Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The Southern Ocean (SO) is of significant interest in the understanding of the global carbon cycle and therefore many studies have been conducted to determine the limiting factors controlling the biological pump within the region. During photosynthesis phytoplankton require various nutrients such as NO3, PO4, inorganic carbon and the micronutrient Fe. The SO is a High-Nutrient Low-Chlorophyll region, therefore no macronutrient limitation is experienced by resident phytoplankton but instead the micronutrient Fe is a significant limiting factor within these waters due to limited inputs. Due to deep mixed layer depths, ice cover, low sun angles and cloud cover throughout parts of the year, light is also considered a limiting factor in the SO. Fe and light limitation cause a decrease in photosynthetic efficiency and therefore a decrease in carbon fixation capabilities. During this study we conducted five bioassay shipboard incubation experiments during two cruises along the Greenwich meridian between South Africa and the ice edge, SOSCEx during March and SAFePool during January to February, in which we varied Fe concentrations and light levels to determine the effects of Fe and light limitation or co-limitation within resident phytoplankton. Spatial and temporal variations in phytoplankton response were studied to determine varying effects of limitation across water masses and different stages of bloom decline within the study area. The combined addition of Fe and light gave the largest increase in biomass, photosynthetic capacity and nutrient uptake. In support of the hypotheses tested changes in the photosynthetic apparatus led to changes in the photosynthetic efficiency and growth of the SO phytoplankton, as a result of variations in Fe and light availability. Variability was also observed in the response of phytoplankton to Fe and light amendments due to spatial and temporal variation in resident phytoplankton communities. It was therefore concluded that both Fe and light are significant controls in the resident phytoplankton photosynthetic apparatus, photosynthetic capabilities, organic carbon fixation and therefore the biogeochemical cycles within the Atlantic sector of the SO. / AFRIKAANSE OPSOMMING: Die Suidelike Oseaan (SO) is van beduidende belang in die begrip van die globale koolstofsiklus en dus is baie studies gedoen om die beperkende faktore te bepaal wat die biologiese pomp in die streek beheer. Tydens fotosintese benodig fitoplankton verskillende voedingstowwe soos NO3, PO4, anorganiese koolstof en die mikrovoedingstof Fe. Die SO is 'n High-Nutrient Lae-Chlorofil streek, dus word geen makrovoedingstof beperking ervaar deur inwoner fitoplankton maar in plaas daarvan is die mikrovoedingstof Fe 'n beduidende beperkende faktor binne hierdie waters weens beperkte insette. As gevolg van diep gemengde laag dieptes, ysbedekking, lae son hoeke en wolkbedekking deur dele van die jaar, word lig ook beskou as 'n beperkende faktor in die SO. Fe en lig beperking veroorsaak 'n afname in die fotosintetiese doeltreffendheid en dus 'n afname in koolstof binding vermoëns. Tydens hierdie studie het ons vyf biotoets inkubasie eksperimente aan boord die skeep gedoen tydens twee vaarte langs die Greenwich meridiaan tussen Suid-Afrika en die ys rand, SOSCEx gedurende Maart en SAFePool gedurende Januarie tot Februarie, waarin ons Fe konsentrasies en lig vlakke gewissel het om die gevolge van Fe en lig beperking, of medebeperking, binne inwoner fitoplankton te bepaal. Ruimtelike en temporale variasies in fitoplankton reaksie was bestudeer om wisselende gevolge van die beperking oor watermassas en verskillende stadiums van bloei afname in die studie area te bepaal. Die gekombineerde byvoeging van Fe en lig het die grootste toename in biomassa, fotosintetiese kapasiteit en voedingsopname gegee. Ter ondersteuning van die getoetste hipoteses, veranderinge in die fotosintetiese apparaat het gelei tot veranderinge in die fotosintetiese doeltreffendheid en groei van die SO fitoplankton, as 'n gevolg van variasies in Fe en lig beskikbaarheid. Veranderlikheid is ook waargeneem in die reaksie van fitoplankton om Fe en lig wysigings weens die ruimtelike en tydelike variasie in inwoner fitoplankton gemeenskappe. Dus was dit by die gevolgtrekking gekom dat beide Fe en lig beduidende kontrole in die inwoner fitoplankton fotosintetiese apparaat, fotosintetiese vermoëns, organiese koolstof binding en daarom die biogeochemiese siklusse binne die Atlantiese sektor van die SO.

This thesis addresses time-series analyses of microbial (i.e. marine heterotrophic bacteria and phytoplankton) and microbially relevant ecosystem variables at two ocean time series stations - Palmer Station in the coastal Western Antarctic Peninsula (WAP) and the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea. Using a diverse spectrum of statistical analyses and models, the aim of this thesis is to gain the better insight into 1) variability of microbial and ecosystem processes across varying time scales, from seasonal to interdecadal, and 2) how each process is influenced by variability of surrounding local physical forcing factors as well as regional and global-scale climate variability along the study region. Chapter 1 provides an introduction to the two study sites as well as a brief history of the ocean time-series programs there. Chapter 2 deals with phytoplankton and nutrient drawdown variability over an interdecadal (1993-2013) period using seasonal time-series variables collected at Palmer Station during full 6-months of Austral growing seasons (October-March). Specifically, the linkage between large-scale climate modes relevant to the WAP area and phytoplankton and nutrient patterns is explored to establish the underlying mechanisms of the observed ecosystem variability, which is ultimately triggered by climate conditions via mediatory physical forcing factors. Chapter 3 addresses a decadal (2002-2014) variability of heterotrophic bacterial variables collected at Palmer Station in Antarctica. This Chapter 3 provides an insight into why bacterial activity was shown to be restricted in this very productive ecosystem from diverse aspects gained using different statistical approaches. Furthermore, the linkage between bacterial properties and surrounding environmental conditions is explored. Finally, Chapter 4 concerns an event-scale phenomenon - the frequency of winter storms - and its impact on bacterial dynamics and ecological processes at the BATS site. Using a previously developed storm tracking algorithm, this study benefits from establishing a mechanistic connection between storm forcing and bacterial processes via storm-induced variability of physical environments - the extent of wind-mixing and entrainment of cold water into the upper mixed-layer. The finding of Chapter 4 is novel in the aspect that prevalent negative North Atlantic Oscillation (NAO) conditions, which lead to frequent arrivals of winter storms over the BATS region, in part, explain a significant decreasing bacterial trend over the past 24-year period. Overall, my thesis, in conjunction with work performed by fellow microbial oceanographers, aims to provide evidence of microbial responses to physical forcings across varying time scales in the strongly coupled climate-biogeochemical systems at two contrasting ocean sites based on a variety of statistical approaches.

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Physical-biological interactions in the Southern Ocean were investigated using remote sensing data from several different satellite sensors. Satellite sea surface temperature data were used to study the dynamics of the Antarctic Polar Front (PF). Satellite ocean color data were used to estimate surface chlorophyll concentrations and their relation to various physical forcings within the Southern Ocean. A detailed study of phytoplankton blooms at the Antarctic Polar Front revealed that elevated chlorophyll concentrations (phytoplankton blooms) occur most often in areas where the PF interacts with large topographic features within the Southern Ocean. The physical dynamics of the PF are strongly influenced by the topography, and in turn strongly influence phytoplankton bloom dynamics. The analysis of satellite data from the modern Southern Ocean indicates that phytoplankton are limited by the availability of the micronutrient iron in most areas. This iron-limitation implies that the elevated iron inputs during glacial periods would have led to increased phytoplankton primary and export production and a stronger sink for atmospheric CO��� in the Southern Ocean. / Graduation date: 2000

Thalassiosira oceanica, a marine centric diatom, possesses an extracellular reductase that reduces iron (Fe(III)) bound to organic complexes as part of a high-affinity Fe transport mechanism. A number of Fe(III) organic complexes are reduced, including siderophores---effective Fe chelates produced by microorganims in response to Fe stress. Reduction rates are inversely related to the relative stability constants of the oxidized and reduced Fe chelates (log Kox/Kred), and vary by a factor of 2.4 in accordance with theoretical predictions. Under Fe-limiting conditions, reduction rates increase and the ability of T. oceanica to transport Fe from siderophores is enhanced. Iron bound to the siderophore desferrioxamine B (DFB) is reduced 2 times faster than it is taken up, suggesting that the reductase is well coupled to the Fe transporter, and can provide all the inorganic Fe to account for the measured Fe uptake rates in the presence of excess DFB. The efficacy of the reductase in providing inorganic Fe for uptake and growth is ultimately dependent on the relative concentrations of excess ligands in solution and cell surface Fe transporters competing for inorganic Fe. The rates of Fe reduction and uptake are twice as fast in cells grown in NO3- compared to those grown in NH 4+, suggesting a link with cellular N metabolism and with NO3- utilization in particular. Enhanced Fe reductase activity in NO3--grown cells enables them to maintain a 1.6-fold higher cellular Fe concentration under low Fe conditions. / Experiments conducted in the subarctic Pacific, an Fe-limited oceanic region, demonstrated that even indigenous plankton (both prokaryotic and eukaryotic plankton) have the ability to acquire Fe bound to strong organic chelates. Large phytoplankton species (>3 mum) reduce Fe bound to siderophores extracellularly. Because the predominant form of dissolved Fe in the sea is bound to strong organic complexes, a reductive mechanism as described here may be a critical step in Fe acquisition by phytoplankton.

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Productivity in large areas of the world's oceans is limited by low concentrations of dissolved iron in surface waters. Phytoplankton have adapted to persist in these environments by reducing their requirements for iron (Fe) in key metabolic pathways, in some cases by replacing Fe-containing catalysts with their iron-free functional equivalents. This thesis examines the requirements and biochemical roles for copper (Cu) and manganese (Mn) in Fe-limited centric marine diatoms. A major finding of my research is that diatoms have elevated requirements for Mn and Cu when grown in Fe-deficient seawater. Iron deficiency induces oxidative stress and increases the cellular concentrations of toxic oxygen radicals and damage products in Thalassiosira pseudonana. The increased Mn-requirement is used, in part, to activate Mn-containing isoforms of the antioxidant enzyme superoxide dismutase. Cultures co-limited by Fe and Mn exhibit high levels of oxidative stress and an inefficient detoxification pathway that further reduces cell growth. Diatoms isolated from the metal poor open ocean require more Cu to divide than related species from metal-rich coastal waters. This pattern is in stark contrast to all other known nutritive trace metals. One part of the diatom Cu requirement that is independent of provenance is for efficient Fe transport. The additional Cu requirement of oceanic species appears to be due to the constitutive expression of a Cu-containing electron transport protein, possibly plastocyanin. Coastal species, which have higher Fe-requirements for growth, retain the Fe-containing functional homologue cytochrome c6. By employing metals other than Fe within photosynthesis and antioxidant pathways, marine diatoms are able to increase their fitness in Fe-deficient environments. However, Mn and Cu also occur in low concentrations in the open ocean and thus may co-limit growth of natural populations of phytoplankton. Metal enrichment experiments i

Three species of photosynthetic flagellates capable of phagotrophy (mixotrophic species) were tested for their abilities to use inorganic iron colloids for growth. Ochromonas sp., Chrysochromulina ericina (a coastal strain) and C. ericina (an oceanic strain) were grown in iron-free seawater supplemented with 1 muM goethite, hematite, magnetite/maghemite or ferrihydrite (90°) in the presence and absence of desferrioxamme B, an iron-binding siderophore. Both strains of Chrysochromulina grew at 35--70% of their maximum rates with goethite, hematite, and magnetite/maghemite, but were unable to use ferrihydrite. Ochromonas, however, grew well with ferrihydrite, but could not use any of the other forms. All the flagellates were able to acquire iron from ingested bacteria. Diatoms that were known only to take up dissolved forms of iron, Thalassiosira oceanica (clone 1003) and T. pseudonana (clone 3H), were unable to use any of the colloids tested. The mechanism of iron acquisition by the flagellates appeared to involve ingestion of the iron colloids as DFB had no effect on colloidal iron availability and bacteria resident in the cultures were unable to use the iron contained in the colloids. Variations in the size of the colloids were hypothesized to account for differences in their availability, independent of colloid chemical stability. The results provide the first strong evidence for direct utilization (i.e. without prior dissolution) of colloidal iron by mixotrophic phytoplankton and document a new pathway of iron acquisition that may be important for their survival in low-iron waters of the sea.

Graduation date: 1997