Fermentation of Microalgal Biomass for Innovative Food Production

Fermentation is an ancient method used worldwide to process and preserve food while enhancing its nutraceutical profile. Alga‐based fermented products have recently been developed and tested due to growing interest in healthy sustainable diets, which demands the development of innovative practices in food production, operating for both human health and Earth sustainability. Algae, particularly microalgae such as Arthrospira platensis, Chlorella vulgaris, and Dunaliella salina, are already cultivated as sources of food due to their valuable compounds, including proteins, pigments, lipids, carotenoids, polyunsaturated fatty acids, steroids, and vitamins. Due to their nutritional composition, functional diversity, and flexible metabolism, microalgae represent good fermentation substrates for lactic acid bacteria (LAB) and yeasts. This review presents an overview of the scientific studies on microalga fermentation underlining microalgae’s properties and health benefits coupled with the advantages of LAB and yeast fermentation. The potential applications of and future perspectives on such functional foods are discussed

Ecophysiological behaviour of different diatoms in response to copepod signals

Phytoplanktonic communities have been moulded from a higher level in the food web by zooplankton predation. Diatoms, which are the major primary producers in present day oceans, have evolved many strategies to thrive in the presence of copepods, their predators. Chemical signals are released in the aquatic environment by microarthropods and elicit morphological, behavioural and/or physiological modifications in diatom cells which allow them to cope with predators. Other than the most studied effects of copepod signals on the release of toxins by diatoms, the role of chemical cues on growth and biology of non-toxic diatoms has been little investigated, although it may broaden our knowledge of coevolutive physiological mechanisms. To examine the response of diatoms to copepod-derived signals, Phaeodactylum tricornutum, Chaetoceros muelleri and Cylindrotheca fusiformis (Bacillariophyceae) were grown in the presence of copepod cues. Physiological modifications occurred and differed in sign and magnitude between species: i) in P. tricornutum cell density was higher, and dry weight lower, suggesting energy allocation into cell division at the expense of lipid and protein quotas; ii) C. muelleri showed a very homeostatic behaviour; and iii) in C. fusiformis cell density was lower and lipid content higher, showing energy allocation into lipids, which likely act as deterrent to feeders. Copepod cues are here suggested to have ruled diatom species composition in the ocean by affecting species-specific growth performance and the energy allocation into macromolecules

Diatoms versus copepods : could frustule traits have a role in avoiding predation?

Predation is one of the strongest selection pressures phytoplankton has evolved strategies to cope with. Concurrently, phytoplankton growth must deal with resource acquisition. Experiments on mono- and mixed cultures of morphologically different diatoms exposed to copepods were performed to assess if size and shape were primary drivers in avoiding predation. Additionally, frustule silicification was investigated as a potential factor affecting prey selection by copepods. Thalassiosira pseudonana, Conticribra weissflogii, Cylindrotheca closterium, and Phaeodactylum tricornutum were exposed to the presence of Temora longicornis, a calanoid copepod. The physiological response in terms of growth, elemental composition and morphology was determined. The power of Image Flow Cytometry allowed functional single-cell analyses of mixed cultures in the presence and absence of copepods. Results highlighted that T. pseudonana although the most eaten by copepods in monospecific cultures, was not the preferred prey when the bigger C. weissflogii was added to the culture. When pennates were co-cultured with centric diatoms, their growth was unaffected by predators. Our data suggested that the frustule morphology contributes to long-term prey-predator interaction since the elongated thinner frustule, which evolved more recently, benefited cells in escaping from predators also when facing competition for resources

Sulfur and phytoplankton: acquisition, metabolism and impact on the environment

Sulfur emission from marine phytoplankton has been recognized as an important factor for global climate and as an entry into the biogeochemical S cycle. Despite this significance, little is known about the cellular S metabolism in algae that forms the basis of this emission. Some biochemical and genetic evidence for regulation of S uptake and assimilation is available for the freshwater model alga Chlamydomonas . However, the marine environment is substantially different from most freshwaters, containing up to 50 times higher free sulfate concentrations and challenging the adaptive mechanisms of primary and secondary S metabolism in marine algae. This review intends to integrate ecological and physiological data to provide a comprehensive view of the role of S in the oceans

Integrative effects of morphology, silicification, and light on diatom vertical movements

Diatoms represent the most abundant and diversified class of primary producers in present oceans; their distinctive trait is the ability to incorporate silicic acid in a silica outer shell called frustule. Numerous adaptative functions are ascribed to frustules, including the control of vertical movements through the water column; this indirectly determines cell access to fundamental resources such as light and nutrients, and favors diatom escape from predators. At the same time, light guides phototroph movements in the water column by affecting cell density (e.g., by modulating Si deposition in diatoms, vacuole volume, and/or solution). We investigated how the tremendous diversity in morphology and silicification that characterizes the frustule and the crucial role of light in diatom spatial distribution govern diatom sinking capacity. To test their integrative effects, we acclimated four diatoms distinguished by frustule traits (Chaetoceros muelleri, Conticribra weissflogii, Phaeodactylum tricornutum, and Cylindrotheca fusiformis) to different light conditions and evaluated their physiological performance in terms of growth, elemental composition, morphological changes, and their in vivo sinking capacity. What emerged from this study was that silicification, more than other morphological characteristics, controls species vertical movements, while a higher energy availability enhances cell floating independently from the silica content

Role of phosphoenolpyruvate carboxylase in anaplerosis in the green microalga Dunaliella salina cultured under different nitrogen regimes

Anaplerosis plays a very important role in providing C for N assimilation. In green algae and higher plants, phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) is the main anaplerotic carboxylase. On this basis we hypothesize that N availability affects PEPC expression. In order to test this hypothesis, the model organism Dunaliella salina was cultured under a variety of N growth regimes. Our results show that the level of PEC activity was unaffected by the N form in which N was supplied to the cells, when N concentration was low (0.5–0.01 mM). When cells were adapted to growth at 5 mM N, however, PEPC activity on a per cell basis was substantially higher in NH4+-adapted cells as compared to their NO3–-adapted counterparts; however, the same difference was not observed on a protein basis. This notwithstanding, even at low N, PEPC of cells cultured in the presence of either NH4+ or NO3– appeared to differ in their molecular masses. These results suggest that cells adapted to different N-form express distinct PEPC isoforms. In addition to this, we observed that, in algae adapted to high (5 mM) NH4+ concentration, a PEPC isoform was induced that differed from the isoforms observed in algae adapted to lower concentrations of the same N-source. These findings lead us to conclude that the expression of PEPC isoforms in D. salina responds to the variation in the C-skeleton demand deriving from changes in the chemical form and availability of N

impact of inhibitors of amino acid protein and rna synthesis on c allocation in the diatom chaetoceros muellerii a ftir approach

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Screening for tolerance to natural phenols of different algal species: Toward the phycoremediation of olive mill wastewater

Olive Mill Wastewater (OMWW) is a by-product of olive oil production and it is rich in nutrients (e.g. P, N and K) and phenolic compounds. These latter are aromatic compounds, and their concentrations can reach up to 11 g L−1 in OMWW. A complete remediation of OMWW is required since phenols are known to cause toxicity once released in the environment: particularly, their effect on microorganisms is species-specific and primarily depends on the chemical structure of the compound. Microalgae have already been tested to remediate OMWW, data are promising but how different phenols affect algal growth is still poorly known. In this work, ten microalgal species belonging to different phylogenetic groups and natural habitats, were grown in the presence of three phenolic compounds found in OMWW (tyrosol, coumaric acid, caffeic acid). Algal growth and removal of phenolic compounds were assessed. Tyrosol was the only compound allowing growth of each tested microalga similarly to what observed in control media. Growth of microalgae and removal of phenolic compounds were not always related, and a multi-step phenolic removal mechanism was suggested. Species such as Nannochloropsis salina and Porphyridium purpureum rapidly died after the addition of coumaric acid or caffeic acid but a high removal percentage (60–100 %) of the phenols was still observed and it was likely due to their absorbance onto the cell surface. On average, freshwater species showed a higher growth performance compared to the one of marine species; in particular, Tetradesmus obliquus and Anabaena sp. showed the best results. This work elucidates a species-specific effect for each phenolic compound on algal growth and it also highlights that growth and removal are not related phenomena

Influence of the nitrogen source and metabolites on phosphoenolpyruvate carboxylase activity in the unicellular green alga Dunaliella parva ccap 19/9

The activity of phosphoenolpyruvate carboxylase (PEPC) was measured in cell-free extracts of the salt-tolerant unicellular green alga Dunaliella parva Lerche CCAP 19/9. For cells grown in batch cultures with 5 mM NaNO3 as the sole source of nitrogen, the optimum pH for PEPC activity was 8 and the reaction was saturated by 1 mM phosphoenolpyruvate (PEP). The K0.5 for PEP was 150 μM and the Vmax was 0.18 ± 0.04 μmol h−1 mg−1 soluble protein. The effect of key metabolites on PEPC activity was determined under optimum assay conditions. Glycerol, the main osmoticum of Dunaliella, had little or no effect on PEPC activity, but dihydroxyacetone phosphate, pyruvate, α-ketoglutarate, orthophosphate, glutamine, glutamate and aspartate all stimulated activity. Surprisingly, the largest stimulation was exerted by aspartate, generally an inhibitor of PEPC, which enhanced PEPC activity up to 30-fold. Oxaloacetate slightly inhibited PEPC activity, but the most effective inhibitor was malate, which caused an 80–90% decrease in the carboxylation, even at low concentration. The response of PEPC activity to replacement of NaNO3 with NH4Cl was tested in continuous culture. The activity of PEPC increased when NH4+ was added to the medium and, even if it decreased somewhat during acclimation to the new nitrogen source, it stayed at levels that were appreciably higher than those observed in NO3−-grown cells. These results are discussed with regard to the role that PEPC may play under different nitrogen growth regimes