Plant characteristics associated with widespread variation in eelgrass wasting disease

Abstract

Seagrasses are ecosystem engineers of essential marine habitat. Their populations are rapidly declining worldwide. One potential cause of seagrass population declines is wasting disease, which is caused by opportunistic pathogens in the genus Labyrinthula. While infection with these pathogens is common in seagrasses, theory suggests that disease only occurs when environmental stressors cause immunosuppression of the host. Recent evidence suggests that host factors may also contribute to disease caused by opportunistic pathogens. In order to quantify patterns of disease, identify risk factors, and investigate responses to infection, we surveyed shoot density, shoot length, epiphyte load, production of plant defenses (phenols), and wasting disease prevalence in eelgrass Zostera marina across 11 sites in the central Salish Sea (Washington state, USA), a region where both wasting disease and eelgrass declines have been documented. Wasting disease was diagnosed by the presence of necrotic lesions, and Labyrinthula cells were identified with histology. Disease prevalence among sites varied from 6 to 79%. The probability of a shoot being diseased was higher in longer shoots, in patches of higher shoot density, and in shoots with higher levels of biofouling from epiphytes. Phenolic concentration was higher in diseased leaves. We hypothesize that this results from the induction of phenols during infection. Additional research is needed to evaluate whether phenols are an adaptive defense against Labyrinthula infection. The high site-level variation in disease prevalence emphasizes the potential for wasting disease to be causing some of the observed decline in eelgrass beds.

Seagrass wasting disease varies with salinity and depth in natural Zostera marina populations

“…To our knowledge, few other re gions have shown equally high prevalence of lesions in Z. marina during recent years. Lesion prevalence has previously been found to vary be tween 0 and 95% (x -= 23%) in Z. marina meadows in northern Europe (Bockelmann et al 2013), and between 6 and 79% (x -= 44%) in the San Juan archipelago, USA (Groner et al 2014(Groner et al , 2016. The prevalence of L. zosterae cells in Z. marina is also higher in our study compared to the few areas where this has previously been measured.…”

“…No significant difference in L. zosterae infection was found between the 3 high-salinity meadows, but a significant small-scale variation among shoots within meadows was detected. Overall, our results show that high-salinity meadows of Z. marina from the study area have high prevalence of L. zosterae infection compared to what has been reported from other regions (Bockelmann et al 2013, Groner et al 2016. We found that 87 to 100% of the shoots had lesions and that 61 to 91% carried L. zosterae cells.…”

“…So far, the degree of infection has been found to vary spatially within and between meadows, and temporally within and between years (Hily et al 2002, Bockelmann et al 2013. The infection rate has also been shown to correlate with several seagrass parameters, including shoot length (Groner et al 2014), biomass (Trevathan-Tackett et al 2013, epiphyte load (Groner et al 2016), and seagrass density (Bull et al 2012, Groner et al 2016. Furthermore, pathogen prevalence has been connected to water depth in intertidal Z. marina meadows, suggesting that the disease is more prevalent on shoots at shallower depths (Groner et al 2014), but it is presently unknown whether this pattern also applies to non-tidal systems such as the ones found on the Swedish west coast.…”

Oysters and Eelgrass: Potential Partners in a High pCO2 Ocean

“…The ranges of EWD severity observed in phase I (natural infection) are typical of those observed at sites in the San Juan Archipelago during the summer (which our experimental conditions mimicked, Groner et al 2014Groner et al , 2016. It is unclear whether the host, the pathogen or both are driving this shift.…”

“…This site was chosen because it has a relatively low EWD prevalence for this region (mean AE SE: 19 AE 3% diseased on 9 August 2014; Groner et al 2016) and a stable eelgrass population has existed since at least 2010 (Wyllie-Echeverria et al 2010, Groner et al 2014, 2016. This site was chosen because it has a relatively low EWD prevalence for this region (mean AE SE: 19 AE 3% diseased on 9 August 2014; Groner et al 2016) and a stable eelgrass population has existed since at least 2010 (Wyllie-Echeverria et al 2010, Groner et al 2014, 2016.…”