390/geosciences9040178 ◽

2019 ◽

Vol 9 (4) ◽

pp. 178 ◽

Cited By ~ 1

Author(s):

Raquel Somavilla ◽

Carmen Rodriguez ◽

Alicia Lavín ◽

Amaia Viloria ◽

Elena Marcos ◽

...

Keyword(s):

Mixed Layer ◽

Phytoplankton Bloom ◽

Mixed Layer Depth ◽

Reanalysis Data ◽

Global Ocean ◽

Deep Chlorophyll Maximum ◽

Near Surface ◽

Convective Mixing ◽

Chlorophyll Maximum ◽

Heat Exchanges

The evolution of the near-surface phytoplankton bloom towards a Deep Chlorophyll Maximum (DCM) in mid-latitudes and subpolar regions of the global ocean is a well-known biological feature. However, our knowledge about the exact mechanism that determines the end of the bloom and its irreversible evolution towards a DCM is still limited. In this work, combining satellite and in-situ oceanographic data together with reanalysis data, we investigate why and when this transition between the near-surface phytoplankton bloom and the development of a DCM occurs. For this aim, we investigate the links between changes in air-sea heat exchanges, the near-surface signature of phytoplankton bloom, and the water column vertical structure by calculating the mixed layer depth (MLD) and depth of the DCM on hydrographic and chlorophyll profiles. We find that the occurrence of the last convective mixing event (heat loss by the ocean surface) at the end of the spring which is able to reach the base of the MLD and inject new nutrients into the mixed layer marks the end of the near-surface bloom and its transition towards a DCM. Identified in this way, the spring bloom duration and the start of the transition towards a DCM can be systematically and objectively determined, providing sensitive indexes of climate and ecosystem variability.

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