Biomass decomposition and heavy metal release from seaweed litter, Gracilaria lemaneiformis, and secondary pollution evaluation

The seaweed Gracilaria lemaneiformis can bioremediate heavy metals and improve the environmental quality of mariculture zones. However, the seaweed litter that is produced in the growth and harvest processes becomes one of the important bottlenecks and causes secondary pollution that restricts the development of sustainable seaweed cultivation. Seaweeds exist widely in the coastal areas of the world and are cultivated on a large scale in Asia, but their decomposition process is rarely studied. Experiments that compared decaying dry (dead) and fresh (falling and dying) Gracilaria were conducted to quantify the differences in decomposition rates and heavy metal release in different physiological states. The heavy metals in the seawater and sediment were investigated. The litterbag technique under controlled laboratory conditions was used. The results indicated that the decomposition rates (k) and decay times in 50% (t50%) and 95% (t95%) values varied between dry and fresh Gracilaria. Fresh Gracilaria exhibited a weight loss rate of 15%, and the dry weight loss was 44%. The variations in MAIs (accumulation index of metals) and M_R (release rate of metals) between the dry and fresh Gracilaria litters differed significantly, which provides evidence that metals are released back into the environment from Gracilaria litters. The contacted sediments could accelerate the heavy metal release from Gracilaria. Based on our estimates obtained from a 45 d experiment, at least 27.5％ of Cd, 16％ of Cu, 60.1％ of Pb, 72.3％ of Zn, 49.4％ of Fe, 38.6％ of Mn, 68.1％ of Cr, and 67.5％ of Ni present in the fresh Gracilaria and 37.4％ of Cd, 46.2％ of Cu, 77.7％ of Pb, 53.7％ of Zn, 42.7％ of Fe, 67.2％ of Mn, 75.1％ of Cr, and 73.5％ of Ni present in the dried Gracilaria were released back into the water when the biomass was left to decay. This study simulates and underscores that Gracilaria has an strong effect on the heavy metal cycles in marine environments and offers a theoretical basis for the development of sustainable seaweed industries in mariculture zones.