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.

The need to reduce the CO₂ footprint of human activities calls for the utilization of new means of production and new sources of products. Microalgae are a very promising source of a large variety of products, from fuels to chemicals for multiple industrial applications (e.g., dyes, pharmaceutical products, cosmetics, food and feed, new materials for high tech manufacture), and for processes such as wastewater treatment. Algae, as photosynthetic organisms, use light to energize the synthesis of organic matter and differently from most terrestrial plants, can be cultured on land that is not used for crop production. We describe the main factors contributing to microalgae productivity in artificial cultivation systems and discuss the research areas that still need investigation in order to pave the way to the generation of photosynthetic cell factories. We shall comment on the main caveats of the possible mode of improving photosynthetic efficiency and to optimize the partitioning of fixed C to products of commercial relevance. We address the problem of the selection of the appropriate strain and of the consequences of their diverse physiology and culture conditions for a successful commercial application. Finally, we shall provide state of the art information on cell factories chassis by means of synthetic biology approaches to produce chemicals of interest.

Carotenoids extracted from microalgae have a considerable economic interest in numerous high-value markets. Natural astaxanthin has gained much interest in its powerful antioxidant properties, however, its commercial-scale production is still challenging. In this study, a simple and economical way to cultivate Haematococcus pluvialis (CCAP 34/1D) by a two-step process was investigated by exploring alternative strategies to maximise algal growth and astaxanthin yield. During the first step—improving biomass production—four nitrogen sources were tested (NH₄Cl, NaNO₃, Fe(III)NO₃, and urea). The second step—carotenogenesis induction—was achieved by using a mix of moderate stressors that worked in synergy (i.e., mild light, nitrogen limitation, the addition of sodium acetate at 0.25% w/v), thereby minimising potential losses of the accumulated biomass caused, for example, by photobleaching or nitrogen starvation. Results showed that urea was the nitrogen source, allowing the highest cell density and growth rate. In terms of carotenogenesis induction, the use of mild stressors resulted in three out of four treatments having a relative increase in cell number (13.8–26.7%) and a concomitant increase in astaxanthin yield. Simple low-cost strategies, such as small adjustments to media recipes and synergism between mild stressors, could bring a disproportionate effect on the future successes of making algal biotechnology a widespread reality.

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.

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.

In today’s oceans, diatoms are abundant and diverse primary producers distinguished by their silica shells. Although molecular clocks suggest that diatoms arose as much as 250 million years ago (Ma), the earliest known diatom fossils date from 190 Ma, leading to the suggestion that early diatoms were at best lightly silicified. By the Cretaceous Period, large circular (in cross section) diatoms with highly silicified frustules thrived in surface oceans, only later to be joined by species with elongated and thinner frustules, as well as lower SiO₂ content. Decreased Si availability in surface oceans has been proposed as a principal driver of diatom evolution. Here, we investigate this through physiological experiments assessing the functional acclimation response of diatoms to reconstructed paleo-seawater. Four diatom species, differing in size and shape, were acclimated to reconstructed paleoenvironments mimicking Mesozoic/Cenozoic concentrations of nutrients in the presence of different Si regimes. When exposed to 500 µM Si, all populations, save for that of Conticribra weissflogii, became more highly silicified; the higher Si content per cell at 500 µM Si coincided with slower growth in small-sized cells. All species except C. weissflogii also showed lower photosynthetic efficiency as well as greater cell volume in comparison with diatoms acclimated to 205 or 25 µM Si. Average cell stoichiometry correlates with cell shape, but not size; pennates, in particular Phaeodactylum tricornutum, showed an acclimatory response to Si regimes, modulating Si use efficiency (the lower the external Si concentrations, the higher the C and N quotas per Si).Experimental data suggest that in the densely silicified and bigger C. weissflogii grown at higher Si, diffusion of silicic acid across membranes made a larger contribution to Si uptake, saving energy which could be reallocated into growth. In contrast, for less highly silicified and smaller species, high energy costs of Si homeostasis needed to prevent the overaccumulation of intracellular Si limited growth. While our experimental species reacted individualistically to changing silica availability, with distinct levels of plasticity, selective pressure associated with the temporal decline in Si availability may well have favored elongated shapes. Modern, less silicified species are unable to exploit high Si concentrations.