A fluorometric method is described which provides sensitive measurements of extracted chlorophyll a free from the errors associated with conventional acidification techniques. Fluorometric optical configurations were optimized to produce maximum sensitivity to Chl a while maintaining desensitized responses from both Chl b and pheopigments. Under the most extreme Chl b:Chl a ratio likely to occur in nature (1 : 1 molar), the new method results in only a 10% overestimate of the true Chl a value, while estimates from older acidification methods are 2.5-fold low. Under conditions of high pheopigment concentrations (pheo a: Chl a = 1 : 1 molar), the new method provides Chl a estimates that are equivalent to those determined from the acidification technique. The new simple method requires a single fluorescence determination and provides adequate sensitivity for small sample sizes (<200 ml) even in the most oligotrophic marine and freshwater environments.

Fluorometric analysis of chlorophyll a in the presence of chlorophyll b and pheopigments

While it has been recently demonstrated that both iron (Fe) and manganese (Mn) control Southern Ocean (SO) plankton biomass, how in particular Mn governs phytoplankton species composition remains yet unclear. This study, for the first time, highlights the importance of Mn next to Fe for growth of two key SO phytoplankton groups at two locations in the Drake Passage (West and East). Even though the bulk parameter chlorophyll a indicated Fe availability as main driver of both phytoplankton assemblages, the flow cytometric and microscopic analysis revealed FeMn co-limitation of a key phytoplankton group at each location: at West the dominant diatom Fragilariopsis and one subgroup of picoeukaryotes, which numerically dominated the East community. Hence, the limitation by both Fe and Mn and their divergent requirements among phytoplankton species and groups can be a key factor for shaping SO phytoplankton community structure. 

/pdf/iron-and-manganese-co-limit-the-growth-of-two-phytoplankton-c1btv0jq.pdf

Iron and manganese co-limit the growth of two phytoplankton groups dominant at two locations of the Drake Passage

Spring and summer Southern Ocean phytoplankton communities have been well characterized, but winter communities are often overlooked. Diatoms are a major contributor to Southern Ocean particulate organic carbon (POC) production and export, and exert a strong control on Antarctic surface and Subantarctic thermocline nutrient concentrations, thus influencing the low-latitude nutrient supply. Understanding diatom distribution, seasonal community progression, and diatom-nutrient interactions is vital for improving biogeochemical models, particularly as climate change alters polar phytoplankton communities. We investigated the distribution of nanophytoplankton (≥3 μm) and their associated biogeochemical environments along 30°E across the Indian Southern Ocean (Subtropical Zone; STZ to Antarctic Zone; AZ) in July 2017. Phytoplankton productivity inferred from chlorophyll-a was low compared to previously-published summertime values. Mixed-layer nitrate (NO3−) and phosphate (PO43−) concentrations were similar to existing summer measurements, likely due to a combination of deep mixing and biological uptake, while silicic acid (Si(OH)4) was high relative to summer, particularly south of the Polar Front (PF). The PF emerged as an important biogeochemical boundary separating relatively high chlorophyll-a, flagellate-dominated northern waters from southern waters characterized by low chlorophyll-a and diatom-dominated communities. Mixed layer-integrated biogenic silica (bSi) decreased 12-fold from the southern AZ to the STZ, resulting in a strong south-north gradient in bSi-per-chl-a (from 3.6 to 0.1 mol/g) and bSi-per-POC (from 0.22 to 0.016 mol mol−1). We attribute this to a high abundance of heavily-silicified diatom species (e.g., Fragilariopsis spp., which dominated the AZ diatom community) and a limited contribution of other phytoplankton to chlorophyll-a and POC to the south – indeed, diatoms constituted 5–67% of the total POC south of the PF and only 3–7% to the north. While mixed-layer Si(OH)4 concentrations decreased more than NO3− across the PF, likely due to preferential Si(OH)4 consumption by iron-limited diatoms, our data imply a lower ratio of Si(OH)4 to NO3− uptake compared to summer. This suggests that iron limitation may be less severe in the AZ in winter, at least in the west Indian sector. We conclude that AZ diatoms impact the low-latitude nutrient supply and are potentially important for carbon export in winter, despite the lower productivity of the Southern Ocean during this season.

Winter biogenic silica and diatom distributions in the Indian sector of the Southern Ocean

Central to the Southern Ocean's role in setting atmospheric CO2 is the seasonal alternation between upward mixing of nutrients and their subsequent consumption by phytoplankton. Active nutrient cycling within the mixed layer, including the release of ammonium (NH4+) and its removal by phytoplankton and nitrifiers, also affects Southern Ocean CO2 drawdown, yet remains poorly understood. We conducted kinetics experiments across the Southern Ocean south of Africa to investigate the dependence of NH4+ uptake (summer, winter) and oxidation (winter) on NH4+ concentration. NH4+ uptake followed a Michaelis–Menten function in both seasons, with the maximum rate (Vmax) decreasing poleward, apparently controlled mainly by light in winter and temperature in summer. The half‐saturation constant (Km) increased poleward with increasing ambient NH4+ ([NH4+]amb) and was threefold higher in winter (150–405 nM) than in summer (41–115 nM), suggesting that summertime phytoplankton are adapted to low‐NH4+ conditions while winter communities typically receive a higher NH4+ supply. NH4+ oxidation showed a high affinity for NH4+ (Km = 28–137 nM), suggesting a dominant role for ammonia‐oxidizing archaea, and followed a Michaelis–Menten curve only when [NH4+]amb was ≤ 90 nM. Vmax was near‐constant across the region regardless of [NH4+]amb, temperature, or light. From coincident mixed‐layer NH4+ oxidation and iron measurements, we hypothesize that iron availability may (co‐)limit the Vmax of NH4+ oxidation. If verified, this suggestion has implications for models that parameterize nitrification as a linear function of [NH4+]amb. Additionally, iron depletion may limit the role of mixed‐layer nitrification, which is dominant in the winter Southern Ocean, in offsetting phytoplankton CO2 drawdown annually.

The kinetics of ammonium uptake and oxidation across the Southern Ocean

Zinc (Zn) is vital to marine organisms. Its active uptake by phytoplankton results in a substantial depletion of dissolved Zn, and Zn bound to particulate organic matter replenishes dissolved Zn in the ocean through remineralization. However, we found that particulate Zn changes from Zn bound to phosphoryls in cells to recalcitrant inorganic pools that include biogenic silica, clays, and iron, manganese, and aluminum oxides in the Southern Ocean water column. The abundances of inorganic pools increase with depth and are the only phases preserved in sediments. Changes in the particulate-Zn speciation influence Zn bioavailability and explain the decoupling of Zn and phosphorus and the correlation of Zn and silicon in the water column. These findings reveal a new dimension to the ocean Zn cycle, implicating an underappreciated role of inorganic Zn particles and their impact on biological productivity. 

Biogenic-to-lithogenic handoff of particulate Zn affects the Zn cycle in the Southern Ocean

CITATION: Viljoen, J. J., et al. 2019. Links between the phytoplankton community composition and trace metal distribution in summer surface waters of the Atlantic Southern Ocean. Frontiers in Marine Science, 6:295,
doi:10.3389/fmars.2019.00295.

/pdf/links-between-the-phytoplankton-community-composition-and-3roi4u4jup.pdf

Links Between the Phytoplankton Community Composition and Trace Metal Distribution in Summer Surface Waters of the Atlantic Southern Ocean

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Winter biogenic silica and diatom distributions in the Indian sector of the Southern Ocean

Links Between the Phytoplankton Community Composition and Trace Metal Distribution in Summer Surface Waters of the Atlantic Southern Ocean