Detection of a Planktothrix agardhii Bloom in Portuguese Marine Coastal Waters

Abstract

Cyanobacteria blooms are frequent in freshwaters and are responsible for water quality deterioration and human intoxication. Although, not a new phenomenon, concern exists on the increasing persistence, scale, and toxicity of these blooms. There is evidence, in recent years, of the transfer of these toxins from inland to marine waters through freshwater outflow. However, the true impact of these blooms in marine habitats has been overlooked. In the present work, we describe the detection of Planktothrix agardhii, which is a common microcystin producer, in the Portuguese marine coastal waters nearby a river outfall in an area used for shellfish harvesting and recreational activities. P. agardhii was first observed in November of 2016 in seawater samples that are in the scope of the national shellfish monitoring system. This occurrence was followed closely between November and December of 2016 by a weekly sampling of mussels and water from the sea pier and adjacent river mouth with salinity ranging from 35 to 3. High cell densities were found in the water from both sea pier and river outfall, reaching concentrations of 4,960,608 cells·L^-1 and 6810.3 × 10⁶ cells·L^-1 respectively. Cultures were also established with success from the environment and microplate salinity growth assays showed that the isolates grew at salinity 10. HPLC-PDA analysis of total microcystin content in mussel tissue, water biomass, and P. agardhii cultures did not retrieve a positive result. In addition, microcystin related genes were not detected in the water nor cultures. So, the P. agardhii present in the environment was probably a non-toxic strain. This is, to our knowledge, the first report on a P. agardhii bloom reaching the sea and points to the relevance to also monitoring freshwater harmful phytoplankton and related toxins in seafood harvesting and recreational coastal areas, particularly under the influence of river plumes.

Keywords: Planktothrix agardhii; cyanobacteria; halotolerance; harmful algal blooms; mcyA; microcystins; salinity.

Figure 1

Light microscopy photographs of Planktothrix agardhii strains observed in this study. (a,f)—P. agardhii filaments observed in marine water from the sea pier; (b,c)—P. agardhii filaments observed in freshwater from S. Domingos reservoir; (d,e)—P. agardhii strain IPMA2; (g,h)—P. agardhii strain IPMA5; (i,j)—P. agardhii strain IPMA3. The arrow indicates the calyptra. Scale bar 5 µm.

Figure 2

P. agardhii cell concentrations in water samples. (a)—Sampling point 1: monitoring station of the Portuguese National Shellfish Monitoring System at the sea pier; (b)—Sampling point 2: River runoff at the beach between the sea and the river; (c)—Sampling point 3: river outfall near the beach. The arrow indicates the first observed occurrence; (*)—No sampling.

Figure 3

Morphometry of Planktothrix agardhii strains isolated in this study. Narrow lines indicate the range of the measurements; (♦)—indicates the average with the value and the standard deviation.

Figure 4

Phylogenetic tree of P. agardhii strains and closely related Planktothrix retrieved from GenBank, inferred by using the Maximum Likelihood method based on the Kimura 2-parameter model of the rpoC1 gene sequences. The percentage bootstrap values of 1000 replicates are given at each node. GenBank accession numbers are indicated after the species designation. The tree is unrooted and drawn to scale, with branch lengths measured in the number of substitutions per site. The analysis involved 23 nucleotide representative sequences. There were a total of 452 positions in the final dataset. The grey box indicates the origin and strain number of the cultures obtained in this study. Accession numbers: IPMA1—MG452723; IPMA2—MG452724; IPMA3—MG452725; IPMA4—MG452726; IPMA5—MG452727; IPMA6—MG452728.

Figure 5

Growth curves of the Planktothrix agardhii strains exposed to several salinities. (a) P. agardhii IPMA2 collected at the sea pier; (b) P. agardhii IPMA3 collected at the river outfall; (c) P. agardhii IPMA5 collected at the freshwater reservoir. Range of salinities tested: (∆) 0; (○) 2.5; (□) 5; (▲) 10; (●) 20; (■) 30.

Figure 6

Schematic representation at scale of the sampling site at the northwest of the Portuguese coast. (●1)—Monitoring station of the Portuguese National Shellfish Monitoring System at the sea pier (39°21′00.4″ N 9°22′19.4″ W), (●2)—Sampling point 2, river run-off at the beach between the sea and the river (39°21′03.0″ N 9°22′04.7″ W), (●3)—Sampling point 3 at the river outfall near the beach (39°21′04.6″ N 9°21′58.6″ W), (●4)—Sampling point 4 near the water dam discharged at the freshwater reservoir located 5 km upstream of the river S. Domingos (39°19′58.1″ N 9°19′07.5″ W), (−)—Sea Piers and water dam, grey lines represent water lines.