Manganese is a naturally occurring trace element found throughout the environment, including air, water, and soil. It exists in various chemical forms, with the divalent manganese ion (Mn2+) being the most common dissolved form in water. This element is essential for numerous biological processes in various organisms, playing a significant role in both human health and aquatic ecosystems.
Manganese: An Essential Element
Manganese is an essential trace mineral, serving as a cofactor for many enzymes important for human health. These enzymes participate in metabolic activities, including carbohydrate, fat, and protein breakdown. It is a component of superoxide dismutase (SOD), an enzyme protecting cells from free radical damage.
Manganese also contributes to bone and cartilage formation by supporting proteoglycan synthesis, components of connective tissues. It supports neurological health, influencing neurotransmitter synthesis and synaptic function. The recommended daily intake for adults is approximately 2.3 mg/day for men and 1.8 mg/day for women.
Manganese deficiency in humans is uncommon due to its presence in many foods, but symptoms can include impaired bone health, reduced growth, and altered glucose and lipid metabolism. Conversely, manganese toxicity is typically associated with environmental exposure, such as through contaminated drinking water or occupational inhalation, rather than dietary intake. High levels can lead to neurological issues, resembling Parkinson’s disease, and cognitive impairments.
Fish as a Source of Manganese for Human Health
Fish and seafood are a dietary source of manganese for human nutritional needs. Shellfish like mussels, clams, and oysters are particularly rich in this mineral. For example, a three-ounce serving of mussels can provide around 5.8 milligrams of manganese, exceeding the daily requirement for adults.
Finfish like mackerel or tuna also contain manganese, though levels vary. Certain seafood offers a concentrated amount of manganese compared to other common dietary sources like whole grains, nuts, legumes, and leafy greens. Consuming these foods helps meet daily manganese requirements.
Concerns about excessive manganese intake from fish alone are generally low. Dietary manganese absorption in humans is typically limited, with only 1% to 5% of consumed manganese absorbed. The body regulates manganese levels through excretion, primarily via bile into the feces, reducing the risk of accumulating toxic amounts from food sources.
Manganese’s Role in Fish Biology
Manganese is an essential micronutrient for fish, supporting their growth, development, and physiological health. It acts as a cofactor for enzyme systems in fish, influencing carbohydrate and lipid metabolism. It is also involved in bone and cartilage formation, particularly relevant for skeletal development in fish.
Adequate manganese contributes to reproductive health and immune function in fish, aiding disease resistance. Dietary manganese deficiency can lead to adverse effects in fish, including reduced growth rates, skeletal deformities like shortened or curved spines, and impaired reproduction. For instance, juvenile yellow catfish may experience inhibited growth if dietary manganese drops below 3.1 mg/kg.
Fish absorb manganese from both their diet and the surrounding water. In aquaculture, ensuring appropriate dietary manganese levels in feeds optimizes fish health and productivity. Optimal dietary manganese requirements vary among species, ranging from approximately 2.4 mg/kg for channel catfish to around 17.35-19.75 mg/kg for coho salmon post-larvae.
Manganese in the Aquatic Environment and Fish
Manganese is present in aquatic environments from natural processes, such as rock and soil weathering, and human activities like industrial discharges and agricultural runoff. Its bioavailability in water, the amount available for uptake by organisms, is influenced by factors like water pH, oxygen levels, and the presence of other minerals. For example, manganese solubility generally increases at lower pH and under anaerobic (low oxygen) conditions, making more dissolved Mn2+ available.
Fish absorb manganese from their environment, primarily through gills and skin, and from their diet. While bioaccumulation, the uptake and retention of manganese in fish tissues, occurs, especially in organs like gills and liver, manganese generally does not biomagnify to harmful levels up the food chain like some other heavy metals such as mercury. Manganese concentrations in fish gills can indicate environmental manganese levels.
Excessive environmental manganese can harm fish health, although it is typically less toxic than other heavy metals. High concentrations can lead to gill damage, neurological effects, and altered blood parameters. For instance, water manganese concentrations exceeding 4.5 mg/L have been shown to decrease growth in brown trout, and concentrations above 15.5 mg/L can be lethal.