**1. Introduction**

As the global economy unfolds and the population increases, environmental concerns associated with greenhouse gas emissions and wastewater discharges, as well as issues involving food security, become unavoidable problems [1]. Given this devastating and worrying scenario, the challenges of sustainable development aroused interest in defining a potentially promising target that was recognized, consensually, as the future salvation.

Under this slant, the macro- and microalgae—known as the most primitive form of life on Earth—become the hype in the academic world. Over years, the discoveries surrounding these green microorganisms formed a glorious path in the biotechnological industry. So, its robust adaptability and metabolic plasticity, beyond its highly flexible capacity for genetic and environmental changes, have reiterated the microalgae, such as gold coins in the global market [2].

Without delay, macro- and microalgae were seen as a biological pump that not just could drive, balance, and maintain global ecosystems, but could also be used in many different technological fields. From this perspective, pioneering studies were traced to the application of microalgae as the main feedstock for bioenergy production [3]. However, obstacles involving low productivity, fractionation of biomass, as well as technical questions of scalability, have repercussions on technical-economic attributes, delaying the consolidation and commercialization of biofuels [4]. Later, the clean technologies concept applied to microalgal biotechnology was extended to the microalgae application as an effective tool for environmental processes, such as wastewater bioremediation and also the capacity to mitigate polluting gases. However, more than once the limitations of the process, already advanced in various aspects, have continued to haunt the microalgae consolidation [5].

Since then, the research objective has changed over time. To rectify this gap, it was necessary to understand basic aspects involving the biological fundamentals of microorganisms and extend the research to optimization of the production stages. Forthwith, the new research efforts return for advances in genetic engineering, mapping from to complex evaluation of the upstream and downstream stages until the ascertainment of new methods of biomass harvesting and extraction, gave the path to new erudition [6].

However, the current plans, at the time, to establish the consolidation of microalgal biotechnology in the fields of bioenergy and environmental bioremediation were still not sufficiently developed, and research agencies and private companies have shown interest in returning to microalgal biotechnology as a viable economy in the immediate future. Consequently, the green gold could not lose its brilliance, and the hype of microalgal biotechnology could not disappear without more, nor less.

Therefore, the most sublime strategy to intertwine the microalgae application was to establish a key connection for the hungry reduction, giving weight to health aspects, concomitant to a sustainable environment [7]. Thus, microalgae as the future food were launched as a promising game against the fulfillment of the sustainable development goals [8]. This is because its excellent nutritional content, attributing prominence to its high protein, lipid, and carbohydrate fractions, is considered a food with high value-added. Besides, the attention paid to fine chemical products, such as pigments, fatty acids, and nucleic acids contained in their biomass would attract the food and pharmaceutical industries as a platform for marketing the function of their biological properties [9].

However, although microalgae have been strategic resources for sustainability and promising market trends, the process challenges remained. Among them, the problems associated with excessive demand for nutrients and energy imputed by microalgae processes urgently need a solution [10]. In this way, it is the need to align the optimization and integration of processes, which turned into the key point for success. Thus, the triumph of microalgae-based processes would consist of a technological advance that would lead to a significant degree of self-sufficiency [11].

In this sense, in order for the microalgae processes in conjecture with biotechnology to grow larger and, in fact, become self-sufficient, it is imperative that public and private research tread exploration paths that lead to the discovery of new scientific studies that break the barriers hitherto imposed. The truth is that these jewels of nature, regardless of which way they emerge, will always play a vital role in the transformation of a sustainable modern world. This is because, in this scientific universe, there are tens of thousands of other green microscopic singularities, with a variety of shapes and functions to breathtaking, still waiting to be explored. For science, microalgae are not just a scum from lakes and seas, but they are in a universe of possibilities. From a commercial point of view, they are considered the feedstock of the future, this is, the jack of all trades.

So, perhaps, it would be appropriate to think that—in a period that is difficult to predict—all the benefits of microalgae for new processes and products are just waiting to be harvested. However, now, it is important that we become part of a revolution in the way we consider and conduct our research, ensuring that the results obtained will eventually, indeed, be used more and more for our benefit and applied to our planet. In this regard, we expect that the next chapters found in this book will serve as building blocks for the advancement of microalgal biotechnology, to build on top of a robust theoretical knowledge, an important practice contribution to the transition to a more sustainable society.
