A Review on Toxic and Harmful Algae in Greek Coastal Waters (E. Mediterranean Sea)

The Greek coastal waters are subjected to harmful algal bloom (HAB) phenomena due to the occurrence of species characterized as toxic (TX), potentially toxic (PT), and non-toxic, high biomass (HB) producers causing harm at multiple levels. The total number of (TX), (PT) and (HB) algae reported in this work are 61, but only 16 species have been associated with the occurrence of important HABs causing damage in the marine biota and the water quality. These phenomena are sporadic in time, space and recurrence of the causative species, and are related to the anthropogenically-induced eutrophication conditions prevailing in the investigated areas.


Introduction
The coastline (18,000 km) of the Greek mainland is located in the Eastern Mediterranean Sea, it is surrounded by the Aegean, Ionian and Cretan Seas and its morphological regime shows a variety of gulfs and semi-enclosed gulfs. All these basins are eutrophic [1] since they receive the waters and fertile material from large rivers and/or smaller water outfalls derived from agricultural and industrial activities.

OPEN ACCESS
Eutrophication triggers various physical and chemical changes in the marine environment and exerts a pressure on algal populations, allowing the intensive growth of certain harmful-toxin producing species or nuisance blooms that may create problems in the structure of the ecosystem and public health. These blooms are collectively called Harmful Algal Blooms (HABs). The greatest number of toxic species is found among dinoflagellates, but evidence has been provided for several species of other taxa (diatoms, flagellates, cyanobacteria, prymnesiophytes, rhaphidophytes) suggesting that they belong in this category [2][3][4].
Concern about harmful algae in Greek coastal waters has been growing since the late 1970s, when the first symptoms of -fish kills‖ due to the increased anthropogenic effects led to the fact that HABs-often quoted as the phenomenon of red tides-acquired the attention of scientists and the public. Since then, routine records of phytoplankton samples from almost all major gulfs along the Greek coastline during the last 30 years have revealed the presence of toxic and potentially toxic algae (those producing and/or potentially producing toxins) and non-toxic, high biomass producing species (non-toxic producers, but causing harmful blooms at multiple levels), although their destructive effects were occasional.
The European Commission has funded a number of projects such as EUROHAB (European Initiative on Harmful Algal Blooms) to generate the required research to better manage the effects of toxic/harmful marine microalgae that have caused problems in European marine waters [5]. This paper is the first comprehensive presentation of these species in the Eastern Mediterranean Sea, based on a synopsis of all published information for the period 1977-2009.

Sampling Areas and Data Collection
The investigated area ( Figure 1) is located in the Eastern Mediterranean and presents the sampling regions along the coastlines of the North Aegean Sea (I), the Western Aegean Sea (II), the Southern Aegean Sea (III), the Ionian Sea (IV) and the Mytilini Island, Eastern Aegean Sea (V). These sites include nine major Gulfs (a: Thermaikos; b: Kavalas; c: Pagassitikos; d: Malliakos; e: Evoikos; f: Saronikos; g: Messiniakos; h: Amvrakikos and i: Kalloni), as well as harbors, docks and marinas.
The collection of data covers the period 1977-2008. The methodology of sampling, preservation of samples, quantitative-qualitative analysis and the toxicity detection/evaluation of each one of the phytoplanktonic species under investigation are given in the literature cited in Table 1. The characterization of species as toxic (TX), potentially toxic (PT) and high biomass (HB) harmful blooms in this work was based on publications providing comprehensive descriptions of the current status of knowledge in the field as well as the IOC-UNESCO Taxonomic Reference List of Harmful Micro Algae [4]. The specifications of toxins were also determined from the literature.

Results and Discussion
A traditional system has been adopted for the eukaryotic species taxonomy [6]. Cyanobacteria are prokaryotes that may create problems producing diverse neurotoxins hazardous for human health; they have been classified among the HAB species [7] and are therefore included here. The majority of species are autotrophic (photosynthetic algae), but certain species (mostly dinoflagellates) are heterotrophic (feeding on particulate or dissolved organic matter) and their mode of nutrition (phagotrophy, osmotrophy) has been also taken into consideration [8]. It is interesting to notice that species of the same family differ in toxic properties.  Family Catenulaceae. A strain of the species Amphora coffeaeformis (from Canada) was found to produce Domoic acid. Other strains examined so far were non-toxic. However, the precise identification of the Canadian strain has been questioned [4,9]. It is also a mucilage producer [10].

Order Peridiniales
Family Goniodomataceae. This family comprises six species of the genus Alexandrium and one of the genus Gambierdiscus that are among the well known harmful algae. A. catenella is a producer of c1-c4 toxins, Saxitoxins and Gonyautoxins [16,17]. A. tamarense, A. minutum and A. taylori produce Gonyautoxins [18,19]. A. balechii and A. insuetum have been characterized in the literature as species of unknown toxicity, but they have been associated with harmful algal blooms [14,20] and Gambierdiscus sp. is known to be toxic producing Ciguatoxin and Maitotoxine [21].

Order Noctilucales
Family Noctilucaceae. Noctiluca scintillans is the single species of this family. It is a non-toxic bloom forming species [57] responsible for harmful outbursts (water discoloration, anoxic events).

Order Nostocalles
Family Oscillatoriaceae. Lyngbya agardhii is a high biomass forming species [62] and Trichodesmium erythraeum produces Saxitoxin [63]. Table 1 presents alphabetically the list of species, their toxic properties and the area of their occurrence given in the literature.

The ecological role of toxic, potentially toxic and bloom forming species in Greek coastal waters
In the present article (Table 1) we nominate toxic (TX) as the species producing blooms associated with evident toxic symptoms in the marine ecosystem, e.g., fish and shellfish death, or in humans consuming the poisoned fish and shellfish populations. Thus, consumption of contaminated shellfish by (a) the diatom Pseudonitzschia seriata, a domoic acid producer, caused [13] amnesic shellfish poisoning (ASP), (b) the dinoflagellate Dinophysis sacculus, an okadaic acid producer, caused [48] diarrhetic shellfish poisoning (DSP) and (c) the cyanobacterium Microcystis aeruginosa, a microcystin-LR producer, caused [61] extensive liver damage.
Potentially toxic (PT) are characterized as species carrying the toxigenic potential according to toxicological analyses, but their presence in an area has not been accompanied by toxic blooms and the relevant symptoms. A noticeable example is the toxic dinoflagellate (GTX1-4) Alexandrium minutum, whose presence did not produce toxic symptoms in the Greek coastal waters since their nutritional status did not favor blooms of this species [64].
Certain non-toxic species create high biomass (HB) blooms that have been characterized as harmful, because their occurrence produces discoloration of the water, undesirable aesthetic symptoms and anoxic harmful conditions to the ecosystem. They also cause severe economic problems due to loss to fisheries and tourism operations [65]. Massive growth of the dinoflagellates Noctiluca scintillans (late winter-early spring), Chatonella globosa (spring) and several species of the genus Prorocentrum in autumn (P. micans, P. triestinum, P. obtusidens and P. rostratum) caused severe water discoloration in Thermakos Gulf during the years 2000-2004 [43].
The total numbers of (TX), (PT) and (HB) algae reported in this work are 61 species. Dinoflagellates included 46 species contributing the 75% of total number (Table 1). Among them, three species are toxic (Dinophysis acuminata, Gyrodinium aureolum, Karenia brevis), seven species are forming high biomass (HB) harmful blooms and the rest (36) are potentially toxic species. Dinoflagellates are referred [66] as the group producing the most potent biotoxins known and with the largest number of HAB species, and the present data (75% dinoflagellates of total number of HAB species) are in accordance with this information.
Diatoms were represented by only six species-all potentially toxic-and this might be attributed to their nutrition requirements for a well balanced ratio (N:P:Si) of all nutrients. This necessity makes them poorer competitors than the non-siliceous dinoflagellates that seem to have a competitive advantage over diatoms if the stoichiometry of nutrients is deviated from its normal status in seawater [67].
Among the 61 species presented in Table 1, certain algae (16) have been associated with the occurrence of important HAB incidents in the investigated areas during the last 30 years, and six among these are heterotrophic species. Table 3 presents the seasonal and spatial distribution of the HAB incidents and the associated impact in the biotic community and water quality. The present data demonstrate that HAB episodes in Greek coastal waters are sporadic in time, space and recurrence of the causative species. Blooms However, the presence of P. minimum in the Kalloni Gulf did not cause any undesirable incidents [29]. Mass occurrence (4.7 × 10 5 -2.5 × 10 6 cells.L −1 ) of Alexandrium insuetum caused water discoloration in the Amvrakikos Gulf in the spring of 2003 and 2004, but in Kalloni Gulf did not create harmful effects [29].
From the ecological point of view, most (TX), (PT) and (HB) algae (Table 1) are -normal‖ components of inshore waters [72,73]. However, major gaps still exist in our understanding of the factors triggering only certain species to initiate and develop harmful populations. There is evidence that HABs are eutrophication-induced phenomena thriven by anthropogenic activities. Records on the trophic status of the Aegean and Ionian Gulfs [1] proved that the investigated areas ( Figure 1) were characterized -eutrophic‖ because the chl α concentrations were higher (>>1.0 mg chlα. m −3 ) in relation to the values (<<0.5 mg chlα. m −3 ) prevailing in the oligotrophic open oceanic waters [74]. The information available on the eutrophication-HAB relationship has recently increased, regarding the general explanation of the competition of phytoplankton species in relation to overall nutrient availability and the ratio between different nutrient species [65].
It is interesting to notice that the species Alexandrium insuetum, A. tamarense, Gymnodinium catenatum, Gyrodinium aureolum, Coolia monotis, Ostreopsis ovata and O. siamensis are not indigenous, but alien species of the Mediterranean Sea. They have been introduced via ship traffic for the Atlantic, Pacific and Indian Oceans [75] and it is obvious that the -ballast water‖ problem needs urgent attention [76].

Conclusions
The available data indicate that 61 identified HAB species (toxic, potentially toxic and high biomass producing algae) have spread across the Greek coastline during the last 30 years. Among these, certain algae (16) were associated with the occurrence of important HAB incidents causing damage in the marine biota and the water quality. There is a strong indication that these incidents were eutrophication-induced phenomena, but sporadic in time, space and recurrence of the causative species.