Heavy metal toxicity has proven to be a major threat and there are several health risks associated with it. The toxic effects of these metals, even though they do not have any biological role, remain present in some or the other form harmful for the human body and its proper functioning. They sometimes act as a pseudo element of the body while at certain times they may even interfere with metabolic processes. Few metals, such as aluminium, can be removed through elimination activities, while some metals get accumulated in the body and food chain, exhibiting a chronic nature. Various public health measures have been undertaken to control, prevent and treat metal toxicity occurring at various levels, such as occupational exposure, accidents and environmental factors. Metal toxicity depends upon the absorbed dose, the route of exposure and duration of exposure, i.e. acute or chronic. This can lead to various disorders and can also result in excessive damage due to oxidative stress induced by free radical formation. This review gives details about some heavy metals and their toxicity mechanisms, along with their health effects.

Ruminants are cloven-hoofed mammals of the Artiodactyla order, with domesticated cattle, sheep, and goats comprising 95% of the total ruminant population [14]. They obtain their food by browsing or grazing, subsisting on plant material using their specialized digestive system [15] with a sophisticated symbiotic web of microorganisms [16]. The digestive system of ruminants consists of four compartments -rumen, reticulum, omasum, and abomasum [17].In the rumen, the intricate community of bacteria (10 10 -10 11 cells/ml), ciliate protozoa (10 4 -10 6 cells/ml), methanogenic archaea (10 6 -10 8 cells/ml), and fungi (10 3 -10 6 cells/ml) synthesizes enzymes which breakdown complex macromolecules derived from feed [18,19]. This fermentative process produces short volatile fatty acids (SVFAs) and microbial crude protein, which is an essential source of energy and protein for the host, while the rumen provides the microbes a suitable environment for survival and growth [16,20]. Acetate (~65%), propionate (~20%), and butyrate (~15%) form the major part of SVFAs in the anaerobic rumen fermentation, which supplies 80% of the animal's total energy requirement [21]. Subsequently, methanogens in the gastrointestinal tract produce methane as a by-product of anaerobic fermentation [22].Human activities account for approximately two-thirds of global methane emissions, wherein the livestock sector is the single massive methane emitter. Methane is a potent greenhouse gas of over 21 times the warming effect of carbon dioxide. In the rumen, methanogens produce methane as a by-product of anaerobic fermentation. Methane released from ruminants is considered as a loss of feed energy that could otherwise be used for productivity. Economic progress and growing population will inflate meat and milk product demands, causing elevated methane emissions from this sector. In this review, diverse approaches from feed manipulation to the supplementation of organic and inorganic feed additives and direct-fed microbial in mitigating enteric methane emissions from ruminant livestock are summarized. These approaches directly or indirectly alter the rumen microbial structure thereby reducing rumen methanogenesis. Though many inorganic feed additives have remarkably reduced methane emissions from ruminants, their usage as feed additives remains unappealing because of health and safety concerns. Hence, feed additives sourced from biological materials such as direct-fed microbials have emerged as a promising technique in mitigating enteric methane emissions.

Development of bioconversion technology for efficient conversion of crude glycerol to non-natural acetins.

The rumen plays an indispensable role in ruminants to utilize ligno-cellulosic material and convert non-protein nitrogen into nutrients otherwise unavailable for human consumption. Recent advancements in the sequencing technology and omics approach have provided profound insights into the rumen world, wherein a consortium of archaea, bacteria, protozoa, fungi, and viruses exist and interact. These ruminal microbes alter the ruminal environment and execute several interlinked metabolic cascades that produce substrates for the host’s energy and body requirements. Methane is emitted as a by-product during this complex fermentation process in ruminants leading to a loss in productivity while negatively impacting the environment. As ruminants play an ever-increasing role in our food supply chain, manipulating the rumen is the critical step towards maximizing the ruminant product’s nutritional value while reducing its carbon footprint. Diet is the most straightforward way to alter the rumen microbiome, possibly in conjunction with phytobiotics and probiotics as feed supplements. Early life interventions allow the manipulation of microbial population structure and function that could persist later on in adult life. It has also been proven that the host exerts influence on the rumen microbiome as a heritable trait. The goal of this review is to provide a better understanding of the rumen, its key organisms, and its development to better identify, characterize, and engineer the rumen microbiome for efficient feed conversion and methane reduction.