Microalgal Response Research Articles

Using PyCaret to model Chlorella vulgaris's growth response to salinity and oil contamination for crude oil bioremediation

ABSTRACT Crude oil spills significantly impact aquatic ecosystems, necessitating innovative remediation strategies. Microalgae-based bioremediation, particularly with Chlorella vulgaris, offers a promising solution. This study introduces a novel framework that evaluates the combined effects of selected environmental stressors on microalgal adaptability, advancing beyond traditional isolated factor analyses. By integrating a factorial experimental design with a machine learning approach using PyCaret AutoML and SHAP values, we provide a detailed examination of how crude oil concentration, salinity, and exposure duration affect C. vulgaris growth. The Extra Trees Regressor model emerged as highly accurate in predicting biomass concentration, a crucial adaptability indicator, achieving an MAE of 0.0202, RMSE of 0.029, and an R² of 0.8875. SHAP analysis highlighted salinity and crude oil as significant growth influencers, with exposure duration playing a minor role. Notably, C. vulgaris exhibited more sensitivity to salinity than to crude oil, indicating potential high-salinity challenges but also a strong tolerance to oil pollutants. These findings enhance our understanding of microalgal responses in polluted environments and suggest improved bioremediation approaches for saline waters affected by oil spills, leveraging the synergy of environmental factors and machine learning insights.

Mitigating salinity stress through interactions between microalgae and different forms (free-living & alginate gel-encapsulated) of bacteria isolated from estuarine environments

Salinity stress in estuarine environments poses a significant challenge for microalgal survival and proliferation. The interaction between microalgae and bacteria shows promise in alleviating the detrimental impacts of salinity stress on microalgae. Our study investigates this interaction by co-cultivating Chlorella sorokiniana, a freshwater microalga, with a marine growth-promoting bacterium Pseudomonas gessardii, both of which were isolated from estuary. In this study, bacteria were encapsulated using sodium alginate microspheres to establish an isolated co-culture system, preventing direct exposure between microalgae and bacteria. We evaluated microalgal responses to different salinities (5 PSU, 15 PSU) and interaction modes (free-living, gel-encapsulated), focusing on growth, photosynthesis, cellular metabolism, and extracellular polymeric substances (EPS) properties. High salinity inhibited microalgal proliferation, while gel-fixed interaction boosted Chlorella growth rate by 50.7 %. Both attached and free-living bacteria restored Chlorella's NPQ to normal levels under salt stress. Microalgae in the free-living interaction group exhibited a significantly lower respiratory rate compared to the pure algae group (−17.2 %). Increased salinity led to enhanced EPS polysaccharide secretion by microalgae, particularly in interaction groups (19.7 %). Both salt stress and interaction increased the proportion of aromatic proteins in microalgae's EPS, enhancing its stability by modulating EPS glycosidic bond C-O-C and protein vibrations. This alteration caused microalgal cells to aggregate, free-living bacteria co-culture group, and fixed co-culture group increasing by 427.5 %, 567.1 %, and 704.1 %, respectively. In gel-fixed bacteria groups, reduced neutral lipids don't accumulate starch instead, carbon redirects to cellular growth, aiding salt stress mitigation. These synergistic activities between salinity and bacterial interactions are vital in mitigating salinity stress, improving the resilience and growth of microalgae in saline conditions. Our research sheds light on the mechanisms of microalgal-bacterial interactions in coping with salt stress, offering insights into the response of estuarine microorganisms to global environmental changes and their ecological stability.

Meta-analysis of transcriptomic profiles in Dunaliella tertiolecta reveals molecular pathway responses to different abiotic stresses.

Microalgae are photosynthetic organisms and a potential source of sustainable metabolite production. However, different stress conditions might affect the production of various metabolites. In this study, a meta-analysis of RNA-seq experiments in Dunaliella tertiolecta was evaluated to compare metabolite biosynthesis pathways in response to abiotic stress conditions such as high light, nitrogen deficiency and high salinity. Results showed downregulation of light reaction, photorespiration, tetrapyrrole and lipid-related pathways occurred under salt stress. Nitrogen deficiency mostly induced the microalgal responses of light reaction and photorespiration metabolism. Phosphoenol pyruvate carboxylase, phosphoglucose isomerase, bisphosphoglycerate mutase and glucose-6-phosphate-1-dehydrogenase (involved in central carbon metabolism) were commonly upregulated under salt, light and nitrogen stresses. Interestingly, the results indicated that the meta-genes (modules of genes strongly correlated) were located in a hub of stress-specific protein-protein interaction (PPI) network. Module enrichment of meta-genes PPI networks highlighted the cross-talk between photosynthesis, fatty acids, starch and sucrose metabolism under multiple stress conditions. Moreover, it was observed that the coordinated expression of the tetrapyrrole intermediated with meta-genes was involved in starch biosynthesis. Our results also showed that the pathways of vitamin B6 metabolism, methane metabolism, ribosome biogenesis and folate biosynthesis responded specifically to different stress factors. Since the results of this study revealed the main pathways underlying the abiotic stress, they might be applied in optimised metabolite production by the microalga Dunaliella in future studies. PRISMA check list was also included in the study.

Protein reserves elucidate the growth of microalgae under nitrogen deficiency

Nitrogen (N) starvation is commonly used in the microalgae industry to induce biosynthesis of value-added products, such as astaxanthin and triacylglycerols. Microalgal adaptation to N starvation not only has an effect on biomass yield but also relates to a topic of N storage and utilization. Here, we show that only 30 % of saturated N availability and 47 % of maximum protein content is necessary to maintain a chemostat of microalga Graesiella emersonii growing at 0.96 d−1. The additionally absorbed nitrogen are transformed mainly as proteins to maintain reserves in metabolic capacity, as demonstrated by the proteomic resources allocation analysis. Such intracellular nitrogen reserve can be considered as an advanced nitrogen storage and utilization strategy as it shows a highly efficient utilization of nitrogen resources in cells. The results allow us to identify kinetics of microalgal response to N availability that is basic to future metabolic engineering.

The algal microbiome protects Desmodesmus intermedius from high light and temperature stress

The mutualistic impacts of bacterial communities on algal growth and biomass accumulation are well documented, but it is unknown how temperature and light stress influence these mutualistic interactions. Here, we generated a bacteria-free (axenic) culture of the green alga Desmodesmus intermedius C046 and compared its growth and yield in the laboratory to its native xenic counterpart - featuring a microbiome that has been maintained with its algal host for at least 5 years – under unstressed and high light, high temperature stress conditions. We then added exogenous microbiomes from high light marine environments to the axenic culture to quantify the contribution of these newly generated microbial consortia in improving D. intermedius growth, yield, and resilience to high light and heat. The native microbiome increased growth and biomass accumulation, under both unstressed and stressed conditions. Three of the five newly derived microbiomes had increased growth compared to the axenic and negligible, decreased, or increased growth impacts compared to the xenic culture. Stress conditions led to decreased algal exudation, larger algal cells when grown alone, but smaller algal cells with an associated microbiome. Our results suggest that the algal microbiome plays a role in microalgal response to heat and light stress, and efforts for screening new algal strains for high biomass productivity under those stress conditions should include a strategy for microbiome optimization.

Short-term physiologic response of the green microalga Picochlorum sp. (BPE23) to supra-optimal temperature

Photobioreactors heat up significantly during the day due to irradiation by sunlight. High temperatures affect cell physiology negatively, causing reduced growth and productivity. To elucidate the microalgal response to stressful supra-optimal temperature, we studied the physiology of Picochlorum sp. (BPE23) after increasing the growth temperature from 30 °C to 42 °C, whereas 38 °C is its optimal growth temperature. Cell growth, cell composition and mRNA expression patterns were regularly analyzed for 120 h after increasing the temperature. The supra-optimal temperature caused cell cycle arrest for 8 h, with concomitant changes in metabolic activity. Accumulation of fatty acids was observed during this period to store unspent energy which was otherwise used for growth. In addition, the microalgae changed their pigment and fatty acid composition. For example, palmitic acid (C16:0) content in the polar fatty acid fraction increased by 30%, hypothetically to reduce membrane fluidity to counteract the effect of increased temperature. After the relief of cell cycle arrest, the metabolic activity of Picochlorum sp. (BPE23) reduced significantly over time. A strong response in gene expression was observed directly after the increase in temperature, which was dampened in the remainder of the experiment. mRNA expression levels associated with pathways associated with genes acting in photosynthesis, carbon fixation, ribosome, citrate cycle, and biosynthesis of metabolites and amino acids were downregulated, whereas the proteasome, autophagy and endocytosis were upregulated.

A new kinetics model to predict the growth of micro-algae subjected to fluctuating availability of light

Light is a key environmental factor for the growth of micro-algae, and optimizing the capture of light is critical for high efficiency production systems. As the density of the population of micro-algae increases, the availability of light decreases, leading to a reduction in the growth rate because of mutual shading, while other effects, such as photo-inhibition, might be especially frequent when the population density is low. Several models in the literature have been developed to take into account light phenomena and predict micro-algal growth, particularly in a mono-culture. With the help of a simple expression for the attenuation of the light, we propose and justify a new growth function that incorporates both photo-inhibition and photo-limitation. In agreement with the experimental data, this new formulation describes the micro-algal response to a wide range of situations of excessive or insufficient light intensities through an explicit dependence on both the incident light and the biomass concentration. While simple, the proposed expression can be satisfactorily applied to practical cases under nutrient replete conditions in photo-bioreactors with different sizes and geometries. It extends naturally to the growth of different species, providing a dynamic model which can simulate experiments in a mono-culture as well as in poly-cultures. The investigation of the competition for light-limited growth shows that the model predicts competitive exclusion, which has also been experimentally demonstrated. This leads to new perspectives for the control and optimization of mixed micro-algal cultures.

Phytotoxic effect and molecular mechanism induced by nanodiamonds towards aquatic Chlorella pyrenoidosa by integrating regular and transcriptomic analyses

The growing diverse applications of nanodiamonds (NDs), especially as adsorbents and catalysts for wastewater treatment, have significantly increased their discharge and potential risk towards aquatic ecosystems. Although NDs have been certified for superior biocompatibility and lower toxicity towards numerous human cell lines, the characteristic response and underlying mechanism of aquatic microalgal response remains unclear. Here, the response of Chlorella pyrenoidosa to five concentrations of NDs was thoroughly investigated by comprehensive phenotypic and transcriptional examinations. Results indicated that higher concentration of NDs (50 mg/L) induced 75.4% growth inhibition, exacerbated oxidative stress and malformed morphology of microalgae after 48 h exposure. Meanwhile, the aggregated microalgae formed several flocs, apparently under 50 mg/L NDs. Noticeably, photosynthesis was susceptible to the NDs exposure. Although, the chlorophyll content and genes involved in photosynthesis were significantly improved by NDs, the results obtained from the photochemical parameters indicated that the excessive electrons during photosynthesis might be a pivotal reason for oxidative stress generation. Additionally, the genes included in amino acids metabolism and protein synthesis were up-regulated to alleviate the oxidative stress. Collectively, this work discloses the explicit molecular mechanisms of aquatic microalgae and provides comprehensive insights of potential aqueous environmental risk of gradually emergent NDs.

Relating microalgal response to nutrient status in a mangrove-dominated estuary

Mangrove-dominated estuaries are highly dynamic and productive systems. The Mngazana Estuary occurs in the sub-tropical biogeographical region along the east coast of South Africa. The present study assessed the water column dynamics in the mangrove-dominated estuary by investigating the response of phytoplankton to the variability in environmental factors and nutrient concentrations in 2014–2015. Freshwater flow into the estuary was variable with a peak at 0.90 m3 s−1 and a monthly average flow of approximately 0.86 m3 s−1. Nutrient concentrations showed seasonal fluctuations in the estuary and are likely derived from both allochthonous (terrestrial runoff) and autochthonous (internal cycling) sources. Results indicate significant, yet unpredictable spatial and temporal variability in phytoplankton, with the main channel exhibiting higher phytoplankton biomass (~ 15 µg l−1) compared to the tidal creeks (~ 8 µg l−1) and phytoplankton composition shifting from pronounced diatom presence (summer 2015) along the main channel to dinoflagellate dominance along the tidal creeks (winter 2015). The potential influences of human activities, ranging from livestock presence to road bridge construction during this study are noted. This study confirms the need for further investigation on mangrove nutrient fluxes to understand their role on the water column microalgal dynamics particularly when freshwater abstraction is high.

Transcriptomic Response of Nannochloropsis oceanica to Benzo[a]pyrene

In this study, the RNA sequencing was used to describe the response of Nannochloropsis oceanica, a marine microalga, to benzo[a]pyrene (BaP), a polycyclic aromatic hydrocarbon, in order to elucidate the metabolic pathways (or processes) involved in microalgal response to this stubborn pollutant. N. oceanica was exposed to BaP at a concentration of 90 μ-1 for 72 h, and its transcriptome was sequenced through the Illumina HiSeq™ 2500 platform. This concentration of BaP was selected as it is the lowest for modeling the most appropriate growth inhibition of N. oceanica for transcriptomic analysis. We found that N. oceanica responds to BaP through degrading proteins and repairing DNA damaged by BaP. In addition, superoxide dismutase (SOD) strengthened its performance by increasing its transcript abundance. The physiological mechanism underlining the response of N. oceanica to BaP as revealed by transcriptomic analysis was consistent with the biochemical insights documented previously.

Interactive effects of temperature and copper toxicity on photosynthetic efficiency and metabolic plasticity in Scenedesmus quadricauda (Chlorophyceae)

Warming and copper (Cu) toxicity are two key abiotic stressors that strongly affect cell growth, photosynthetic rate, and metabolism in microalgae. In this study, a freshwater chlorophyte, Scenedesmus quadricauda, was exposed to various concentrations of copper sulfate (300, 600, and 1000 μM nominal concentrations of CuSO4·5H2O) at 25 and 35 °C. The changes in cell density, photosynthetic parameters, in vivo absorption spectra, reactive oxygen species (ROS) levels, and metabolic profile were analyzed. The effects of copper toxicity on the physiology and biochemistry of microalgae were highly dependent on water temperature. The interactive effects of both stressors induced significant impact on the photosynthetic parameters such as maximum quantum yield (Fv/Fm), saturation irradiance (Ek), and non-photochemical quenching (NPQ). Temperature induced significant impact on cell density, Ek and NPQ, while the Cu toxicity significantly affected the Fv/Fm and NPQ. Changes in the in vivo absorption spectra and high levels of reactive oxygen species (ROS) were observed across different treatments. Overall, S. quadricauda adapted to the two abiotic stresses via NPQ and metabolic restructuring. Key metabolites including glycine, proline, hexadecanoic acid, propanoic acid, octadecanoic acid, galactose, lactose, and sucrose were involved in the microalgal response. The synergistic effects of temperature and Cu stresses on microalgae might affect community tolerance and species distribution in the long run.

RESPONSE OF MICROALGAE IN A CHANGING CLIMATE AND ENVIRONMENT

Microalgae are ecologically important as a major primary productivity driver via photosynthetic carbon fixation. As the primary producers and food sources for higher trophic organisms, microalgae play a crucial role in maintaining the equilibrium of food webs in the aquatic ecosystem. The current shifts of global climate due to anthropogenic release of greenhouse gases have been reported to pose numerous impacts on microalgae. Extreme fluctuations in atmospheric temperature, light intensity, ultraviolet (UV) radiations, carbon dioxide (CO2) levels, and salinity can lead to alterations in growth, disruption of homeostasis, photosynthetic rate, respiration, enzymatic activity, protection to oxidative damage, and trophic transfer in microalgae. Various studies on microalgal responses to these environmental changes are ongoing to provide a deeper insight into the relationship between microalgal growth, metabolic adjustment and community structure. In this review the authors aim to highlight the recent findings on the responses of microalgae in the changing environment. Keywords: microalgae, climate change, environmental factors

Investigation of light/dark cycles effects on the photosynthetic growth of Chlamydomonas reinhardtii in conditions representative of photobioreactor cultivation

In microalgae cultivation, the combination of light attenuation and culture mixing exposes the microalgae to fluctuating light regimes, which is a factor that can interact with cell metabolism and affect photosynthetic conversion. Here we investigate the effect of light/dark cycles (L/D cycles) on the growth of microalga Chlamydomonas reinhardtii using a photobioreactor equipped with a light-emitting diode (LED) panel to simulate L/D cycles. The L/D cycles applied were engineered to mimic L/D cycles obtained in photobioreactor operating conditions, i.e. L/D cycle frequencies and light attenuation conditions applied during the light period. Long-term experiments were run in continuous mode to investigate whether cells adapt to applied L/D cycles.For cycles of duration higher than 40s, microalgal response was close to the response assuming no L/D cycle effect (no light integration). Biomass growth rate showed responses at durations below 12s, which points to partial light integration resulting in an increase of measured biomass growth rate. This increase is known as the “L/D cycle effect”, where L/D cycle frequencies are found to generate positive effects for photosynthetic conversion. To investigate whether the growth increase was explained by L/D cycle effects, results were compared against the predictions of a kinetic model assuming no light integration but time-solved for light regimes corresponding to L/D cycles applied experimentally. The model was found to very accurately represent any of the light regimes applied, including those leading to an increase in biomass growth rate. It was then concluded that expected primary coupling between L/D cycles and photosynthetic conversion was negligible. The main influence was on pigment adaptation, which when integrated into the kinetic model assuming no L/D cycle effects was found to very accurately predict culture growth under all light regimes.

Biological system development for GraviSat: A new platform for studying photosynthesis and microalgae in space

Microalgae have great potential to be used as part of a regenerative life support system and to facilitate in-situ resource utilization (ISRU) on long-duration human space missions. Little is currently known, however, about microalgal responses to the space environment over long (months) or even short (hours to days) time scales. We describe here the development of biological support subsystems for a prototype “3U” (i.e., three conjoined 10-cm cubes) nanosatellite, called GraviSat, designed to experimentally elucidate the effects of space microgravity and the radiation environment on microalgae and other microorganisms. The GraviSat project comprises the co-development of biological handling-and-support technologies with implementation of integrated measurement hardware for photosynthetic efficiency and physiological activity in support of long-duration (3–12 months) space missions. It supports sample replication in a fully autonomous system that will grow and analyze microalgal cultures in 120 μL wells around the circumference of a microfluidic polymer disc; the cultures will be launched while in stasis, then grown in orbit. The disc spins at different rotational velocities to generate a range of artificial gravity levels in space, from microgravity to multiples of Earth gravity. Development of the biological support technologies for GraviSat comprised the screening of more than twenty microalgal strains for various physical, metabolic and biochemical attributes that support prolonged growth in a microfluidic disc, as well as the capacity for reversible metabolic stasis. Hardware development included that necessary to facilitate accurate and precise measurements of physical parameters by optical methods (pulse amplitude modulated fluorometry) and electrochemical sensors (ion-sensitive microelectrodes). Nearly all microalgal strains were biocompatible with nanosatellite materials; however, microalgal growth was rapidly inhibited (∼1 week) within sealed microwells that did not include dissolved bicarbonate due to CO2 starvation. Additionally, oxygen production by some microalgae resulted in bubble formation within the wells, which interfered with sensor measurements. Our research achieved prolonged growth periods (>10 months) without excess oxygen production using two microalgal strains, Chlorella vulgaris UTEX 29 and Dunaliella bardawil 30 861, by lowering light intensities (2–10 μmol photons m−2 s−1) and temperature (4–12 °C). Although the experiments described here were performed to develop the GraviSat platform, the results of this study should be useful for the incorporation of microalgae in other satellite payloads with low-volume microfluidic systems.

Microalgal responses to physico-chemical variability in the small temporarily open/closed Seteni Estuary, South Africa

The Seteni Estuary is a small temporarily open/closed estuary (TOCE) in South Africa under the influence of agricultural practices. While the general significance of microalgae to estuarine production is widely recognised, the factors regulating microalgal biomass in these heterogeneous systems are less well understood, particularly when man-induced pressures are superimposed on natural variability. This study investigated microalgal responses to physico-chemical variability in the nutrient-enriched Seteni Estuary in 2008–2009. Microphytobenthic biomass ranged from 1.1 to 91.7 mg Chl a m-2, while phytoplankton biomass varied between 0.22 and 18 mg Chl a m-3. Despite the high dissolved inorganic nitrogen (DIN) concentrations recorded, salinity and rainfall were identified as the main environmental drivers, highlighting the importance of allochthonous inputs. While this system appears to function as a typical TOCE, the relatively low microalgal biomass recorded may be an artifact of several factors such as phosphorus limitation, herbicide treatment, etc., acting singly or together to depress biomass levels. However, should the balance be upset, the most probable result would be a dramatic increase in microalgal biomass, to the point where harmful algal blooms may ensue. The system therefore needs to be closely monitored to prevent further degradation.

Virus infection of Haptolina ericina and Phaeocystis pouchetii implicates evolutionary conservation of programmed cell death induction in marine haptophyte-virus interactions.

The mechanisms by which phytoplankton cope with stressors in the marine environment are neither fully characterized nor understood. As viruses are the most abundant entities in the global ocean and represent a strong top-down regulator of phytoplankton abundance and diversity, we sought to characterize the cellular response of two marine haptophytes to virus infection in order to gain more knowledge about the nature and diversity of microalgal responses to this chronic biotic stressor. We infected laboratory cultures of the haptophytes Haptolina ericina and Phaeocystis pouchetii with CeV-01B or PpV-01B dsDNA viruses, respectively, and assessed the extent to which host cellular responses resemble programmed cell death (PCD) through the activation of diagnostic molecular and biochemical markers. Pronounced DNA fragmentation and activation of cysteine aspartate-specific proteases (caspases) were only detected in virus-infected cultures of these phytoplankton. Inhibition of host caspase activity by addition of the pan-caspase inhibitor z-VAD-fmk did not impair virus production in either host–virus system, differentiating it from the Emiliania huxleyi-Coccolithovirus model of haptophyte–virus interactions. Nonetheless, our findings point to a general conservation of PCD-like activation during virus infection in ecologically diverse haptophytes, with the subtle heterogeneity of cell death biochemical responses possibly exerting differential regulation on phytoplankton abundance and diversity.

Nitrogen Limitation in Neochloris oleoabundans: A Reassessment of Its Effect on Cell Growth and Biochemical Composition

The aim of this work was to reassess the effect of nitrogen limitation (from 0 to 1 mM nitrate), on the growth and the biochemical composition of Neochloris oleoabundans cultures, where only the CO2 available in the air was provided. Slight differences in the initial nitrate concentration, even minimal increments of 0.2 mM, significantly modify the microalgal response towards nitrogen limitation. This stress condition reduced cell proliferation, but increased cell mass values due to the simultaneous accumulation of two storage compounds: lipids, which contained up to a 55.9 % of total fatty acids; and carbohydrates, which may be primarily composed by starch. The highest biomass and lipid productivities of 98.24 and 43.24 mg/l/day, respectively, were attained at an initial nitrate concentration of 0.6 mM. The theoretical annual projection, based on these productivities, allowed the estimation of the liquid fuel energy yields, which are comparable or even higher than those calculated for several biomass feedstocks such as corn, oil palm, sugarcane, or even fast growing grasses, confirming the potential of nitrogen-limited N. oleoabundans biomass as an appropriate feedstock for biofuel purposes.

Characterization of cell response in Chlamydomonas moewusii cultures exposed to the herbicide paraquat: Induction of chlorosis

The use of herbicides constitutes the principal method of weed control, but the introduction of these compounds into the aquatic environment can provoke severe consequences for non-target organisms such as microalgae. Effects of the widely used herbicide paraquat were assessed on the green freshwater microalga Chlamydomonas moewusii by means of the analysis of its photosynthetic pigment content, using a traditional spectrophotometric technique that provides population bulk measurements, and by means of flow cytometry, which allowed characterizing the microalgal response at a single-cell level. Results obtained reveal that paraquat concentrations above 50 nM induce chlorosis in a percentage of microalgal cells depending on herbicide concentration and exposure time, as reflected by a reduced cell chlorophyll autofluorescence and pigment content of the biomass. Cell viability in these cultures was also reduced in a concentration dependent way. The possibility of analysing chlorotic and non-chlorotic subpopulations separately allowed the study of morphological properties and physiological status of both cell types, leading to the conclusion that chlorotic cells are non-viable cells, based on their reduced size and complexity and their inability to be stained in the fluorescein diacetate assay. In the case of non-chlorotic cells, cell viability was reduced with the increase of paraquat concentration. Non-chlorotic cells in these cultures showed an increased size and complexity in comparison with control cells, probably due to a growth inhibition. Chlorophyll fluorescence was the most sensitive parameter since even cells exposed to the lowest concentration assayed, 50 nM, although not chlorotic, showed a significantly reduced chlorophyll fluorescence with respect to control cells, reflected also by a reduced chlorophyll content of the biomass.

Sediment and waternutrients and microalgae in a coastal shallow lagoon, Ria Formosa (Portugal): Implications for the Water Framework Directive

Coastal shallow lagoons are considered to be highly important systems, which have specific biogeochemical cycles and characteristics. The assessment of sediment-water interfaces is essential to understand nutrient dynamics and to evaluate the vulnerability to eutrophication, especially in regions of restricted water exchange (RRE), such as the Ria Formosa, which have natural conditions for the accumulation of nutrients. Water samples were collected during the years of 2006 and 2007-08 for nutrients, chlorophyll a and dissolved oxygen. Sediment samples were also collected for pore water nutrients and microphytobenthic chlorophyll a. Measurements of temperature, salinity and photosynthetic active radiation were also taken. The lagoon salinity is affected by occasional strong rainfall events. From comparison with previous work, a decrease in the nitrogen concentration in the water column can be observed, which may indicate an improvement of the water quality. Pore water nutrient concentrations were significantly larger than in the water column. Sediment-water exchanges are considered to be the most important processes in nutrient dynamics of the lagoon. Benthic microalgal biomass was also large compared with that of the phytoplankton. It represents about 99% of the total microalgal chlorophyll biomass of the system. The lagoon also contains (discontinuous) meadows of intertidal seagrass, but we did not study these. Due to the importance of sediments, the standard monitoring plans required by the Water Framework Directive may fail to track changes in the nutrient conditions and the microalgal responses to them.

Response of micro-algae in the Kromme Estuary to managed freshwater inputs

The Kromme is a permanently open estuary that receives little freshwater input because the capacity of the dams is equivalent to the mean annual runoff from the catchment. The estuary is marine dominated and phytoplankton chlorophyll a is low because of the low freshwater pulses that introduce nutrient-rich freshwater. Water released (2 x 106 m3) from the Mpofu Dam in 1998 produced little micro-algal response in the estuary. The study reported here addresses further runoff scenarios to see which might be beneficial in stimulating microalgal production. Recent surveys together with past research were used to describe the present state and reference condition of the estuary. Average intertidal chlorophyll a was 12.9 ± 2.5 μg·g-1 of sediment and 4.9 ± 0.4 μg·g-1 of sediment during November 2003 and July 2004. These concentrations are relatively low but comparable to those found in intertidal sediments in other South African estuaries and might indicate that intertidal microalgal biomass is not severely limited by low freshwater inputs. Average water column chlorophyll a concentrations have ranged from 0.6 ± 0.1 to 5.6 ± 0.3 μg·ℓ-1. Present state conditions can thus be described as those where water column chlorophyll a seldom exceeds 5 μg·ℓ-1 and small flagellates (3.5 μm x 2.8 μm) dominate the phytoplankton. The diatoms introduced via freshwater have been lost. Under reference conditions before the Mpofu Dam was built, baseflow would have been greater than 1 m3·s-1 for approximately 8 months of the year. The flocculation of fine particles associated with the mixing of fresh and saline waters would have resulted in phytoplankton peaks (chl a >10 μg·ℓ-1) in the middle reaches of the estuary. A more suitable habitat might also have been present for the epipelic (mud associated) benthic microalgae. An assessment of the future runoff scenarios indicated that the most beneficial for the microalgae would be a flow release from the Mpofu Dam of 5 x 106 m3 in October and then again in January. This would stimulate a 25 to 33% increase in phytoplankton chlorophyll a and a doubling in intertidal benthic chlorophyll a for a period of two months following the releases. Water SA Vol 32(1)pp:71-80