Humic substances (HS) are organic compounds that dominate the dissolved organic carbon (DOC) in most of the aquatic ecosystems. Despite the variable chemical functionality and ubiquity of HS, their direct effects to organisms have been largely neglected up to the last decade. However, HS are uptaken and cause typical stress responses in organisms, such as oxidative stress. By causing a usually mild stress, HS may extend longevity and confer multiple stress resistance of aquatic organisms. In this study, we tested the effects of natural HS from a tropical polyhumic lagon on life-history traits and salt stress resistance of the cladoceran Moina macrocopa. First, a life-table experiment with the whole lifespan was performed with female neonates along a HS gradient with of 0, 5, 10, 20, 50 and 100 mg DOC L-1. Further, a 1-week life-table experiment was performed exposing animals to HS (10 mg DOC L-1) and/or salt (5 g L-1), in a 2x2 factorial design. This experiment was run simultaneously with animals from 2 different pre-treatments: non-pre-exposed (regular culture) or pre-exposed to HS (10 mg L-1). This allowed the test of transgenerational (maternal) effects of HS.
HS exposure at concentrations between 5 and 20 mg DOC L-1 extended both the mean and the maximum lifespan of M. macrocopa, with maximum extensions of 32% (mean lifespan) and 42% (maximum lifespan) at 10 DOC mg L-1. HS also increased body length at first reproduction and enhanced male offspring production, but there were no changes in the total offspring produced. Salt significantly reduced somatic growth and offspring production. Nevertheless, HS promoted the resistance to salt stress, alleviating these symptoms. These effects were observed even when the HS exposure was only maternal (salt treatment for the pre-exposed population). Our results suggest that, under environmentally realistic concentrations, natural HS modify crucial life-history traits of M. macrocopa by acting as mild chemical stressors, favoring its survival and maintenance on ecological and evolutionary timescales. Some of the effects of HS are transgenerational and thus may apply to the timescale of multiple generations. These effects of HS may help aquatic organisms to persist in fluctuating environments, such as costal lagoons, where salinity is widely variable over time.